ACT Science: Calculate Percent Error Like a Pro - Formula and Common Traps

Percent error measures how far an experimental result is from an accepted or theoretical value. Formula: (|Experimental Value-Accepted Value|/Accepted Value)×100%. The absolute value bars ensure the result is always positive. ACT Science uses percent error to test whether you can quantify experimental accuracy; this appears in 2-3 questions per section.

Example: A student measured the boiling point of water as 98°C when the accepted value is 100°C. Percent error=(|98-100|/100)×100%=(2/100)×100%=2%. The experiment was 2% off. If the student measured 102°C, percent error=(|102-100|/100)×100%=(2/100)×100%=2% (same, because of the absolute value). This tells you how accurate the measurement was, regardless of direction.

Trap 1: Using the experimental value as the denominator instead of the accepted value. Wrong: (|Experimental-Accepted|/Experimental)×100%. Right: (|Experimental-Accepted|/Accepted)×100%. The denominator is always the accepted/theoretical/known value, never the experimental one. Trap 2: Forgetting to use absolute value and getting a negative percent error. Percent error should be positive; the negative sign is ignored. Trap 3: Not multiplying by 100 at the end (leaving answer as a decimal instead of a percentage). Students lose points for flipping the denominator or forgetting the multiplication step.

Fix: before you calculate, identify which value is "accepted" (the correct/known value) and which is "experimental" (the measured value). Then apply the formula rigidly: (|measured-accepted|/accepted)×100%. Do not deviate from this order or you'll invert the fraction.

Problem 1: Accepted density of aluminum=2.7 g/cm³. Student measured 2.65 g/cm³. Percent error=(|2.65-2.7|/2.7)×100%=(0.05/2.7)×100%≈1.85%. Problem 2: Accepted gravitational constant g=9.8 m/s². Student measured 9.6 m/s². Percent error=(|9.6-9.8|/9.8)×100%=(0.2/9.8)×100%≈2.04%. Problem 3: Accepted melting point=150°C. Student measured 152°C. Percent error=(|152-150|/150)×100%=(2/150)×100%≈1.33%. Problem 4: Accepted volume=50 mL. Student measured 48.5 mL. Percent error=(|48.5-50|/50)×100%=(1.5/50)×100%=3%. All four use the same formula; once you identify accepted and experimental values, the calculation is straightforward.

Time yourself on these four. You should calculate each in under 30 seconds, including setup. If you're slower, practice the formula until it's automatic.

Percent error appears in 2-3 ACT Science questions per section, usually in data evaluation or experimental accuracy contexts. These are "easy" questions worth 1 point each, but only if you know the formula and apply it correctly. Students who don't remember the formula or mix up the numerator/denominator miss these free points. The percent error formula is one of the easiest formulas to memorize and apply, netting you 2-3 guaranteed points per test.

Memorize the formula: (|measured-accepted|/accepted)×100%. Write it on a flashcard. Drill it once daily for a week. By test day, you'll calculate percent error faster than 90% of test-takers and guarantee these points.