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How to calculate percentages (the plain-English way)
Percentages trip people up because the word hides three or four different questions. Here is each one, with a formula you can remember and a worked example — plus a free calculator when you would rather just type the numbers.
Ask someone to “work out the percentage” and you’ll get a blank look — not because the maths is hard, but because percentage quietly means four different questions. Sort out which question you’re asking and each one becomes a single line of arithmetic. Here they are, with the shortcuts worth memorising. When you’d rather skip the mental maths, our free percentage calculator does all of these instantly.
1. A percentage of a number
The everyday one: tips, tax, “20% off”. Divide the percentage by 100 and multiply.
15% of 200 → 15 ÷ 100 = 0.15 → 0.15 × 200 = 30
The shortcut worth knowing: 10% is just the number with its decimal point moved one place left (10% of 240 is 24). From there, 5% is half of that, 20% is double, and you can assemble most everyday percentages in your head.
2. Percentage increase and decrease
For price rises, pay bumps, or a stat that moved. Take the difference, divide by the original, times 100.
From 80 to 100 → (100 − 80) ÷ 80 × 100 = 25% increase From 100 to 80 → (80 − 100) ÷ 100 × 100 = −20% (a decrease)
The trap is dividing by the wrong number: always divide by where you started, not where you ended up. That’s why a 50% drop needs a 100% rise to recover.
3. Reverse percentages (find the original)
When a number already includes the percentage — a total with tax, a sale price — you work backwards by dividing.
£120 including 20% tax → 120 ÷ 1.20 = £100 before tax £80 after a 20% discount → 80 ÷ 0.80 = £100 original price
Add the rate for something included (1.20), subtract it for a discount (0.80). Reaching for “20% of 120” here is the classic mistake — it gives the wrong original every time.
4. Percentage difference
When you’re comparing two values and neither is the “before”, measure against their average instead:
40 vs 60 → |60 − 40| ÷ ((40 + 60) ÷ 2) × 100 = 20 ÷ 50 × 100 = 40%
Use this for “how far apart are these two readings?” — not for change over time, which belongs in section 2.
When to just use a calculator
Mental shortcuts are great for a tip or a rough sanity-check. For anything with money, deadlines, or more than two steps, type it in — a dedicated tool removes the “divided by the wrong number” errors entirely. Our percentage calculator covers all four questions above (plus percentage points and fractions) on one page, free and with nothing to install.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. For 15% of 200: 15 ÷ 100 = 0.15, then 0.15 × 200 = 30. A quick shortcut for round numbers: 10% is the number with the decimal moved one place left, and you can scale from there.
How do I work out a percentage increase or decrease?
Subtract the old value from the new value, divide by the old value, then multiply by 100. Going from 80 to 100 is (100 − 80) ÷ 80 × 100 = 25% increase. A negative answer means a decrease.
What is 15% of 200?
30. Multiply 200 by 0.15 (which is 15 ÷ 100). The same method works for any percentage of any number.
How do I reverse a percentage to find the original price?
If a price already includes a percentage, divide by 1 plus the rate. A total of £120 that includes 20% tax was £120 ÷ 1.20 = £100 before tax. To remove a discount instead, divide by 1 minus the rate.
What is the difference between percentage change and percentage difference?
Percentage change has a clear before and after and is measured against the starting value. Percentage difference compares two values with no natural order and is measured against their average — useful when neither number is the baseline.