The percentage math you actually use at a restaurant, a checkout, or a sale rack. Includes the mental shortcuts, an international tipping guide, and the discount trap that quietly costs shoppers money.
Fast mental methods for tips, how to back out pre-tax price from a total, an international tipping guide, and why a 20% + 10% coupon stack is 28% off, not 30%.
See also: Percentage calculator · Formula cheat sheet
The reliable trick is to build every tip out of 10%, which you get by moving the decimal one place left. On a $60 bill, 10% = $6.
To split a bill and tip per person, total first (bill + tip), then divide by the number of people — dividing before tipping makes the arithmetic messier.
Norms vary widely; treat this as a starting point, not a rule, and check locally.
| Region | Customary restaurant tip |
|---|---|
| United States & Canada | 15–20%, expected; often 18–20% for good service |
| United Kingdom & Ireland | 10–12.5%, and often already added as a "service charge" |
| Continental Europe | Service frequently included; round up or add 5–10% for good service |
| Japan & South Korea | No tipping; it can cause confusion or offense |
| Australia & New Zealand | Not expected; round up for exceptional service |
Adding tax: multiply the pre-tax price by (1 + rate). An $80 item at 8% tax: 80 × 1.08 = $86.40.
Backing out the pre-tax price from a tax-inclusive total is the one people get wrong. Don't subtract the tax rate — divide:
Pre-tax = Total ÷ (1 + rate)
A $108 total at 8% tax: 108 ÷ 1.08 = $100.00 pre-tax, so $8 was tax. Subtracting 8% from $108 would wrongly give $99.36, because the 8% was charged on the smaller pre-tax figure, not the total.
A "20% off, then an extra 10% off" coupon is not 30% off. The second discount applies to the already-reduced price, not the original.
On a $100 item: 20% off → $80, then 10% off $80 → $72. You paid $72, a total discount of 28%, not 30%. The shortcut is to multiply the remaining fractions:
0.80 × 0.90 = 0.72 → 28% off
Because it's multiplication, the order doesn't matter — 10% then 20% also lands at $72. And the more discounts you stack, the further the true total drifts below the naive sum: 20% + 20% + 10% feels like 50% but is actually 1 − (0.8 × 0.8 × 0.9) = 42.4% off.
| Task | Do this |
|---|---|
| Add P% tax/tip | multiply by (1 + P/100) |
| Take P% off | multiply by (1 − P/100) |
| Pre-tax from total | divide total by (1 + rate) |
| Stack two discounts | multiply the remaining fractions, don't add the discounts |
No. The second discount applies to the already-reduced price, so 20% then 10% off $100 gives $72 — a 28% total discount. Multiply the remaining fractions (0.80 x 0.90 = 0.72) to get the true total.
Divide the total by (1 + tax rate), don't subtract the rate. A $108 total at 8% tax is 108 / 1.08 = $100 pre-tax. Subtracting 8% would incorrectly give $99.36.
Find 10% by moving the decimal one place left, then scale it: 20% is double that, 15% is 10% plus half of 10%. On a $60 bill, 10% = $6, so a 20% tip is $12.
No. Because the discounts multiply, 20% then 10% gives the same result as 10% then 20% — both land at 72% of the original price.
Links point to primary sources and standards bodies. Tax rates and official formulas change over time — verify against the source for current figures.