Guides
Step-by-step explanations that go deeper than a single calculator — from building a timesheet in Excel to understanding how loans are amortized.
Math 10
- How to Add and Subtract Fractions With Unlike DenominatorsRewrite both fractions over a common denominator, then add or subtract the numerators. Step-by-step method, shortcuts and mixed-number examples.
- How to Calculate a Percentage: 3 Formulas With ExamplesEvery percentage problem is one of three questions about a part, a whole and a rate. Learn the formula for each, plus fast mental shortcuts.
- How to Calculate Percentage Change, Increase and DecreaseOne formula covers every percentage increase and decrease. Learn it, plus how to reverse a change, chain several changes and avoid the classic traps.
- How to Convert a Decimal to a Fraction (Including Repeating Decimals)Write the decimal over a power of 10 and simplify, or use a short algebra trick for repeating decimals. Includes an inch-fraction chart.
- How to Do Long Division: Step-by-Step With ExamplesLong division is one four-step cycle repeated: divide, multiply, subtract, bring down. Worked examples cover remainders, two-digit divisors and decimals.
- How to Find the GCF and LCM: 4 Methods and When to Use EachFour ways to find the greatest common factor and least common multiple, the shortcut that links them, and how to tell which one a word problem is asking for.
- How to Simplify Fractions: 3 Methods and Common MistakesReduce any fraction to lowest terms by dividing the top and bottom by their greatest common factor. Three methods, divisibility shortcuts and worked examples.
- How to Write Numbers in Scientific Notation (and Calculate With Them)Write any number as a value from 1 to 10 times a power of ten, convert back, and do arithmetic in scientific notation without losing track of the exponent.
- Order of Operations (PEMDAS) Explained With ExamplesPEMDAS tells you which part of an expression to calculate first. Learn the rules, the left-to-right tie-breaker and the traps that cause most errors.
- Significant Figures Rules: Counting, Rounding and CalculatingWhich digits count as significant, how to round to a given number of sig figs, and how many to keep after adding, multiplying or taking a logarithm.
Algebra 6
- Exponent Rules: The Laws of Exponents With ExamplesThe seven laws of exponents, why each one works, and how to combine them to simplify expressions with negative and fractional powers.
- How to Factor Trinomials: x² + bx + c and ax² + bx + cFind two numbers that multiply to one value and add to another. The product-sum and AC methods, sign patterns, and how to tell when a trinomial will not factor.
- How to Simplify Square Roots and Other RadicalsPull the largest perfect-square factor out from under the root sign. Step-by-step examples for square roots, cube roots, variables and denominators.
- How to Solve a System of Equations: Substitution, Elimination and Cramer's RuleFour ways to solve two or three linear equations at once, how to choose between them, and how to spot systems with no solution or infinitely many.
- How to Solve Quadratic Equations: Factoring, Completing the Square and the FormulaThree methods solve every quadratic equation. Learn when each one is fastest, how the discriminant predicts the answers, and how to check your roots.
- Logarithm Rules: Product, Quotient, Power and Change of BaseEvery logarithm rule in one place, why each works, and how to use them to expand and condense logs and solve exponential and log equations.
Statistics 8
- How to Calculate a Confidence Interval for a Mean or ProportionA confidence interval is an estimate plus or minus a margin of error. Step-by-step examples for means and proportions, critical values and sample size.
- How to Calculate a Weighted Average (Grades, Prices and More)Multiply each value by its weight, add the products and divide by the sum of the weights. Worked examples for grades, prices, speeds and percentages.
- How to Calculate a Z-Score and Turn It Into a PercentileA z-score counts how many standard deviations a value sits from the mean. Learn the formula, how to read it as a percentile and where it breaks down.
- How to Calculate Standard Deviation by Hand (Step by Step)Five steps from raw data to standard deviation, with a fully worked example, the n − 1 rule for samples, a faster shortcut formula and grouped data.
- How to Calculate Your GPA (Semester, Cumulative and Weighted)Convert grades to points, weight them by credit hours, and divide. Step-by-step semester and cumulative examples, weighted GPAs and how to raise yours.
- How to Find Quartiles and the Interquartile Range (IQR)Sort the data, split it at the median, and find the median of each half. Worked odd and even examples, outlier fences, and why methods disagree.
- Mean vs Median vs Mode: Which Average Should You Use?The mean, median and mode each answer a different question about the middle of your data. Learn how to find them and which one tells the honest story.
