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Percentage Calculator
Everyday percentage maths in one place: what is X% of Y, X is what percent of Y, the percentage change between two numbers, and the original value before a percentage was added. No formulas to remember.
100% private — everything runs in your browser, nothing is ever uploaded.
What this tool does
A shop discounts a 79 dollar item by 15 percent, a report says revenue rose from 42,000 to 51,000, and a price tag includes VAT that needs stripping out to compare suppliers. These are four different calculations that people often try to force into one formula, and the commonest mistake is dividing by the wrong number. This tool separates them into four modes: what is X percent of Y, X is what percent of Y, the percentage change between two values, and the original value before a percentage was added. Answers update as you type, so a quick what-if takes seconds. It is built for everyday arithmetic — discounts, tips, tax, growth rates and survey results — rather than for financial modelling, and it removes the need to remember which denominator belongs to which question.
How to use it in three steps
- Pick the calculation that matches your question from the four modes. Choosing the right mode matters more than the arithmetic, especially for reverse percentages.
- Type the two numbers. The answer updates as you type, so you can compare a 10 percent and a 20 percent case without clearing the form.
- Copy the result, or switch modes and keep going. For a change calculation, check which value you entered as the original, since reversing them flips the sign. Write down which number is the base before choosing a mode, since that single choice determines the answer.
Questions people ask
Why is percentage increase not symmetric with decrease?
The change is measured against the starting value, so going from 100 to 120 is a 20 percent rise, while going back from 120 to 100 is a 16.7 percent fall. The denominator changes with the direction.
How do I remove tax from a total that already includes it?
Use the original-value-before-a-percentage-was-added mode with the tax rate. Dividing the total by the rate as though it were a discount gives the wrong answer, because the tax was applied to the smaller pre-tax price.
Can I use it for compound growth over several years?
No. It handles single-step percentage arithmetic. Compound growth needs an exponent for the number of periods, which is a different calculation.
Why does my discount calculation disagree with the shop's receipt?
Shops often round to the nearest cent at each step and may apply tax after the discount rather than before. The calculator gives the exact arithmetic result; a receipt that differs by a cent or two usually reflects rounding order, not a wrong percentage.
Privacy: Every percentage result is calculated by JavaScript in this browser tab, so prices, margins, salaries and survey figures you type are never sent to a server or saved anywhere.
How to use it
- Pick the calculation you need from the four modes.
- Type the numbers — the answer updates as you type.
- Copy the result, or switch modes and keep going.
Why it stays private
A trivial tool, and deliberately so: it computes with your browser's own arithmetic on numbers you typed. Salary figures, invoice totals and discount maths never reach a server.
Questions
How do I calculate a discount?
Use Percent of a number: enter the discount rate and the price, then subtract the result from the price. Or use Percent change from the original to the sale price to see what percentage you are actually saving.
What is the difference between percent change and percent of?
Percent change measures the difference between an old and a new value relative to the old one (a rise from 80 to 100 is +25%). Percent of just multiplies (20% of 80 is 16).
What is reverse percentage useful for?
Undoing a markup or tax: if 121 is a price including 10% tax, reverse percentage tells you the pre-tax figure was 110.