Formulas
- B% of A = A × B ÷ 100
- A as a percent of B = A ÷ B × 100
- Percent change = (new − old) ÷ old × 100
- Add B% = A × (1 + B ÷ 100); take off B% = A × (1 − B ÷ 100)
Percent vs percentage points
A difference between two rates is measured in percentage points. If an interest rate goes from 3% to 4%, that is a 1 percentage point rise, or a (4 − 3) ÷ 3 ≈ 33.3% rise. Both describe the same change.
Two discounts in a row
20% off and then another 10% off is 28% off, not 30%, because the second discount applies to the already reduced price.
Total discount = 1 − (1 − 0.2) × (1 − 0.1) = 0.28 → 28%
For the same reason, a value that falls 50% and then rises 50% ends at 75% of where it started.
FAQ
How do I find the original price before a discount?
Divide the sale price by (1 − discount). ₩8,000 after 20% off was ₩8,000 ÷ 0.8 = ₩10,000.
How do I add or remove Korea’s 10% VAT?
Use the VAT Calculator to split an amount into supply value, VAT and total.
What if the starting value is 0?
Percent change from 0 is undefined, because you cannot divide by zero.
For reference only. Actual amounts and decisions depend on your contract, your workplace and how the authorities apply the rules.