- What Is a P-Value? A Plain-English Explanation With ExamplesA p-value measures how surprising your data would be if nothing interesting were going on. Learn how to calculate it, read it and avoid the classic misinterpretations.
Geometry 5
- How to Calculate Volume: Formulas for Boxes, Cylinders, Cones and SpheresMost volume formulas are base area times height, with a factor of one-third for pointed shapes. Formulas, worked examples and unit conversions.
- How to Find the Area of a Circle (From Radius, Diameter or Circumference)Area equals pi times the radius squared. Learn the formula, the diameter and circumference versions, real-world examples and partial circles.
- How to Find the Area of a Triangle: 5 Formulas and When to Use EachPick the triangle area formula that matches what you know: base and height, three sides, two sides and an angle, or corner coordinates.
- How to Find the Slope of a Line (From Two Points or an Equation)Slope is rise over run. Find it from two points, a graph or an equation, and convert it to a percent grade or angle for ramps, roofs and roads.
- How to Use the Pythagorean Theorem (a² + b² = c²)Find any missing side of a right triangle with a² + b² = c². Worked examples for ladders, screens, square corners and 3D diagonals.
Trigonometry 2
- How to Convert Degrees to Radians (and Radians to Degrees)Multiply degrees by π/180 to get radians, or radians by 180/π to get degrees. Exact forms, a full reference table and where radians are required.
- SOHCAHTOA Explained: Sine, Cosine and Tangent in Right TrianglesSOH CAH TOA names the three trig ratios of a right triangle. Learn to pick the right one, solve for sides and angles, and avoid degree-mode errors.
Finance 22
- 15-Year vs. 30-Year Mortgage: Payments, Interest and Which to ChooseA 15-year mortgage costs far less interest but has a much higher payment. See the numbers side by side and the questions that decide which term fits you.
- APR vs. APY: What's the Difference and Which One to UseAPR is a simple annual rate that ignores compounding (and on loans includes fees); APY includes compounding. Learn when each applies and how to convert.
- CAGR vs. Average Annual Return: Why the Numbers DifferThe average of yearly returns overstates real growth whenever returns vary. CAGR gives the true compound rate. See the formulas, examples and when each applies.
- Debt Snowball vs. Avalanche: Which Pays Off Debt Faster?The avalanche method saves the most interest; the snowball method delivers quicker wins. See both run on real numbers and how to pick one.
- How a 401(k) Match Works: Formulas, Vesting and 2026 LimitsAn employer match adds free money to your 401(k) when you contribute. Learn to read your plan's formula, capture the full match and avoid losing it.
- How Compound Interest Works, with Formulas and ExamplesCompound interest earns interest on past interest. See the formula, how often compounding happens, what regular deposits add and how fast money doubles.
- How Credit Card Interest Is Calculated (Average Daily Balance)Card interest is the daily periodic rate times your average daily balance times the days in the cycle. Learn each step and how to avoid paying it.
- How Much Do Extra Mortgage Payments Save? Real Numbers by AmountExtra principal payments cut interest and years off a mortgage. Compare common strategies on a $300,000 loan and weigh prepaying against other uses of money.
- How Much Do I Need to Retire? The 4% Rule and Your NumberYour retirement number is roughly your yearly spending gap times 25. Work through spending, Social Security, withdrawal rates and the monthly savings needed.
- How Much House Can I Afford on a $100,000 Salary?Apply the 28/36 rule to a $100,000 income step by step, then see how rates, debts, down payment and taxes move the price you can afford.
- How to Calculate a Break-Even Point in Units and DollarsBreak-even units equal fixed costs divided by contribution margin per unit. Learn both break-even formulas, profit targets, sensitivity checks and product mixes.
- How to Calculate Mortgage Interest (Monthly, Yearly and Total)Work out the interest in any mortgage payment, the interest you pay each year and the total over the life of the loan, with formulas and a worked example.
- How to Calculate Net Pay (Take-Home Pay) from Gross PayNet pay is gross pay minus taxes and deductions, taken in a specific order. Follow the steps with 2026 federal figures and a worked biweekly paycheck.
- How to Calculate ROI (Return on Investment) and Annualized ROIROI is net gain divided by cost. Learn what belongs in each part, how to annualize ROI so different holding periods compare fairly, and where ROI misleads.
