Percentages calculators
Percentages show up in prices, test scores, interest, growth and measurement error, and most mistakes come from using the right numbers in the wrong formula. These calculators separate the common percentage questions: percent of a number, percentage change, discounts, markup and margin, and percentage error.
9 tools · Updated October 21, 2026
Which one do I need?
The three basic percentage questions
Almost every percentage problem is one of three questions built on the same relationship, part = percent ÷ 100 × whole. You might want the part (what is 20% of 80?), the percent (16 is what percent of 80?) or the whole (16 is 20% of what?). The Percentage Calculator answers all three, so start there when you are not sure which formula applies.
Percentage change is a different question: it compares a new value with an old one, and the old value is always the base. Going from 40 to 50 is a 25% increase, but going from 50 back to 40 is a 20% decrease. The Percentage Change Calculator also distinguishes percentage points, which is what you need when a rate moves from 4% to 5% (one percentage point, or a 25% relative increase).
Discounts, markup and margin
Stacked discounts multiply rather than add. Twenty percent off followed by another 10% off leaves you paying 0.8 × 0.9 = 72% of the price, a 28% total discount rather than 30%. The Percentage Discount Calculator handles single and stacked discounts and shows the money saved.
Markup and margin describe the same profit against different bases. Markup divides profit by cost, margin divides it by selling price. An item that costs $60 and sells for $90 has a 50% markup but a 33.3% margin. Confusing the two is a common way to underprice, so use the Markup Calculator whenever you set prices from a target margin.
Percentage error in measurements
Percentage error compares a measured or estimated value with an accepted value: the absolute difference divided by the accepted value, times 100. It tells you how far off a result is relative to the size of the thing measured, so a 2 g error matters far more on a 10 g sample than on a 10 kg one.
Use the Percentage Error Calculator for lab reports and estimation checks. If you are comparing two measurements where neither is the reference, a percentage difference based on their average is usually the fairer comparison.