Calculate percentages, percentage changes, increases, decreases, and find original values. Multiple calculation modes with formulas.
What is 15% of 230? What percentage is 45 of 300? How much does a price change from 80 to 100? This tool answers all three types of percentage questions with a single input form.
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You need three related answers fast: 15% of 240, what percent 36 is of 240, and the change from 240 to 300.
Inputs
15% of 240 · 36 is what % of 240 · 240 → 300
Results
36 15% +25% increase
The three percentage questions people constantly mix up, "of", "is what percent of", and "percent change", are handled as distinct modes so you do not have to remember which formula goes where. Each shows the calculation, not just the number.
What is 15% of 230? What percentage is 45 of 300? How much does a price change from 80 to 100? This tool answers all three types of percentage questions with a single input form.
The calculator exposes six distinct modes you switch between as buttons: "Percentage of" (X% of Y, computed as (X ÷ 100) × Y), "What Percent" (X is what % of Y, as (X ÷ Y) × 100), "% Change" (from X to Y), "Add %" and "Subtract %" (X ± X·Y/100), and "Find Original" (recover the starting value before a known % change). Selecting a mode relabels both input fields to match - for example "From (Original)" and "To (New)" for percent change, or "Current Value" and "After Change (%)" for find-original - and clears any prior values so you never carry numbers across an unrelated calculation. Each result is accompanied by the literal formula it used, shown in a monospace "Formula" panel (e.g. ((125 - 100) ÷ |100|) × 100), so the math is auditable rather than a black box.
Two formulas are worth calling out because they handle cases naive calculators get wrong. The percent-change mode divides by the absolute value of the original - ((new - old) ÷ |old|) × 100 - so a move from -50 to -25 is correctly reported as a +50% change rather than flipping sign because the base is negative; the result is also color-coded green for an increase and red for a decrease, with a leading + sign on positive changes and an "Increase"/"Decrease" caption underneath. The "Find Original" mode inverts a percentage change with X ÷ (1 + Y/100), so 115 after a 15% increase resolves to 100 (i.e. dividing by 1.15, not subtracting 15%, which is the common mistake). Several divide-by-zero and undefined cases are guarded explicitly and return no result: "What Percent" when the denominator is 0, percent-change when the original is 0, and find-original at exactly -100% (which would divide by zero).
Output is formatted via the browser's locale number formatting with up to six decimal places, and values whose absolute magnitude falls below 0.01 (but aren't exactly zero) switch automatically to scientific notation via toExponential(4) - a mantissa with four digits after the decimal point - so tiny results stay readable instead of rounding to 0. Input is sanitized live as you type - the field strips anything that isn't a digit, allows only a single decimal point, and permits only one leading minus sign - and the result recomputes instantly on every keystroke through a memoized calculation with no calculate button. For increase and decrease modes the calculator additionally breaks out the raw amount added or subtracted in its own green or red panel, and a copy button writes just the numeric result (result.value.toString()) to the clipboard for pasting elsewhere.
Choosing "% Change" and entering From -50 to -25 yields +50% (green), because the formula divides by |original|: ((-25 - -50) ÷ |-50|) × 100.
In "Find Original", entering Current Value 115 and After Change 15% returns 100 - it computes 115 ÷ (1 + 15/100) = 115 ÷ 1.15, not 115 minus 15%.
A calculation whose result is below 0.01 in magnitude (e.g. 0.002% of 1 = 0.00002) is shown as 2.0000e-5 via toExponential(4) rather than rounding down to 0.
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Change from X to Y is computed as (Y - X) divided by the absolute value of X, times 100. The result is shown in green with a plus sign for an increase and in red for a decrease.
Yes. The Find Original mode takes a current value and the percent change applied to it, then returns the value before that change. For example, 115 after a 15% increase gives an original of 100.
Yes, both inputs accept decimal points and a leading minus sign. Results are shown to up to 6 decimal places, and very small numbers switch to scientific notation.
Every calculation runs locally in your browser. Your numbers and expressions are not transmitted or stored.