Percentage Calculator
🔒 Runs in your browser
Four formulas, one calculator
Pick the question that matches what you already know: a% of b multiplies
(e.g. 21% of 1000 is 210); a is what % of b divides a by b; percentage
change from a to b expresses the difference as a share of the starting value; and
a increased or decreased by b% scales a by (1 ± b/100) — the same
formula a discount or a markup uses. Each mode relabels the two number fields so it is always
clear which value plays which role.
When a result is undefined
Two of the four modes have a mathematical blind spot: "a is what % of b" cannot answer when b is 0 (dividing by zero), and neither can "percentage change from a to b" when a is 0. Both cases are caught explicitly and shown as a clear error instead of a broken or misleading number. Every calculation runs instantly in your browser as you type — nothing is sent to a server.
Frequently asked questions
What are the four percentage calculations?
a% of b (e.g. 21% of 1000 = 210); what percent a is of b (a/b × 100); the percentage change from a to b ((b−a)/a × 100); and a increased or decreased by b% (a × (1 ± b/100)).
How do I calculate a percentage increase or decrease?
Increase: multiply the starting value by (1 + percent/100). Decrease: multiply it by (1 − percent/100) — e.g. 1000 increased by 21% is 1210, and 1210 decreased by 21% is 955.9.
Why does the calculator show an error for some inputs?
"a is what % of b" is undefined when b is 0, and "percentage change from a to b" is undefined when a is 0 — both would require dividing by zero.
Is this calculation sent to a server?
No — every calculation runs instantly in your browser as you type; nothing is transmitted or stored.