How to Calculate a Percentage: Change, Increase and Decrease

To calculate a percentage, open the Percentage Calculator, pick what you're solving for from the Calculate dropdown, and type your two numbers into X and Y — the answer and the formula behind it appear instantly, no formula to remember.

What the Percentage Calculator does

"Percentage" math is really five different questions wearing the same word: what is X% of Y, what percent is X of Y, how much did Y change from X, and what is Y after increasing or decreasing it by X%. Each one uses a different formula, and it's easy to grab the wrong one under pressure — the Percentage Calculator separates them into five explicit modes so you pick the question you're actually asking instead of reverse-engineering a formula.

How to calculate a percentage with Toolbench

1. Open the Percentage Calculator.
2. Choose your question from the Calculate dropdown: "What is X% of Y?", "X is what % of Y?", "% change from X to Y", "Y increased by X%", or "Y decreased by X%".
3. Type your first number into X and your second into Y.
4. Read the answer in the large result box, labeled with your question. Click it to copy the value.
5. Below it, a row shows the Formula used (or the Difference/Change, depending on the mode) — click any row to copy that value too.
6. Click Copy results to copy everything — the question, the answer, and every row — as plain text.

Example: what is X% of Y?

The tool opens on this mode with X set to 15 and Y set to 200:

Calculate	What is X% of Y?
X	15
Y	200
15% of 200	30
Formula	X ÷ 100 × Y

15% of 200 is 30, from 15 ÷ 100 × 200.

Example: X is what % of Y?

Switch the mode and try X = 30, Y = 120:

30 is what % of 120	25%
Formula	X ÷ Y × 100

30 ÷ 120 × 100 = 25%. This is the mode to reach for when you know two amounts and want the rate between them — like figuring out what share of a $120 bill a $30 tip represents.

Example: percent change, increase and decrease

The "% change from X to Y" mode tells you how much Y moved relative to X. Going from 80 to 100:

Increase	+25%
Difference	20
Formula	(Y − X) ÷ |X| × 100

And the reverse, 100 down to 80:

Decrease	-20%
Difference	-20
Formula	(Y − X) ÷ |X| × 100

Notice the increase and decrease aren't mirror images (+25% up, but only -20% back down) — percent change is always relative to the starting number, so the same 20-point gap produces different percentages depending on which number you started from.

The "Y increased by X%" and "Y decreased by X%" modes work the other direction: give them a starting amount and a known rate, and they return the new total. Increasing 50 by 20% gives 60 (a change of +10); decreasing 50 by 20% gives 40 (a change of -10).

Reference: the five formulas

Question	Formula
What is X% of Y?	X ÷ 100 × Y
X is what % of Y?	X ÷ Y × 100
% change from X to Y	(Y − X) ÷ |X| × 100
Y increased by X%	Y × (1 + X ÷ 100)
Y decreased by X%	Y × (1 − X ÷ 100)

Tips and common mistakes

• Percent change always divides by the starting value, not the ending one. That's why an increase and a decrease of the same raw amount give different percentages — a jump from 80 to 100 is +25%, but dropping back from 100 to 80 is only -20%.
• "X% of Y" and "X is what % of Y" are opposites, not the same question. The first gives you an amount (15% of 200 is a number), the second gives you a rate (30 out of 120 is a percentage) — mixing them up is the most common percentage mistake.
• A negative X in the change mode still works, since the formula divides by the absolute value of X — useful if you're tracking a change from a negative starting number.
• The increase/decrease modes don't need you to compute the new total by hand. If you already know the rate (say, a 20% markup), use "Y increased by X%" directly instead of calculating X% of Y and adding it yourself.

Related tools

• Discount Calculator — apply a percentage discount to a price and see the amount saved.
• Margin Calculator — work out profit margin and markup from cost and price.
• Average Calculator — mean, median, mode, range and standard deviation of a list.
• Unit Converter — convert length, weight, temperature, volume, data, time and speed.

Frequently asked questions

What's the formula for percentage change?

Percentage change from X to Y is (Y − X) ÷ |X| × 100. The result is positive for an increase and negative for a decrease, and it's always relative to the starting value X, not the ending value Y.

How do I calculate what percent one number is of another?

Divide the part by the whole and multiply by 100: X ÷ Y × 100. For example, 30 is 25% of 120 because 30 ÷ 120 × 100 = 25.

Why isn't a 25% increase undone by a 25% decrease?

Because each percentage is calculated against a different starting number. Increasing 80 by 25% gives 100, but decreasing 100 by 25% gives 75, not 80 — the base amount changed in between.

Is my data sent to a server when I use the Percentage Calculator?

No. The calculation happens in your browser as you type. If you're signed in, your inputs sync privately to your account; if you're signed out, nothing is saved anywhere.

Can X or Y be negative or zero?

Yes for most modes, though "X is what % of Y" and the increase/decrease modes divide by Y, so a Y of zero has no defined result. The % change mode divides by the absolute value of X, so a negative X still produces a valid answer.