Percentage Calculator — Quick % Calculations
Calculate percentages in three different modes.
Operation
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calculatorAbout Percentage Calculator
Percentage calculations look simple on the surface but trip people up constantly because the question "what does this percentage mean" has several different answers depending on the situation. Is it 15% of a number? Is the result a 15% increase from the starting number? What number, when increased by 15%, gives this result? Percentage Calculator handles all of those — and a few more — in one interface, with each variant clearly labelled so you do not have to remember which formula to use.
The five most common variants are built in as separate calculators on the same page, so you pick the one matching your actual question rather than reaching for a single generic input that requires you to know the formula. "What is X% of Y?" — the basic calculation, used for tips, taxes, and quick proportional estimates. "X is what percent of Y?" — used for "I scored 73 out of 90, what percentage is that?" "What is the percentage change from X to Y?" — used for price changes, growth rates, before/after comparisons. "What is X with a Y% increase/decrease applied?" — used for sale prices, markups, raises. "X is Y% of what number?" — used for working backwards from a known percentage.
Each calculator includes a worked example below the result that shows the full step-by-step calculation, not just the final number. The reason for this is that percentage calculations are a place where you often want to double-check your understanding rather than just trust the number — the worked example makes the math transparent so you can sanity-check the result and learn the formula for the next time you need it without a calculator.
Common real-world scenarios are covered with one-click presets: 15% / 18% / 20% tip percentages, common sales tax rates, percentage-off shopping discounts, and percentage-off-list-price markups. Everything runs locally in your browser; the calculator works offline once the page has loaded once. There is no precision limit beyond JavaScript’s native floating-point math, which is accurate to at least 15 significant digits — more than enough for any percentage calculation outside of specialist scientific contexts.
How it works
- 1Pick the percentage variant that matches your question (X% of Y, X is what % of Y, % change, etc.).
- 2Type the two numbers into the input fields. The calculation runs as you type — no need to hit Calculate.
- 3The result appears with a worked example showing the full step-by-step calculation.
- 4For tips, tax, and sale-price scenarios, click the relevant preset to set common percentages instantly.
- 5The calculator runs entirely in your browser — works offline and nothing is sent to any server.
Common use cases
- Calculate the tip on a restaurant bill quickly
- Work out the sale price after a percentage discount
- Find what percentage one number is of another (test scores, completion rates)
- Calculate the percentage change between two numbers (price increase, growth rate)
- Work out the original price before a percentage discount was applied
- Calculate sales tax on a pre-tax price
FAQ
What calculation modes are available?
Three modes: "X% of Y", "X is what % of Y", and "percentage change from X to Y".
How do I calculate percentage increase?
Use the "percentage change" mode. Enter the old and new values to see the % increase or decrease.
Are decimal results shown?
Yes — results are shown to two decimal places for precision.
How do I calculate a percentage of a number by hand?
To find X% of Y, multiply Y by X/100. So 15% of 80 = 80 × (15/100) = 80 × 0.15 = 12. The shortcut for common percentages: 10% of any number is the number divided by 10; 1% is divided by 100; 50% is divided by 2; 25% is divided by 4. Combine these for quick mental math: 15% = 10% + 5% = (number ÷ 10) + (number ÷ 20).
How do I calculate what percentage one number is of another?
Divide the part by the whole, then multiply by 100. So if you scored 73 out of 90, the percentage is (73 ÷ 90) × 100 = 81.1%. The order matters: it is always (smaller ÷ larger) × 100 to get a percentage between 0 and 100, unless one number is negative or you specifically want a percentage greater than 100.
How do I calculate percentage change between two numbers?
Subtract the old value from the new value, divide by the old value, then multiply by 100. So if a price went from $80 to $100, the change is ((100 - 80) ÷ 80) × 100 = 25%. A negative result means a decrease. The formula always uses the original (old) value as the divisor because that is the baseline you are measuring change from.
How do I add or subtract a percentage from a number?
To add X%, multiply by (1 + X/100). To subtract X%, multiply by (1 - X/100). So adding 20% to 50 = 50 × 1.20 = 60. Subtracting 20% from 50 = 50 × 0.80 = 40. The shortcut for sale prices: subtract the discount percentage from 100 and multiply by that fraction. A 30%-off $80 item is $80 × 0.70 = $56.
How do I calculate the original price before a discount?
If the discount was X% and the discounted price is Y, the original price is Y ÷ (1 - X/100). So if a $56 item was 30% off, the original was $56 ÷ 0.70 = $80. This works for finding any "before" value when you know the "after" value and the percentage applied.
What is the difference between percentage and percentage points?
A common confusion. If interest rates rise from 4% to 5%, that is a 1 percentage point increase but a 25% relative increase ((5-4)/4 = 25%). Percentage points measure absolute change in something already measured in percentages; percent change measures relative change. News headlines often conflate them and produce misleading numbers as a result.
How accurate are the calculations?
They use JavaScript’s standard IEEE-754 double-precision floating-point math, which is accurate to about 15–17 significant decimal digits. For any percentage calculation outside specialist scientific contexts (chemistry molarity, astronomical distances), this is dramatically more precision than the input numbers warrant.
Can I do percentage calculations on negative numbers?
Yes — the calculator handles negative inputs correctly. "What is 20% of -50" returns -10. "What is the percentage change from -10 to -8" returns +20% (the value increased). "What is -10 with a 20% increase" returns -8. The math is the same, but signs require some care to interpret correctly.