Learn

Short, plain-English guides to the math behind the calculators. Each one explains how a method works, where people go wrong, and links to worked examples you can open with the numbers already filled in.

Guides

How to Calculate a PercentageWhat “percent” means, the one formula behind every percentage question, and the mental-math shortcuts worth knowing.Percentage Change vs Percentage DifferenceOne has a direction and a starting point; the other treats both numbers as equals. Which question you’re asking decides which one to use.Percentage Points vs Percent ChangeWhen a rate moves from 20% to 25%, that is +5 percentage points and a +25% relative change. Same move, two numbers — and headlines mix them up.Margin vs MarkupSame profit in dollars, different denominator: markup divides it by cost, margin divides it by price. A 50% markup is only a 33% margin.ROI ExplainedReturn on investment is a gain measured against what you put in — so the same $250 profit is a 25% ROI on $1,000 and a 5% ROI on $5,000.