Percentage calculation
The three most common questions, each with its own input row — change whichever you need.
① What percentage of B is A
② Increase / decrease A by P%
③ Change rate from original value A → current value B
Growth rate has no intuitive meaning when the original value is negative (−100 to −50: up or down?). This is marked as not applicable instead of giving a misleading number.
When to use it
- You need to tell a 20% rise apart from a 20-percentage-point rise, or check a growth rate quoted in a report or news story.
- You are pricing a discount, markup or commission: what share A is of B, what A becomes after a P% change, or how much a value moved from A to B.
When not to
- The original value is negative — a loss, negative growth, an overdrawn balance. Percentage change has no intuitive meaning there, so the tool says so instead of returning a misleading figure.
- You need money rounded to the cent for settlement: this is plain floating-point percentage arithmetic with no currency rounding rules, so the invoice or contract governs.
Inputs are calculated in the browser only — nothing is uploaded or stored, and a refresh clears them. Limits: percentage change is undefined when the original value is 0 or negative, and the tool flags it; results show two decimals without currency rounding; thousands separators are not parsed, so 1,234 is read as 1.
Frequently asked
What is the difference between percent and percentage points?
Going from 20% to 25% is a +25% relative change — (25−20)/20 — but only +5 percentage points in absolute terms. For rates that are themselves percentages, such as interest rates, tax rates, approval ratings or polling, “up 5 percentage points” means 25% − 20% = 5, while “up 25%” means the relative change. Mixing them turns a 5-point gap into 25%, a fivefold exaggeration, so check which unit a report is using.
Why is percentage change undefined when the original value is negative?
The formula is (new − old) / old, and a negative denominator flips the sign: going from −100 to −50 gives −50%, which looks like a 50% fall but is actually a loss halved. Using the absolute value as the denominator gives +50%, which is a different convention. Both can be argued, so this tool refuses to pick one and labels the result “not applicable when the original value is negative”. Describe direction with the absolute difference (+50) and size with words such as “the loss halved”.
Does a 15% rise followed by a 15% fall return to the original price?
No. 100 rises 15% to 115, then falls 15% of 115 to 97.75 — 2.25 below the start. The bases differ: the rise uses 100, the fall uses 115. To get back from 115 to 100 you need a fall of 15/115 ≈ 13.04%, not 15%. Use mode ② to check each step separately and the gap becomes visible.