Percentage Calculator

How it works

The calculator covers three common percentage problems. Pick a mode, enter two numbers, and select Calculate. Results update in place — no page reload — and can be copied with one click.

Find X% of Y

Multiplies the value by the percentage and divides by 100 — the standard way to take a percentage of a number.

Formula: (X ÷ 100) × Y. Example: 25% of 80 is(25 ÷ 100) × 80 = 20.

X is what percentage of Y?

Divides the first number by the second and multiplies by 100 to express it as a share of the whole.

Formula: (X ÷ Y) × 100. Example: 20 out of 80 is(20 ÷ 80) × 100 = 25%.

Percentage change

Measures the increase or decrease from an original value to a new value, as a percentage of the original.

Formula: ((To − From) ÷ From) × 100. Example: from 80 to 100 is ((100 − 80) ÷ 80) × 100 = 25%, a 25% increase.

Assumptions and edge cases

• Results are calculated at full precision and rounded only for display — up to six decimal places, with trailing zeros removed.
• Negative and decimal inputs are allowed and applied literally; nothing is clamped.
• Dividing by zero is undefined: the "What %" mode requires a non-zero whole. Percentage change requires a positive original value — a zero baseline divides by zero, and a negative baseline would produce a misleading increase or decrease. The calculator explains these cases instead of showing Infinity, NaN, or a misleading percentage.
• An increase and a decrease of the same amount are not symmetrical — going from 80 to 100 is a 25% increase, but going back from 100 to 80 is a 20% decrease, because the change is always relative to the original value.