Percentage Calculator

What is a percentage calculator?

A percentage calculator turns the three questions people actually ask about percentages into a couple of taps. Instead of remembering which number to divide by, or whether to multiply before or after, you choose a mode, type two numbers, and read the answer. The tool above handles the three cases that cover the vast majority of everyday percentage maths:

• What is X% of a value? — for tips, discounts, tax, commission and "give me 15% of this."
• X is what percent of a total? — for scores, progress bars, conversion rates and "how much of the budget is this?"
• Percent change from one value to another — for price moves, growth, weight loss, and any before-and-after comparison.

Each mode shows a single big result so there is nothing to misread, and the percent-change mode also tells you whether the move is an increase or a decrease.

How to use it

1. Pick a mode using the three buttons at the top. The labels and input prompts change to match the question you are answering.
2. Type your two numbers. Empty boxes simply leave the result blank — you will never see "NaN" or a broken value while you are mid-typing.
3. Read the result tile. It updates live as you type, so you can try variations without pressing anything.
4. Copy the answer with one click if you need it in a message, spreadsheet or document.

Everything happens on your own device, so it stays instant even if you change the numbers dozens of times, and it works the same offline once the page has loaded.

The three calculations, in plain words

Percent of a value

To find a percent of a number, divide the percent by 100 and multiply by the value. So 20% of 150 is 0.20 × 150 = 30. This is the maths behind sales discounts ("20% off"), restaurant tips, and adding sales tax or VAT to a price. If you want the final price after a discount, find the percent off and subtract it — for a 20% discount on 150, that is 150 − 30 = 120.

One number as a percent of another

To express one number as a percent of a total, divide the part by the whole and multiply by 100. 30 out of 120 is 30 ÷ 120 × 100 = 25%. This is how you turn raw counts into rates: a test score, the share of a budget spent, the proportion of visitors who clicked, or the slice one category takes of a total. The calculator guards against a total of zero, because dividing by zero has no meaningful answer.

Percent change

Percent change tells you how much a value moved, relative to where it started:

change % = (new − old) ÷ |old| × 100

Going from 80 to 100 is (100 − 80) ÷ 80 × 100 = a 25% increase. Going from 100 back down to 80 is a 20% decrease — note that the two are not symmetric, because the starting value (the denominator) is different each time. This asymmetry is the single most common source of percentage confusion, so it is worth remembering whenever you compare a rise and a fall.

Common mistakes to avoid

• Mixing up percent change with percentage points. If a rate goes from 40% to 45%, that is 5 percentage points but a 12.5% increase. Headlines often blur the two; the maths does not.
• Assuming a rise and a fall cancel out. A 50% gain followed by a 50% loss does not return you to the start. After +50% then −50%, 100 becomes 150 then 75 — a net loss of 25%.
• Stacking discounts by adding the percentages. "20% off then 10% off" is not 30% off. Apply them in sequence: 20% off 100 is 80, then 10% off 80 is 72 — a 28% total discount.
• Dividing by the wrong number in percent change. Always divide by the original value, not the new one, or the result will be off.

Tips for getting answers faster

• Reverse a discount to find the original price. If an item costs 72 after 20% off, the original is 72 ÷ 0.80 = 90. You can confirm it with the "percent of value" mode.
• Use the ratio mode for progress. "I have written 1,200 of 2,000 words" is 60% — a quick way to label a progress bar or status update.
• Sanity-check with round numbers. 10% of anything is just the value with the decimal point shifted one place left, which makes a fast mental estimate before you trust the exact figure.
• Watch the sign on change. A green, positive result is a rise; a red, negative result is a fall. That colour cue catches direction mistakes at a glance.

Accurate, private, and always free

Because the percentage calculator runs entirely in your browser, there are no accounts, no limits and nothing uploaded. Keep it open in a tab whenever you are budgeting, shopping a sale, checking growth, or grading — it turns "what percentage is that?" into a number you can see and copy in seconds. For other everyday sums, the age calculator and BMI calculator work the same instant, private way.