- How to Calculate Sales Tax (and Work Backward from a Total)Multiply the taxable price by the combined state and local rate to get sales tax. Learn the reverse formula, how discounts and trade-ins work, and common pitfalls.
- How to Calculate Self-Employment Tax (2026 Rates and Examples)Self-employment tax is 15.3% of 92.35% of your net profit, with a Social Security cap. Learn the steps, the deduction for half and how to pay it quarterly.
- How to Calculate Your Debt-to-Income (DTI) RatioDTI is your monthly debt payments divided by gross monthly income. Learn what counts, what lenders accept and how to bring the ratio down.
- How to Convert Salary to Hourly Pay (and Hourly to Salary)Divide salary by 2,080 work hours to get an hourly rate, or multiply hourly pay by 2,080 for a salary. Learn when to adjust the hours and what the rate leaves out.
- How to Read an Amortization Schedule, Column by ColumnAn amortization schedule lists every loan payment and how it splits between interest and principal. Learn to read and check one, line by line.
- Margin vs. Markup: The Difference and How to Convert Between ThemMarkup measures profit against cost; margin measures it against the selling price. Learn both formulas, convert between them and price products correctly.
- Marginal vs. Effective Tax Rate: What You Really Pay (2026)Your tax bracket is not the share of income you pay in tax. Learn how progressive brackets work, compute both rates with 2026 figures and use each correctly.
- Roth vs. Traditional IRA Explained: Which Is Better for You?Traditional IRAs give a tax break now and tax withdrawals later; Roth IRAs do the reverse. Compare them with 2026 limits and real after-tax numbers.
Time Cards 20
- FLSA Overtime Rules Explained in Plain EnglishA plain-English guide to federal overtime law: who is covered, how overtime pay is calculated, what counts as work time, and where state law goes further.
- How Many Work Hours Are in a Year? (2,080 and the 2026 Count)Payroll uses 2,080 hours a year, but the real number depends on the calendar, holidays and time off. See 2026 figures by month and which number to use when.
- How to Add Hours and Minutes in Excel (Totals Over 24 Hours)Sum shifts, add minutes to a clock time, total hours by employee and keep totals above 24 hours from wrapping back to zero.
- How to Build an Excel Timesheet with Lunch BreaksA ready-to-copy weekly timesheet layout with formulas that subtract unpaid lunches, flag missing punches and total the week for overtime.
- How to Calculate Average Hours in ExcelAverage daily hours, weekly hours and start times in Excel with formulas that handle days off, time formats and shifts that cross midnight.
- How to Calculate Holiday Pay for Hourly and Salaried EmployeesHoliday pay is set by employer policy, not federal law. Learn the common policies, how to calculate each, and how holiday hours interact with overtime.
- How to Calculate Hours Worked Across Midnight (Overnight Shifts)When a shift ends after midnight, add 24 hours to the end time before subtracting. Learn the method, break deductions, day splits and DST adjustments.
- How to Calculate Hours Worked in ExcelSet up a date, time in and time out sheet in Excel, then turn punches into hours and minutes, decimal hours, weekly totals and gross pay.
- How to Calculate Night Shift Hours in ExcelFormulas for shifts that cross midnight, shifts of 24 hours or more, night differential hours, workweek splits and daylight saving changes.
- How to Calculate Overtime in ExcelTurn a week of clock times into regular hours, overtime hours and gross pay with a few Excel formulas, plus daily overtime, double time and bonus variations.
- How to Calculate Overtime Pay Under State Rules (Daily OT and Double Time)Some states require overtime after 8 or 12 hours in a day, not just 40 in a week. Learn the rules and calculate a week's pay step by step.
- How to Calculate PTO Accrual per Hour and per Pay PeriodDivide annual PTO hours by hours worked or pay periods to get an accrual rate. Learn the formulas, rate tables, caps and how part-time accrual works.
- How to Calculate Shift Differential Pay (Including Overtime)A shift differential adds a flat amount or a percentage to the base rate for less desirable hours. Learn to compute it and include it in overtime pay.
- How to Calculate Time and a Half (and Double Time) PayTime and a half is 1.5 times your regular rate for each overtime hour. Learn the formula, a quick rate table and the special cases payroll often gets wrong.
- How to Convert Decimal Hours to Time (h:mm) in ExcelTurn decimal hours such as 7.75 or 41.25 into readable h:mm times in Excel, rounded cleanly to the minute and correct past 24 hours.
