Ohm’s law is the first formula most people meet in electronics, and together with the power law it ties together the four quantities on every circuit diagram: voltage, current, resistance and power. This calculator takes any two of them and returns the other two, with automatic unit prefixes, the conductance, a suggested resistor wattage and a small circuit sketch labeled with your numbers. It is handy for sizing resistors, checking what a heater or lamp draws, or verifying a homework answer.
How to use the Ohm’s law calculator
- In I know the, choose which pair you have: voltage and current, voltage and resistance, voltage and power, current and resistance, current and power, or resistance and power.
- Enter the two values. Voltage (V) takes µV, mV, V or kV; Current (I) µA, mA, A or kA; Resistance (R) mΩ, Ω, kΩ or MΩ; Power (P) µW, mW, W or kW. Every value must be greater than zero.
- Optionally pick a unit for each unknown under Show … in. The default, Best fit (auto), chooses the prefix that puts the number between 1 and 1,000.
- Read all four quantities (the calculated ones are highlighted), the conductance G = 1/R in siemens, the resistor rating to choose, and the circuit diagram. “Show the work” lists the two formulas used.
Ohm’s law and power formulas
Substituting one into the other gives twelve formulas, often drawn as a wheel. Each row lists the three ways to reach one quantity:
| To find | From the other quantities |
|---|---|
| Voltage V | I × R · P ÷ I · √(P × R) |
| Current I | V ÷ R · P ÷ V · √(P ÷ R) |
| Resistance R | V ÷ I · V² ÷ P · P ÷ I² |
| Power P | V × I · I² × R · V² ÷ R |
The calculator picks the pair of formulas that matches the two values you entered, so you never need to remember which wedge of the wheel applies.
Worked examples
12 V across a 220 Ω resistor (the default)
I = 12 V ÷ 220 Ω = 0.05455 A = 54.55 mA
P = (12 V)² ÷ 220 Ω = 0.6545 W (654.5 mW)
Conductance is 1 ÷ 220 = 0.004545 S. Twice the dissipation is 1.31 W, so the suggested rating is 2 W.
An LED current-limiting resistor. A red LED on a 5 V supply drops about 2.0 V, leaving 3 V across the resistor. For 20 mA, choose “Voltage and current”, enter 3 V and 20 mA, and the calculator gives R = 150 Ω and P = 60 mW, with a suggested rating of 0.125 W (1/8 W). If the result is not a stock value, round up to the next standard resistor; that lowers the current slightly, which is the safe direction.
A space heater. Choose “Voltage and power” and enter 120 V and 1,500 W. The heater draws I = 12.5 A and its hot element has R = 9.6 Ω. That uses most of a 15 A household circuit, so a heater should not share one with other large loads. No resistor rating is suggested here, because the list of standard ratings stops at 100 W.
Choosing a resistor power rating
A resistor turns all of its dissipated power into heat. Run one near its full rating and it can get hot enough to discolor a circuit board and drift in value; resistor ratings also assume a modest ambient temperature and fall off in a warm enclosure. The calculator therefore recommends the smallest standard rating (1/8, 1/4, 1/2, 1, 2, 3, 5, 10, 25, 50 or 100 W) that is at least twice the power you calculated. For surface-mount parts, the package sets the rating: typical values are about 1/10 W for 0603, 1/8 W for 0805 and 1/4 W for 1206 sizes.
Where Ohm’s law needs care
- Non-ohmic parts. Diodes, LEDs, transistors and thermistors do not have a fixed resistance, so V = IR applies only to the resistor in series with them, not to the device itself.
- Temperature. Metals become more resistive as they warm; copper’s resistance rises about 0.39% per °C. Filament lamps are the extreme case.
- AC circuits. Use RMS values. A 120 V RMS outlet actually peaks at about 170 V. The calculator’s note limits it to DC and purely resistive AC loads; inductors and capacitors call for impedance and phase angle.
For converting watts to horsepower or BTU per hour, use the power converter, and for juggling milli-, micro- and kilo- prefixes, the metric prefix converter. Energy over time is covered by the power calculator.
Frequently asked questions
What is Ohm's law?
Ohm's law states that the current through a resistor is proportional to the voltage across it: V = IR. One volt across one ohm drives one ampere. Combined with the power relation P = VI, it links voltage, current, resistance and power, so any two determine the other two.
How do I size a current-limiting resistor for an LED?
Subtract the LED's forward voltage from the supply voltage and divide by the current you want. With a 5 V supply, a red LED dropping about 2.0 V and 20 mA, R = 3 V ÷ 0.02 A = 150 Ω, dissipating 60 mW. Check the LED's datasheet for its actual forward voltage and maximum current.
What wattage resistor do I need?
Pick a rating at least twice the power the resistor will dissipate; the calculator suggests the smallest standard rating that meets this. A 220 Ω resistor across 12 V dissipates about 0.65 W, so it recommends 2 W rather than 1 W. The margin keeps the part cooler and longer-lived.
Does Ohm's law work for AC circuits?
For purely resistive loads such as heaters and incandescent lamps, yes, as long as you use RMS voltage and current. US outlets supply about 120 V RMS. Motors, transformers and power supplies with coils or capacitors need impedance instead of resistance, and their real power also depends on the power factor.
Why does my multimeter show a much lower resistance for a light bulb?
A tungsten filament's resistance rises steeply as it heats. A 60 W, 120 V bulb works out to 240 Ω when lit (120² ÷ 60), but typically measures less than a tenth of that when cold. Use operating values, not cold readings, for power calculations.