Percent Error Calculator
Percent error calculator — enter your measured (experimental) value and the true (accepted) value to get the percent error, the absolute error and whether you over- or under-estimated. Essential for science labs and experiments. Runs in your browser.
Percent Error Calculator
How to Use the Percent Error Calculator
Enter your measurement
The experimental value you obtained.
Enter the true value
The accepted or reference value.
Read the percent error
See how far off you were, as a percentage.
Check the direction
See if you over- or under-estimated.
Measuring How Wrong You Were
Percent error is the standard way to express how far a measurement strays from the value it should be — the workhorse calculation of every school science lab and many professional ones. It takes the gap between your measured (experimental) value and the true (accepted) value, expresses that gap as a fraction of the true value, and turns it into a percentage. The formula is |measured − true| ÷ |true| × 100, so a reading of 98 against a true value of 100 carries a 2% error. This calculator returns that headline percentage along with the raw absolute error and a signed version that reveals the direction of the mistake.
Why a percentage rather than the raw difference? Because it standardises errors of very different sizes so they can be compared fairly. Being one centimetre out matters enormously when measuring a coin and not at all when measuring a football pitch; dividing by the true value captures that context. By convention percent error is reported as a positive number — the formula’s absolute values strip the sign — because it measures the magnitude of the error. The signed value this tool also shows is genuinely useful, though: a consistently positive signed error across repeated measurements points to an instrument or method that systematically reads high, and a consistently negative one points to reading low, which is the signature of a calibration problem rather than random scatter.
Two distinctions are worth keeping straight. First, percent error is not percent difference: percent error compares a measurement against a known correct value, while percent difference compares two equally uncertain measurements by dividing by their average — use the former when you have a reference and the latter when you do not. Second, accuracy is not precision: percent error quantifies accuracy, how close you are to the truth, whereas precision is about repeatability, how closely repeated readings agree. You can be precise yet inaccurate, or accurate yet imprecise. What counts as a “good” percent error depends entirely on the field — under five percent is often fine in a teaching lab, while analytical chemistry and precision engineering expect figures well under one percent. The one hard rule is that the true value cannot be zero, since the calculation divides by it. As always, the computation runs entirely in your browser, so your data never leaves your device.
Percent error measures accuracy against a known truth; a consistent signed error is the fingerprint of a miscalibrated instrument.
10 Facts About Percent Error
Percent error = |measured − true| ÷ |true| × 100.
It is usually reported as a positive value.
Lower percent error means a more accurate result.
Accuracy is closeness to truth; precision is repeatability.
A signed error shows over- or under-estimation.
Science labs report percent error to judge experiments.
It needs a known true or accepted value.
Percent error differs from percent difference.
A true value of zero makes percent error undefined.
This calculator runs in your browser — nothing is uploaded.
Frequently Asked Questions
- Percent error is the absolute difference between your measured value and the true (accepted) value, divided by the absolute true value, times 100: |measured − true| ÷ |true| × 100. A measurement of 98 against a true value of 100 has a percent error of 2%. This calculator computes it along with the absolute and signed error.
- Percent error compares a measurement against a known, accepted true value — it tells you how accurate you were. Percent difference compares two measured values when neither is the “correct” one, dividing by their average. Use percent error when there is a reference value, and percent difference when comparing two equally uncertain results.
- Conventionally it is reported as a positive number, because it measures the size of the error regardless of direction — that is why the formula uses absolute values. This calculator shows the positive percent error as the headline, and also a signed version so you can see whether you over- or under-estimated.
- The signed error keeps the direction: a positive value means your measurement was higher than the true value (an over-estimate), and a negative value means it was lower (an under-estimate). This is useful for spotting systematic bias in an instrument or method that consistently reads high or low.
- It depends entirely on the context. In a school physics lab, under 5% is often considered good and under 10% acceptable; in precision engineering or analytical chemistry, errors well below 1% are expected. There is no universal threshold — compare your result against the tolerance appropriate to your field and equipment.
- Accuracy is how close a measurement is to the true value, which percent error quantifies. Precision is how repeatable your measurements are — how closely repeated readings agree with each other, regardless of whether they are correct. You can be precise but inaccurate (consistently wrong) or accurate but imprecise (scattered around the truth).
- Because the formula divides by the true value, and dividing by zero is undefined. Percent error is inherently a relative measure — error as a fraction of the true magnitude — so it has no meaning when the reference value is zero. In that situation you would report the absolute error instead.
- Whenever you have measured something that has a known correct value and want to express how far off you were: validating a science experiment against a textbook constant, checking a sensor against a calibrated reference, or assessing a forecast against the actual outcome. It standardises the error so results of different sizes can be compared fairly.
- Yes — for the values you enter, the percent error, absolute error and signed error are computed exactly. The result is shown to several decimal places; round it to a sensible number of significant figures when reporting, matching the precision of your measurements.
- Completely free, with no account or limit. It works offline once the page has loaded and collects no data.
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