Percent Error Calculator
Measurement Accuracy Tool
Enter your measured (experimental) value and the known (theoretical) value to find the percent, signed, and absolute error.
Understanding Percent Error
Percent error tells you how close a measured or experimental result is to the true, accepted value — a foundational concept for evaluating accuracy in science, engineering, and everyday measurement.
In lab experiments, quality control, and engineering, percent error is the standard way to quantify how accurate a measurement or result is compared to a known reference. It lets you judge whether your instruments, technique, or process are reliable, and makes results comparable across experiments of very different scales.
Signed error keeps the direction of the deviation — positive if you overshot the true value, negative if you undershot it. Absolute error strips the sign, giving just the magnitude. Percent error then expresses that magnitude as a share of the theoretical value, so accuracy can be compared regardless of units.
The Formula
Where E is the experimental (measured) value and T is the theoretical (accepted) value.
Reading Your Results
A percent error close to 0% means your measurement closely matches the accepted value, while a large percent error suggests measurement error, instrument miscalibration, or a flawed procedure. The signed error tells you the direction of the mistake — a positive signed error means your experimental value was higher than expected, while a negative signed error means it was lower. Because the theoretical value sits in the denominator of the formula, it can never be zero; if there's truly nothing to divide by, percent error simply isn't a meaningful metric for that comparison.
Key Takeaways
- Percent error measures accuracy. It expresses how far an experimental value strays from the theoretical value, as a percentage.
- Signed error shows direction. A positive value means you overshot the true value; a negative value means you undershot it.
- The theoretical value can never be zero. Dividing by zero makes percent error undefined for that comparison.
- Want to measure data spread instead? Use the Standard Deviation Calculator to see how spread out a data set is.
Frequently Asked Questions
- Enter your Experimental (Measured) Value — the number you actually observed or measured.
- Enter the Theoretical (Accepted) Value — the known, expected, or accepted correct value.
- Click "Calculate" to see the percent error, signed error, and absolute error.
Percent error measures how far off an experimental or measured value is from the true (theoretical) value, expressed as a percentage of the theoretical value. A smaller percent error means the measurement is closer to the accepted value; a larger percent error signals more measurement inaccuracy or experimental error.
In labs and scientific experiments, percent error is a standard way to quantify measurement accuracy against a known or accepted reference value. It lets students and researchers evaluate whether their experimental technique, instrument calibration, or methodology introduced meaningful deviation, and it makes results from different experiments directly comparable regardless of the units or magnitude involved.
Signed error (experimental − theoretical) keeps its sign, telling you whether the measurement overshot (positive) or undershot (negative) the true value. Absolute error is the magnitude of that difference with the sign removed. Percent error takes the absolute error and expresses it as a percentage of the theoretical value, making it easy to compare accuracy across measurements of very different sizes.
Percent error divides the absolute error by the absolute value of the theoretical value. If the theoretical value is zero, that division is undefined (division by zero), so the calculator cannot compute a meaningful percentage in that case and will show an error instead of an incorrect result like Infinity or NaN.
The calculator computes results using full floating-point precision and displays them rounded to a few decimal places for readability. For lab reports or formal write-ups, follow your instructor's or publication's significant-figure conventions rather than copying every displayed digit.