- How to Convert Hours to Minutes in Excel (and Back Again)Turn decimal hours or h:mm times into total minutes in Excel, total a timesheet in minutes, and convert minutes back to hours.
- How to Convert Time to Decimal Hours in ExcelTurn h:mm entries such as 7:45 into decimal hours such as 7.75 for payroll, invoices and pay formulas, with the right rounding.
- How to Fill Out a Timesheet CorrectlyA practical walkthrough for employees and supervisors on recording time, breaks, overtime and leave, approving timesheets and fixing errors.
- How to Read Military Time (24-Hour Clock)How to read, say and convert military time, the 24-hour clock that runs from 0000 to 2359 with no AM or PM.
- Time Clock Rounding Rules: The 7-Minute Rule and Other IncrementsFederal rules allow rounding punches to 5, 6 or 15 minutes only if it is neutral over time. Learn each rounding rule and how to check your own time.
Time & Date 6
- How Leap Years Work: The Rule, the Reason and the ExceptionsA year is a leap year if it is divisible by 4, unless it is a century year not divisible by 400. Learn why the rule exists and how it affects dates and pay.
- How to Calculate Business Days Between Dates (Excluding Holidays)Business days are weekdays minus holidays. Learn a fast hand method, how to add business days to a deadline, which holidays to skip and the 2026 counts.
- How to Calculate Your Age in Days, Weeks and HoursYour age in days is 365 times your full years, plus one for each leap day you have lived through, plus the days since your last birthday. Here is the method.
- How to Count Days Between Two Dates (Inclusive or Exclusive)Subtract the earlier date from the later one to get elapsed days, and add one if both end dates count. Learn the hand method, the fencepost trap and real uses.
- How to Find the Day of the Week for Any Date (Doomsday and Zeller)Find the weekday of any past or future date with two methods: Conway's Doomsday rule for mental math and Zeller's congruence for a reliable formula.
- ISO Week Numbers Explained: How to Find the Week Number of a DateISO weeks run Monday to Sunday, and week 1 is the week with the year's first Thursday. Learn the formula, 53-week years and how US week numbers differ.
Converters 5
- How to Convert Celsius to Fahrenheit (Exactly and in Your Head)Multiply by 1.8 and add 32, or use a quick mental shortcut. Exact formulas both ways, accuracy of the tricks, oven and weather tables, and Kelvin.
- How to Convert Centimeters to Feet and Inches (Height and Length)Divide centimeters by 2.54 to get inches, then split into feet and inches. Step-by-step method, a height chart, the reverse conversion and fraction-of-an-inch tips.
- How to Convert Kilograms to Pounds (and Pounds to Kilograms)One kilogram is about 2.2 pounds. Exact factors both ways, a mental shortcut accurate to 0.2%, pounds-and-ounces and stone conversions, and a quick chart.
- KB, MB, GB and TB Explained: Decimal vs Binary Storage UnitsStorage units come in two systems, powers of 1,000 and powers of 1,024. Learn which is which, why your drive looks smaller, and how bits and bytes differ.
- Metric Prefixes Explained: Kilo, Mega, Milli, Micro and MoreEvery metric prefix is a power of ten. A full chart of SI prefixes, a reliable way to convert between them, and the traps with squared, cubed and binary units.
Health 9
- How BMI Is Calculated, What It Means and Where It Falls ShortBMI divides weight by height squared to screen for underweight and excess weight. Learn the formula, the adult categories, a height chart and what BMI misses.
- How Much Protein Do I Need per Day? (By Weight, Age and Goal)Most adults need at least 0.8 grams of protein per kilogram of body weight a day; active people, older adults and dieters often benefit from more. Find your number.
- How Much Water Should I Drink a Day? A Practical Way to Calculate ItAdult women need about 2.7 liters and men about 3.7 liters of total water a day from drinks and food. Learn a simple personal estimate and when to drink more.
- How to Calculate a Due Date: Naegele's Rule, Conception and UltrasoundMost due dates are 280 days after the first day of the last menstrual period. Learn Naegele's rule, cycle and IVF adjustments, and why ultrasound may change it.
- How to Calculate BMR: Mifflin–St Jeor, Harris–Benedict and Katch–McArdleBMR is the energy your body burns at rest. Work through the three main equations with examples, see how they differ and turn BMR into daily calorie needs.
