The three percentage formulas
Every percentage question falls into one of three cases. Each has a one-line formula:
The third formula gives a positive result for an increase and a negative one for a decrease. The calculator above applies all three instantly — just pick the matching tab.
Mental math tricks you'll actually use
- 10% of any number: move the decimal point one place left. 10% of 248 = 24.8.
- 1%: move it two places left. 1% of 248 = 2.48.
- 5%: take 10%, then halve it. 5% of 80 = 4.
- 15% (the classic US tip): 10% + 5%. On an $80 bill: 8 + 4 = $12.
- 20%: take 10% and double it. 20% of 65 = 13.
- x% of y = y% of x: 8% of 50 = 50% of 8 = 4. Use whichever is easier.
Common tip & discount calculations
| Amount | 10% | 15% | 18% | 20% | 25% |
|---|---|---|---|---|---|
| $25 | $2.50 | $3.75 | $4.50 | $5.00 | $6.25 |
| $40 | $4.00 | $6.00 | $7.20 | $8.00 | $10.00 |
| $60 | $6.00 | $9.00 | $10.80 | $12.00 | $15.00 |
| $80 | $8.00 | $12.00 | $14.40 | $16.00 | $20.00 |
| $100 | $10.00 | $15.00 | $18.00 | $20.00 | $25.00 |
| $120 | $12.00 | $18.00 | $21.60 | $24.00 | $30.00 |
| $150 | $15.00 | $22.50 | $27.00 | $30.00 | $37.50 |
| $200 | $20.00 | $30.00 | $36.00 | $40.00 | $50.00 |
Percent vs. percentage points
These are not the same, and confusing them is a classic mistake. If an interest rate goes from 10% to 12%, it rose by 2 percentage points — but that's a 20% relative increase (2 ÷ 10). News headlines mix them up constantly; now you won't.
Frequently asked questions
How do I calculate X% of a number?
Multiply the number by the percentage, then divide by 100: X% of Y = X × Y ÷ 100. Example: 15% of 200 = 15 × 200 ÷ 100 = 30.
How do I calculate percentage change?
Percent change = (new value − old value) ÷ old value × 100. Positive means an increase, negative means a decrease. Always divide by the original value.
What is the difference between percent and percentage points?
Percent is a relative ratio; percentage points are the arithmetic difference between two percentages. Going from 10% to 15% is a 5 percentage-point rise — and a 50% relative increase.
Is this percentage calculator free?
Yes — 100% free, unlimited, no sign-up. CalculUnits is funded by non-intrusive ads, never by paywalls.