Percentage Calculator

Three modes in one. Find any percentage, ratio, or change in seconds.

%

e.g. 15% of 200 = 30

X% of Y
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How the Percentage Calculator Works

This calculator covers the three most common percentage calculations in one tool. Switch between modes using the tabs at the top.

  • X% of Y — Find a percentage of a number (e.g. 15% of 200 = 30).
  • What % — Find what percentage X is of Y (e.g. 30 is 15% of 200).
  • % Change — Find the percentage increase or decrease between two values.

Understanding Percentages in Everyday Life

Percentages are everywhere in modern life. Sales tax, restaurant tips, exam grades, smartphone battery levels, mortgage interest rates, investment returns, inflation numbers in the news — all expressed as percentages. The word "percent" itself comes from the Latin "per centum," meaning "per hundred." A percentage is simply a fraction with 100 as the denominator: 25% means 25 out of every 100, or one quarter. This universal reference point of 100 makes it easy to compare quantities that would otherwise be hard to put side by side — like saying a store offers a 20% discount and a competitor offers 1 in 5 items free, which are in fact identical deals.

Understanding percentages is fundamental to financial literacy. Mortgage rates describe how much interest you pay each year as a fraction of what you owe. Inflation tells you by what percentage prices have risen over a year. A salary increase of $3,000 sounds large or small depending on the base — expressed as a percentage, a 3% raise on a $100,000 salary and an 8% raise on a $37,500 salary both deliver roughly the same dollar amount, but their significance is very different in context. Developing an intuitive feel for percentages means you can quickly judge whether a discount is genuinely good, whether an interest rate is competitive, and whether a statistic you read in the news is as dramatic as it sounds.

Mental Math Tricks for Percentages

Once you know a few shortcuts, many percentage calculations become instant mental math. Quick tip: these tricks are especially useful when you're at a restaurant, in a store, or quickly checking a number in your head — no calculator required.

  • The 10% trick — move the decimal one place left. To find 10% of any number, simply move the decimal point one position to the left: 10% of $85 = $8.50, 10% of 320 = 32. This is the single most useful shortcut because 10% is the building block for nearly every other estimate.
  • 5% is exactly half of 10%. Find 10% first, then halve it. So 5% of $60 = half of $6.00 = $3.00. This takes two seconds once you have the 10% anchor and is ideal for calculating small tips or service charges.
  • 15% = 10% + 5% — the standard tip formula. Calculate 10%, calculate 5% (half of that), and add them together. On a $42 meal: 10% = $4.20, 5% = $2.10, total tip = $6.30. This method is fast enough to do at the table without picking up your phone.
  • 20% = double the 10%. Find 10% and multiply by 2. So 20% of $75 = $7.50 × 2 = $15.00. The 20% tip is increasingly standard in many cities, making this trick especially practical for everyday restaurant math.
  • The flip trick — percentages are commutative. X% of Y equals Y% of X. This sounds counterintuitive but is mathematically true: 8% of 25 = 25% of 8 = 2. The flip trick is most powerful when one of the numbers is a multiple of 10 or 25 — mental math becomes trivial. Instead of computing 4% of 50, flip it to 50% of 4 = 2 instantly.

Shareable URLs and a running calculation history

Every calculation now writes its values into the URL — mode, inputs, result. Copy the link and whoever opens it lands on the same calculation, already filled in. Useful when you need to send a quick check to a colleague: 'here's the 18% tip on $84.50, see for yourself.' The URL updates silently as you type, so there's no button to remember. The three modes work as before: X% of Y, X is what % of Y, and % change from X to Y — direction (increase vs. decrease) is inferred from the sign of the result.

A history panel keeps the last 5 calculations with a copy button on each entry. Switch modes and run a few numbers and they stack up automatically — no manual saving. If you close the tab, the history is gone; this tool doesn't use localStorage for history. What isn't here: a running total across entries, currency formatting, or a calculation label field.

Frequently Asked Questions

What's the formula for calculating a percentage?
To find X% of Y, multiply Y by X and divide by 100. For example, 20% of 150 = 150 × 20 / 100 = 30. To find what percentage X is of Y, divide X by Y and multiply by 100. For example, 30 ÷ 150 × 100 = 20%.
How do I calculate percentage change?
Percentage change = ((New Value − Old Value) / |Old Value|) × 100. A positive result means an increase; a negative result means a decrease. For example, going from 80 to 100 is a 25% increase. Going from 100 to 80 is a 20% decrease.
Why doesn't a 50% increase followed by a 50% decrease get you back to the original?
Because percentages are relative to the current value, not the original. If you start at 100 and increase by 50%, you get 150. Then a 50% decrease of 150 gives you 75 — not 100. This is why percentage changes aren't simply reversible and why the order of operations matters.
How do I calculate a discount price?
To find the final price after a discount, subtract the discount percentage from 100%, then multiply the original price by that fraction. For a 30% discount on a $120 item: 100% − 30% = 70%, so the sale price is $120 × 0.70 = $84. Alternatively, calculate the discount amount ($120 × 0.30 = $36) and subtract it from the original price ($120 − $36 = $84).
What's the difference between a percentage point and a percentage change?
These are often confused but mean very different things. A percentage point is an absolute difference: if a tax rate rises from 5% to 8%, it increased by 3 percentage points. A percentage change is relative: that same increase is a 60% change (3 ÷ 5 × 100). Politicians and media sometimes blur the distinction intentionally — "rates rose by 60%" sounds alarming while "rates rose by 3 percentage points" sounds modest, even though both describe the same event.

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