- How to Calculate Calories for Weight Loss (Step by Step)Estimate your daily calorie needs, subtract a moderate deficit and adjust as you lose weight. Worked examples, deficit tables and realistic expectations.
- How to Calculate Macros: Protein, Carbs and Fat in GramsTurn a daily calorie target into grams of protein, carbohydrate and fat. Learn the percentage and grams-first methods, common splits and how to check them.
- How to Calculate Your Target Heart Rate for ExerciseEstimate your maximum heart rate from age, then take a percentage of it, or of your heart rate reserve, to find moderate and vigorous exercise zones.
- How to Measure Body Fat Percentage: Methods, Formulas and AccuracyBody fat can be measured with scans, calipers, smart scales or a tape measure. Compare accuracy, work through the US Navy formula and read your result.
Physics 4
- How to Calculate Density (Solids, Liquids and Irregular Objects)Density is mass divided by volume. Learn to measure both for any object, convert density units, rearrange the formula and use density to identify materials.
- How to Calculate Kinetic Energy (and Potential Energy)Kinetic energy is half the mass times the speed squared. Learn the formula, why speed matters so much, how it trades with potential energy, and unit conversions.
- Ohm's Law Explained: Voltage, Current, Resistance and PowerV = I × R links voltage, current and resistance. Learn to rearrange it, add the power formulas, solve real circuits and know where the law stops applying.
- The Kinematic Equations Explained: How to Choose and Use SUVATFour equations describe any motion with constant acceleration. Learn which variable each one leaves out, how to choose, and how to avoid sign errors.
Chemistry 3
- How to Calculate Molar Mass From a Chemical FormulaAdd up the atomic weights of every atom in the formula. Worked examples with parentheses and hydrates, plus grams-to-moles and percent composition.
- How to Calculate Molarity and Make a Solution of Known ConcentrationMolarity is moles of solute per liter of solution. Step-by-step examples for calculating it, weighing out a solution, diluting stock and converting units.
- How to Calculate pH (Strong Acids, Bases, Weak Acids and Buffers)pH is the negative log of the hydrogen ion concentration. Step-by-step examples for strong and weak acids, bases and buffers, plus the logs behind the scale.
Construction 4
- How Much Concrete Do I Need? Slabs, Footings and Post HolesMultiply length × width × thickness in feet and divide by 27 for cubic yards. Worked examples for slabs, driveways, footings and fence posts, plus bag counts.
- How Much Paint Do I Need? Walls, Ceilings, Trim and ExteriorsWall area minus openings, times coats, divided by coverage per gallon. A worked room example, ceilings, trim, primer, exteriors and buying tips.
- How to Calculate Stairs: Risers, Treads, Run and Stringer LengthDivide the total rise into equal risers under 7¾ inches, then work out treads, run and stringer length. A worked example checked against IRC limits.
- How to Calculate the Square Footage of a Room or HouseMultiply length by width for each rectangle, then add them up. Step-by-step methods for odd-shaped rooms, inches, waste factors and unit conversions.
Tech 2
- How to Calculate Aspect Ratio (and Resize Without Distortion)Reduce width and height by their greatest common divisor to get the aspect ratio. Then resize, crop and compare screens without stretching anything.
- How to Convert Binary to Decimal (and Decimal to Binary and Hex)Add the powers of 2 where the bits are 1, or double as you read left to right. Reverse it by dividing by 2, and group bits in fours for hexadecimal.
Everyday 5
- How Long to Cook a Turkey: Times by Weight, Thawing and Safe TemperatureAt 325°F, a 12- to 14-pound unstuffed turkey roasts in about 3 to 3¾ hours. See times for every size, how long to thaw and how to know it is done.
- How to Calculate Dog Years in Human Years (Beyond Times 7)The first dog year is about 15 human years, the second about 9 more, then 4 to 7 per year depending on size. Compare the chart method with the epigenetic formula.
- How to Calculate the Electricity Cost of an ApplianceConvert an appliance's watts to kilowatt-hours, multiply by your electricity rate, and you have its running cost. Learn the formula, typical costs and how to cut them.
- How to Calculate Unit Price and Find the Better DealUnit price is the price divided by the quantity, such as cents per ounce. Learn to convert units, compare sizes and deals, and avoid common traps.
- How to Scale a Recipe Up or Down (Without Ruining It)Divide the servings you want by the servings the recipe makes, multiply every ingredient, then tidy the units. Learn the method and the exceptions.