Percentage Calculator

How to use the percentage calculator

Pick a calculation and type your numbers — the answer updates instantly, with no calculate button. Under every answer you get the formula with your numbers plugged in, so you can check the logic or copy the working into homework or a report.

Each calculation is saved to History after a moment. Tap an entry to load it again. History lives only in this browser and is never uploaded. Share link creates a URL with your current inputs so someone else can see the same calculation.

• X% of Y — e.g. 15% of 80. Also shows Y plus and minus that percentage.
• X is what % — the share of a part in a whole, e.g. 30 out of 200 is 15%.
• % change — percentage increase or decrease from an old value to a new one.
• Add / subtract % — a value increased or decreased by a percentage, e.g. a 5% raise.
• Discount — sale price from a discount rate, or the discount rate from a sale price. Supports a second, stacked discount.
• Reverse % — the original value before a percentage change, e.g. a pre-tax price.
• % vs pp — compares two rates in percentage points and in relative percent.
• ROI — return on investment and gain/loss, plus the gain needed to break even after a loss.

Percentage formulas at a glance

A percent is a ratio out of 100, so almost every percentage problem comes down to one “÷ 100” or “× 100”.

• X% of Y = Y × X ÷ 100 (15% of 80 = 80 × 15 ÷ 100 = 12)
• A is what % of B = A ÷ B × 100 (30 ÷ 200 × 100 = 15%)
• Percent change = (New − Old) ÷ Old × 100 (80 → 100 is a 25% increase)
• Increase by X% = Y × (1 + X ÷ 100); decrease by X% = Y × (1 − X ÷ 100)
• Discount rate = (Original − Sale) ÷ Original × 100
• Original value = New value ÷ (1 ± X ÷ 100) (107 after a 7% increase → 107 ÷ 1.07 = 100)

Mental math shortcuts

Start from 10% and 1%: 10% moves the decimal point one place left, 1% moves it two places. For 15% of 38, take 3.8 (10%) plus 1.9 (5%) = 5.7.

Percentages are reversible: 8% of 50 equals 50% of 8, which is 4. Pick whichever is easier.

The % key on pocket and phone calculators behaves differently between models, so the same key sequence can give different answers. If in doubt, type × number ÷ 100 instead. In a spreadsheet, =A1*B1/100 gives B% of A and =(B1-A1)/A1 with percent formatting gives the percent change.

Percent vs. percentage points

When comparing two rates, a change in percentage points is the simple difference, while a percent change is relative to the starting rate. If an interest rate goes from 3% to 4%, it rose by 1 percentage point — which is a 33.3% increase. Both are true, but they say very different things, so news and reports distinguish them.

Use the % vs pp mode to see both numbers side by side with a plain-language sentence.

Stacked discounts and returns

A 20% discount followed by an extra 10% is a 28% discount, not 30%, because the second discount applies to the already reduced price: 1 − 0.8 × 0.9 = 0.28. Enter both rates in Discount mode to see the combined rate.

Losses are harder to recover than they look. After a 50% drop you need a 100% gain to get back to even; a 20% loss needs a 25% gain. ROI mode shows that break-even gain automatically. Results are simple returns before fees and taxes.