Percentage Calculator
Four calculators in one: find a percentage of a number, apply an increase or decrease, work out the percent change between two numbers, or reverse-calculate the original value before a percentage was applied. Results update as you type.
The formulas behind each mode
% of a Number: (Percent ÷ 100) × Number.
15% of 200 is (15 ÷ 100) × 200 = 30.
Increase / Decrease: Number × (1 ± Percent ÷ 100).
A $50 item increased 20% becomes 50 × 1.20 = $60.
% Change: ((To − From) ÷ |From|) × 100.
Going from 80 to 100 is a 25% increase.
Find the Original (reverse): Result ÷ (1 ± Percent ÷ 100).
If $45 is the price after a 10% discount, the original price was 45 ÷ 0.90 = $50.
Percentage increase and decrease in practice
The increase/decrease mode is the one to reach for with prices and money: working out a 15% tip, adding sales tax, applying a discount, or marking a product up by a set margin. Enter the number and the percentage, flip the direction toggle, and the result updates as you type. Remember that a 20% discount followed by a 20% markup does not return to the original price: each percentage applies to a different base, which the FAQ above walks through.
Reverse percentage: finding the original number
Reverse percentages answer the question "what was the price before?" when you only know the final number and the percentage that was applied. Divide the result by one plus or minus the rate: a $45 price after a 10% discount means the original was 45 ÷ 0.90 = $50. This is the mode to use for VAT-inclusive invoices, sale prices, and salary figures quoted after a raise. Using the wrong direction here is the classic mistake — dividing by 1.10 instead of 0.90 gives $40.91, which looks plausible but is wrong.
When to use percentage change instead
Percentage change compares two known values — last quarter versus this quarter, this week's traffic versus last week's — rather than applying a rate to a number. It also handles negative starting points correctly by dividing by the magnitude of the original value, so a move from −80 to −100 reads as a 25% increase in size rather than a confusing negative result. For grade averages and weighted scores, the GPA calculator does the credit-weighted version of the same idea, and the salary negotiation calculator turns a percentage raise into its lifetime value.
Frequently asked questions
How do you calculate a percentage of a number?
Divide the percentage by 100 to get a decimal, then multiply by the number. For example, 15% of 200 is (15 ÷ 100) × 200 = 30. The "% of a Number" tab above does this instantly for any two values.
How do I calculate a percentage increase or decrease?
Multiply the original number by 1 plus the percentage (as a decimal) for an increase, or 1 minus the percentage for a decrease. A $50 item increased by 20% becomes 50 × 1.20 = $60; decreased by 20% it becomes 50 × 0.80 = $40.
What is a reverse percentage calculation?
It answers "what was the original number before this percentage was applied?" — useful when you know a discounted price or a grown value but not the starting point. Divide the result by (1 plus or minus the percentage as a decimal): if $45 is the price after a 10% discount, the original price was 45 ÷ 0.90 = $50.
How do I calculate the percentage change between two numbers?
Subtract the original value from the new value, divide by the magnitude of the original value, then multiply by 100. Going from 80 to 100 is ((100 − 80) ÷ 80) × 100 = 25% increase; going from −80 to −100 is also a 25% increase. A negative result means a decrease.
Why do I get a different answer than a percentage increase followed by the same percentage decrease?
Because each percentage applies to a different base. Increasing 100 by 20% gives 120, but decreasing 120 by 20% gives 96, not 100 — the second 20% is taken from a larger number. This is a common source of confusion in discount and markup math.