Percentage Calculator

This percentage calculator answers the six questions people actually ask: what is 15% of 80, what percent is 25 of 200, how much did something go up or down, what is a price plus or minus a percentage, what was the price before tax or a discount, and what is the difference between a percentage point and a percent. Each one sits in its own card, written as a sentence with boxes to fill in, so you never have to remember which formula goes where.

Every answer updates the moment you type and comes with the working in plain words, for example “250 × 10 ÷ 100 = 25”. That makes it useful for checking homework, a receipt, a pay rise or a quote, because you can see how the number was reached.

It is free, has no sign-up, and the sums are done by your browser on your own device, so nothing you type is sent anywhere. It also handles the awkward cases properly: decimals, negative numbers, changes that start from zero, and taking off 100%, each with a clear message instead of a confusing “NaN” or “Infinity”.

1. Find the card that matches your question, for example “Percentage change” or “Reverse percentage”.
2. Type your numbers into its boxes. Decimals like 12.5 are fine, and so are thousands separators like 1,250.
3. Read the answer in large type, with the working and a plain-English explanation underneath.
4. To make a number negative on a phone, tap the box, then tap “± Flip sign”, because most phone number pads have no minus key.
5. Tap “Copy answer” to copy the working and explanation, ready to paste into a message or document.

• Percentage of a number: what is X% of Y.
• What percent: X is what percentage of Y, including answers over 100%.
• Percentage change from an old value to a new one, labelled clearly as an increase or a decrease.
• Add or take off a percentage, showing both the new total and the amount added or removed.
• Reverse percentage to find the original amount before a rise or a discount, with one-tap 10%, 15%, 18% GST and 20% VAT buttons.
• Percentage points vs percent: shows both the point change and the relative change, side by side.
• Live results as you type, with no calculate button, and long decimals rounded to six places so float glitches like 0.30000000000000004 never appear.

• A 20% discount followed by a 20% rise does not bring you back to where you started. £100 less 20% is £80, and £80 plus 20% is £96. Use the reverse percentage card to undo a change correctly.
• To strip GST out of a price that includes it, use the reverse card, not “minus 10%”. Taking 10% off $110 gives $99, which is wrong; dividing by 1.1 gives the real $100.
• When a starting value is negative, the change is measured against its size, so going from −50 to −25 is shown as a 50% increase. The card says so whenever this happens.
• A change from 0 cannot be a percentage, because it would mean dividing by zero. You will see the plain amount it went up or down by instead.
• Interest rates and poll results move in percentage points. A rate going from 4% to 5% is one point higher, but 25% higher in relative terms. Both are true; news stories often mix them up.