No manufactured part, measured dose or electronic component is exactly its nominal value. A tolerance says how far it is allowed to stray. Engineers, machinists, electricians, pharmacists and quality inspectors all write specs this way: a target value plus an allowed deviation, often as a percentage. This calculator converts that spec into hard minimum and maximum limits and, if you enter a measurement, tells you whether it passes.
How to use the percent tolerance calculator
- Enter the nominal value — the target or rated value printed on the part or drawing.
- Choose how the tolerance is written:
- ± percent for a symmetric band such as ±5%.
- + / − percents when the upper and lower allowances differ, such as +10% / −5%.
- ± amount when the spec gives a plain allowance in units, such as ±0.05 mm. The calculator converts it to a percentage.
- Optionally type a measured value to see whether it falls inside the band and how far it deviates from nominal.
- The range diagram marks the limits, the nominal value and your measurement (green inside, red outside).
Tolerance formulas
To go the other way, from a unit allowance to a percentage:
Worked example: checking a resistor
A resistor is labeled 470 Ω with a gold tolerance band, which means ±5%. A multimeter reads 482 Ω.
Allowance: 470 × 0.05 = 23.5 Ω
Minimum: 470 − 23.5 = 446.5 Ω
Maximum: 470 + 23.5 = 493.5 Ω
Measured deviation: (482 − 470) ÷ 470 × 100 ≈ +2.55%
Result: 482 Ω lies inside 446.5–493.5 Ω, so the part passes.
Common tolerance standards
| Context | Typical tolerance | Notes |
|---|---|---|
| Resistor, brown band | ±1% | Precision metal-film parts |
| Resistor, red band | ±2% | |
| Resistor, gold band | ±5% | General-purpose carbon film |
| Resistor, silver band | ±10% | Older or low-cost parts |
| Electrolytic capacitor | ±20% | Often looser on the high side |
| Capacitor code “Z” | +80% / −20% | A classic asymmetric tolerance |
| Machined fit, general | ±0.005 in (±0.13 mm) | Expressed as an amount, not a percentage |
Resistor color bands and tolerance letter codes were standardized by the former Electronic Industries Alliance (EIA) and are defined internationally in IEC 60062, which is why the same gold band means ±5% on parts from different manufacturers.
Percentage or amount?
Percent tolerances scale with the part: ±1% of a 10 kΩ resistor is 100 Ω, while ±1% of a 100 Ω resistor is just 1 Ω. Mechanical drawings usually give an absolute amount instead, because the precision a machine can hold does not grow with the size of the feature. Converting between the two is useful when comparing suppliers or deciding whether a measuring tool is precise enough. A common rule of thumb in metrology is that your instrument should resolve at least one-tenth of the tolerance band.
Stack-ups and safety margins
When several toleranced parts combine — resistors in series, shims in a stack, ingredients in a batch — their deviations can add up. In the worst case, every part sits at the same end of its range, and the total allowance is the sum of the individual allowances. Designers either plan for that worst case or use statistical methods that assume deviations partly cancel. For a single part, though, the min-max range from this calculator is all you need.
If you are comparing a measurement with a known true value instead of a spec, use the percent error calculator. To round a measured limit to a sensible number of digits, see the significant figures rounding calculator.
Frequently asked questions
How do I calculate a tolerance range from a percentage?
Multiply the nominal value by the tolerance percentage to get the allowance, then subtract it for the minimum and add it for the maximum. A 470 Ω resistor rated ±5% has an allowance of 23.5 Ω, so it may measure anywhere from 446.5 Ω to 493.5 Ω.
How do I convert a ± amount into a percent tolerance?
Divide the allowance by the nominal value and multiply by 100. A shaft specified as 25 mm ± 0.05 mm has a tolerance of 0.05 ÷ 25 × 100 = 0.2%.
What is an asymmetric tolerance?
It allows a different deviation above and below the nominal value, such as +10% / −5%. Some capacitors are sold with +80% / −20% tolerance, so a 10 µF part can measure from 8 µF up to 18 µF.
Is a value exactly on the limit within tolerance?
Usually yes — the limits are normally inclusive, so a reading equal to the minimum or maximum passes. Some quality standards define the limits differently, so check the specification you are working to.
How is tolerance different from percent error?
Tolerance is decided in advance and defines an acceptable band around a target. Percent error is measured after the fact and tells you how far a single result landed from the true value. A part can have a 2% error and still pass a ±5% tolerance.