How to Calculate Percentage: The Simple Formulas (With Examples)

Percentages come up everywhere — a 20% tip, a 30%-off sale, a test score, an interest rate, a pay rise. They feel intimidating, but almost every percentage question you'll ever face is one of just three calculations. Learn those three and you can handle the rest in your head.

This guide walks through each formula with plain worked examples, then shows the mental-math shortcuts that let you skip the calculator entirely. And when the numbers get messy, you can use our free percentage calculator to get the answer instantly — it runs entirely in your browser, with nothing uploaded anywhere.

What a Percentage Actually Is

"Percent" literally means "per hundred." When you say 25%, you're saying 25 out of every 100, which is the same as the fraction 25/100 or the decimal 0.25. That single idea is the key to every percentage calculation:

To turn a percentage into a number you can multiply with, divide it by 100.

So 25% becomes 0.25, 7% becomes 0.07, and 150% becomes 1.5. Once a percentage is in decimal form, the math becomes ordinary multiplication and division. Every formula below is just a different arrangement of that one rule.

Calculation 1: Find X% of a Number

This is the most common percentage question — "What is 20% of 80?" It answers tips, discounts, tax, and commission. The formula is:

Result = (Percentage ÷ 100) × Number

Example — what is 20% of 80?

1. Convert the percentage: 20 ÷ 100 = 0.20
2. Multiply by the number: 0.20 × 80 = 16

So 20% of 80 is 16. A 20% tip on an $80 bill is $16; a 20%-off discount on an $80 item saves you $16.

Calculation 2: What Percent Is X of Y?

This one turns a fraction into a percentage — "I scored 42 out of 50, what percent is that?" The formula is:

Percentage = (Part ÷ Whole) × 100

Example — 42 out of 50 is what percent?

1. Divide the part by the whole: 42 ÷ 50 = 0.84
2. Multiply by 100: 0.84 × 100 = 84%

A score of 42/50 is 84%. The same formula tells you what percentage of a budget you've spent, your completion rate, or your conversion rate.

Calculation 3: Percentage Increase and Decrease

This measures how much a number changed relative to where it started — a price rise, a pay bump, a drop in traffic. The formula is:

Percent Change = ((New − Old) ÷ Old) × 100

A positive answer is an increase; a negative answer is a decrease.

Example — a price rose from $50 to $65.

1. Subtract old from new: 65 − 50 = 15
2. Divide by the old value: 15 ÷ 50 = 0.30
3. Multiply by 100: 0.30 × 100 = 30% increase

Example — traffic fell from 1,200 to 900. (900 − 1200) ÷ 1200 × 100 = −25%, a 25% decrease.

The classic trap: a 50% drop followed by a 50% rise does not return you to the start. $100 → −50% → $50 → +50% → $75. Percentages are always taken from the current number, not the original — which is exactly why a tool that shows each step is worth keeping open.

The Reverse Percentage (Working Backwards)

Sometimes you know the final figure and need the original — "I paid $90 after a 10% discount, what was the original price?" Don't just add 10% back; that gives the wrong answer. Instead, divide by what's left:

Original = Final ÷ (1 − Discount%/100)

Example: $90 ÷ (1 − 0.10) = 90 ÷ 0.90 = $100. The original price was $100, not $99. The same logic strips tax out of a total: to remove 13% sales tax from a $113 receipt, divide by 1.13 to get $100.

Mental Math Shortcuts

You can do most everyday percentages without a calculator using these tricks:

• 10% of anything — move the decimal one place left. 10% of 240 is 24.
• 1% of anything — move the decimal two places left. 1% of 240 is 2.40.
• Build other percentages from 10%. Need 30%? Take 10% three times. Need 5%? Halve the 10%. Need 15%? 10% plus half of 10%.
• X% of Y equals Y% of X. 8% of 50 is hard, but 50% of 8 is obviously 4 — same answer. This flip rescues you constantly.
• 20% (a common tip) — take 10% and double it.

For anything beyond mental range — odd decimals, percentage change across big numbers, or back-calculating an original price — drop the figures into the percentage calculator and read the answer in one step.

Quick Reference: Which Formula Do I Need?

Tips for Getting Percentages Right Every Time

• Always identify the "whole" first. Most percentage mistakes come from dividing by the wrong number. The whole is the thing the percentage is "out of."
• Sanity-check against 10%. If 10% of 200 is 20, then your 35% answer should land somewhere near 70 — not 7 or 700.
• Percentage points ≠ percent. Going from 5% to 7% is a 2 percentage-point rise, but a 40% increase. Know which one is being asked.
• Discounts don't stack by adding. 30% off then a further 20% off is not 50% off — it's 44% off, because the second discount applies to the already-reduced price.
• When money or grades are on the line, verify. A free percentage calculator removes arithmetic slips in seconds.

Frequently Asked Questions

Stop Second-Guessing the Math

Three formulas cover almost every percentage you'll ever need: a percentage of a number, one number as a percentage of another, and percentage change. Learn the 10% shortcut and you'll handle most of them in your head. For everything else — reverse percentages, stacked discounts, awkward decimals — our free percentage calculator gives you the answer in one step, no signup and nothing uploaded. Working with other numbers too? The unit converter handles the rest of your everyday conversions.