Percent Error / Accuracy Checker
Quantify the accuracy of your scientific data. Compare your lab results against accepted theoretical values.
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The Truth About Lab Data
In science, getting the "wrong" answer is expected. The goal isn't perfection, but rather understanding how close you are to perfection.
Percent Error is the standard metric for this. It answers the question: "How far off was I?" If you measured the density of water as 0.98 g/mL instead of 1.00 g/mL, you aren't "wrong"—you just have a 2% error. Understanding this number helps you diagnose if your equipment is faulty, your chemicals are impure, or if you just need steady hands.
Accuracy vs. Precision
These are not the same thing!
- Accuracy (Percent Error)How close you are to the Bullseye (True Value). This calculator measures Accuracy.
- Precision (Deviation)How close your shots are to each other. You can represent this with Standard Deviation across multiple trials.
Sources of Error
Equipment flaw. E.g., a scale that wasn't zeroed (tared). It skews all data in one direction. Calculated High Error.
Environmental noise. E.g., a breeze hitting a sensitive scale. Fixed by averaging trials.
Interpreting Your Score
0% - 5% (Excellent)
This is usually the "A grade" zone for high school and college labs. It implies careful technique and properly calibrated equipment.
5% - 15% (Acceptable)
Common in introductory labs where purity of chemicals or equipment quality isn't perfect. Requires explanation in your lab report.
> 15% (Needs Review)
Significant deviation. Check for calculation errors, major spills, or unit conversion mistakes (e.g. measuring in cm but calculating in m).
Frequently Asked Questions
What is the percent error formula?
Percent Error = (|Experimental Value - Theoretical Value| / Theoretical Value) * 100%. The absolute value bars imply the result is always positive.
What is a "good" percent error?
In high school labs, <5% is often considered excellent, and <10% is acceptable. In professional analytical chemistry, errors might need to be <1% or even <0.1%. It depends entirely on the precision of your equipment.
Why is my percent error over 100%?
This happens if your experimental value is more than double the theoretical value. For example, if you should have got 10g but measured 25g, your error is 150%. This usually indicates a major procedural error.
Does percent error measure precision?
No, it measures Accuracy (closeness to truth). Precision is measured by "Percent Deviation" or standard deviation across multiple trials.
What is the difference between Actual and Theoretical?
They are synonyms in this context. "Theoretical" usually implies a calculated value (from stoichiometry), while "Accepted" or "Actual" implies a known constant (like the speed of sound).
Can percent error be negative?
Strictly speaking, no, because of the absolute value bars. However, some teachers ask for "Percent Difference" without absolute value to see if your result was high (+) or low (-).
How do I reduce percent error?
Calibrate your instruments, perform multiple trials and average them, ensure pure reagents, and minimize human error (parallax when reading burettes).
What are systematic errors?
Errors that skew results in one direction (e.g., a scale that always reads 0.5g too high). These cannot be fixed by averaging more trials.
What are random errors?
Unpredictable fluctuations (e.g., air drafts, temperature spikes). These can be minimized by averaging multiple trials.
Is 0% error possible?
It is extremely rare in physical measurements due to the limits of instrument precision. If you get exactly 0%, verify you did not accidentally use the theoretical value as your measurement.
How does this relate to Percent Yield?
They are opposites. Percent Yield = (Experimental / Theoretical) * 100. If you have 95% yield, you have roughly 5% error (ignoring impurities).
How many decimal places should I keep?
Follow Significant Figures rules. Usually, percent error is reported to 2 or 3 sig figs.
What if I don't know the theoretical value?
You cannot calculate percent error without a known standard. You can only calculate standard deviation of your own trials (precision).
Why divide by Theoretical?
To normalize the error. Being off by 1cm when measuring a table is small error. Being off by 1cm when measuring an atom is huge error. Dividing gives context.
What is "Human Error"?
"Human error" is a vague term often rejected in lab reports. Be specific: "Misread meniscus," "Spilled sample," or "Reaction time delay."