ACT Science: Calculate Percent Change Accurately Every Time

Percent change appears on nearly every ACT Science test. The formula is: Percent Change=(New-Old)/Old×100%. Example: If temperature rose from 20°C to 25°C, the percent change is (25-20)/20×100%=5/20×100%=25%. Always use Old as the denominator; reversing this is the most common error. Three variations: (1) Percent increase uses this formula directly. (2) Percent decrease uses the same formula; if the result is negative, it is a decrease. (3) Percent difference (no directional language) sometimes uses |New-Old|/(Old) to get a positive value. Check the question wording to determine which formula applies.

Example: A data table shows pH of 6.5 initially and 7.2 after treatment. Percent change=(7.2-6.5)/6.5×100%=0.7/6.5×100%≈10.8%. The pH increased by about 11%. Another example: Population fell from 1000 to 800. Percent change=(800-1000)/1000×100%=-200/1000×100%=-20%. The population decreased by 20%.

Trap 1: Using New as the denominator instead of Old. (25-20)/25×100%=20%, which is wrong. Trap 2: Forgetting to multiply by 100 when expressing as a percent. (5/20=0.25, but you want 25%, not 0.25). Trap 3: Rounding too early in multi-step calculations. If you round 0.7/6.5 to 0.1 too early, your final answer drifts. Store intermediate decimals and round only at the end. Use the formula exactly, step-by-step, without mental math shortcuts that invite rounding errors.

Double-check by asking: "Does my answer make sense?" If a value doubled, the percent change should be around 100%. If it increased by half, percent change is around 50%. If your answer is wildly off, re-examine your formula application.

Problem 1: A sample mass increased from 50 g to 75 g. Percent change? Problem 2: Pressure decreased from 1.2 atm to 0.9 atm. Percent change? Problem 3: A reaction rate improved from 0.05 mol/min to 0.08 mol/min. Percent change? Problem 4: Temperature went from 100 K to 120 K. Percent change? Problem 5: Concentration fell from 2.5 M to 1.0 M. Percent change? For each, (1) identify Old and New, (2) apply the formula exactly, (3) check your answer against the given choices. Do this drill without a calculator first, then check with one to verify.

Answers: 1) (75-50)/50×100%=50%. 2) (0.9-1.2)/1.2×100%=-25%. 3) (0.08-0.05)/0.05×100%=60%. 4) (120-100)/100×100%=20%. 5) (1.0-2.5)/2.5×100%=-60%.

Percent change questions are among the most straightforward on ACT Science because the formula is mechanical; there is no conceptual ambiguity. Missing a percent change question due to a formula error is a self-inflicted wound that costs you points you should easily earn.

Practice the drill twice this week with strict formula discipline. By test day, percent change will be automatic, and you will unlock 1-2 reliable points every test.