Percentages
Why a percentage calculator on a trades site?
Because percentages are the arithmetic every job runs on. A material estimate gets a 10% waste allowance. A quote gets a markup. A supplier gives you 15% off, then adds sales tax on top. A bid comes in 8% over budget and you need to know whether that is a lot. None of these are hard sums, but they are easy to get backwards — and getting a percentage backwards is how a 15% markup becomes a 13% margin without anyone noticing.
The three questions every percentage problem is
Almost every percentage problem is one of three questions in disguise. What is 15% of 80? is finding a part. 12 is what percent of 60? is finding a rate. What is the change from 50 to 65? is comparing two numbers. The percentage calculator answers all three, plus the fourth one people actually need most often: adding or taking a percentage off a price.
Each answer shows the formula it used. Percentages are worth understanding rather than just computing, because the mistakes — using the wrong base, confusing percent with percentage points, reversing a discount by adding the same percentage back — come from the setup, not the arithmetic.