Percentage Calculator

About this tool

This calculator handles the four percentage questions that cover almost every real situation. Pick a mode, type two numbers, and the result updates immediately — there's no calculate button.

The four modes. A percentage is just a fraction out of 100, so "15%" means 0.15, and every mode is a short calculation on that decimal:

• X% of Y — multiply. 15% of 1250 is 1250 × 0.15 = 187.5. This is the one for tips, discounts, commissions, and tax amounts.
• X is what % of Y — divide, then scale. 45 out of 250 is 45 ÷ 250 = 0.18, or 18%. Use it for a pass rate, a proportion, or "how much of the budget did I spend."
• Increase by X% — multiply by (1 + X/100). 1250 increased by 15% is 1250 × 1.15 = 1437.5. For markups, rate rises, and adding tax.
• Decrease by X% — multiply by (1 − X/100). 1250 decreased by 15% is 1250 × 0.85 = 1062.5. For discounts and depreciation.

Why "off then on" doesn't cancel. Take 15% off 1250 and you get 1062.50. Add 15% back and you get 1221.88, not 1250 — the second percentage is applied to a smaller number, so it's a smaller amount. The same maths is why a 50% loss needs a 100% gain to recover.

Percentage points vs. percent. A rate moving from 4% to 6% is a rise of two percentage points but a 50% increase in the rate itself. The two are often confused in headlines; they're not the same thing.

Reversing a change. To find the original price behind a tax-inclusive total, divide rather than subtract: 1437.50 that already includes 15% started at 1437.50 ÷ 1.15 = 1250.

For a restaurant bill specifically, the tip calculator adds splitting between people; for tax, the sales tax calculator and discount calculator are purpose-built. The guide to calculating percentages works through more examples and mental-maths shortcuts.

Frequently asked questions

What happens if I divide by zero?

In "X is what % of Y" mode, if Y is 0 the calculator shows "N/A" instead of an error, because the calculation is undefined.

Does this round the result?

The displayed result is shown to two decimal places for readability. The underlying calculation is not rounded before display.

Can I use negative numbers?

Yes. Negative values are accepted and calculated normally, which is useful for reversals or for a decrease expressed as a negative percentage.

Why isn't a 20% discount then a 20% surcharge back to the original price?

Each percentage is calculated on the current amount. After the discount the base is smaller, so adding the same percentage back adds less than was taken off.

How do I work out the original number before a percentage was added?

Divide the final amount by (1 + rate). A total of 120 that includes 20% was 120 ÷ 1.2 = 100 before the addition — not 120 minus 20%.

Is "2% higher" the same as "2 percentage points higher"?

No. Two percentage points is an absolute change to the rate; 2% higher is two percent of the current value. For a rate near 4% the first is twice the size of the second.