Percentage Calculator — Percent Of, Percent Change and Percent Difference

1. 1Pick the calculation you needChoose “% of a number”, “X is what % of Y” to express one number as a share of another, or “% change” to compare an old value with a new one.
2. 2Enter your two numbersDecimals and negative numbers work, and both comma and full-stop decimal marks are recognised.
3. 3Read the result and the workingThe answer updates as you type, with the formula underneath showing your own numbers substituted.

A percentage is a fraction with 100 as the denominator. Writing 15% is another way of writing 15/100, or the decimal 0.15. Every percentage question becomes a multiplication or a division once the percent sign is converted to that decimal — the step most people skip doing it in their head.

The three modes cover the three shapes the question takes: you know the percentage and the total and want the part, know the part and the total and want the percentage, or know two totals and want the change between them.

For “what is X% of Y” the calculator multiplies: (X ÷ 100) × Y, so 15% of 250 is 0.15 × 250 = 37.5. For “X is what % of Y” it divides and scales: (X ÷ Y) × 100, so 30 out of 250 is 12%.

Percent change uses ((new − old) ÷ |old|) × 100. Dividing by the absolute starting value means a move from −50 to −25 reads as a 50% increase, not a decrease. Results display to two decimal places; the calculation itself runs in full double precision.

The starting value is always the denominator. A rise from 80 to 100 is a 25% increase, because the 20-unit gain is measured against 80. A fall from 100 to 80 is a 20% decrease, because the same 20 units are measured against 100. This is why a price that drops 50% must then rise 100% to recover.

Percent difference is a different measure, dividing by the average of the two values, for when neither is the obvious baseline. This page reports percent change, which is what price rises, salary increases and exam improvements ask for.

Percentages do not add. A 20% discount followed by a further 10% off is not 30% off: the second reduction applies to the already-reduced price, giving 0.8 × 0.9 = 0.72, or 28% off. The same compounding applies to interest and successive price rises. Separately, a percentage point is not a percent — a rate moving from 4% to 5% has risen one percentage point but 25 percent.

The calculator is arithmetic executed by your browser. Nothing you type is transmitted, logged or retained, and there is no account and no analytics attached to the figures you enter.