5 Percentage Calculations Everyone Should Know
Percentages show up everywhere — payslips, sale racks, restaurant bills, exam results, interest rates. Most of it comes down to a small handful of calculations. Learn these five and you'll rarely need to reach for a calculator, though the Percentage Calculator is there for the moments you'd rather not do the mental math.
1. Finding a percentage of a number
The most common one: working out a discount, a tip, or a tax amount.
Example: a 20% discount on an $85 jacket. (20 ÷ 100) × 85 = $17 off, so you'd pay $68.
2. Working out what percentage one number is of another
Useful for exam scores, project completion, or "what share of my budget did that cost?"
Example: you scored 42 out of 60 on a test. (42 ÷ 60) × 100 = 70%.
3. Percentage increase or decrease
This is the one behind pay rises, price hikes, and "how much did that grow?" questions.
Example: your pay goes from $65,000 to $70,000. ((70,000 − 65,000) ÷ 65,000) × 100 ≈ 7.7% increase. The same formula gives a negative number for a decrease — a drop from $70,000 to $65,000 is about −7.1%, not the same size as the increase, which trips a lot of people up.
4. Reverse percentages — finding the original number
The one people get stuck on: a price already has a discount or tax baked in, and you need to work backward to the original figure. You can't just add the percentage back onto the new number — that gives the wrong answer.
Example: a jacket is on sale for $68 after a 20% discount. $68 ÷ (1 − 0.20) = $68 ÷ 0.80 = $85 original price. For a price that's had tax added instead of a discount, swap the minus for a plus: divide by (1 + tax rate).
5. Splitting a bill with a tip
A specific, very common case of calculation #1 — but worth calling out on its own since it comes up so often. Work out the tip amount, add it to the bill, then divide by the number of people.
Example: a $120 bill, an 18% tip, split four ways: (120 × 0.18) + 120 = $141.60, ÷ 4 = $35.40 per person. ToolboxTree's Tip Calculator handles this one instantly if you'd rather skip the arithmetic at the table.