Percentage Calculator on the Go
Add our free Chrome extension and calculate percentages anywhere you browse.
Mental Math for Percentages
Every percentage calculation builds from two anchors: 10% (move the decimal left once) and 1% (move it left twice). From these, you can construct any percentage in your head.
For any other percentage: find 10% and 1%, then combine. For 17%, that’s 10% + 5% + 1% + 1%. For 35%, that’s 10% + 10% + 10% + 5%.
Tip Reference
Tip amounts for restaurants, bars, salons, and service providers. Find your bill and read across.
| Bill | 15% | 18% | 20% | 25% |
|---|---|---|---|---|
| $20 | $3.00 | $3.60 | $4.00 | $5.00 |
| $30 | $4.50 | $5.40 | $6.00 | $7.50 |
| $40 | $6.00 | $7.20 | $8.00 | $10.00 |
| $50 | $7.50 | $9.00 | $10.00 | $12.50 |
| $75 | $11.25 | $13.50 | $15.00 | $18.75 |
| $100 | $15.00 | $18.00 | $20.00 | $25.00 |
| $150 | $22.50 | $27.00 | $30.00 | $37.50 |
| $200 | $30.00 | $36.00 | $40.00 | $50.00 |
Fraction to Percentage
Fractions show up in recipes, measurements, statistics, and splitting costs. These are the conversions worth memorizing.
| Fraction | Decimal | Percentage | Where You See It |
|---|---|---|---|
| 1/2 | 0.5 | 50% | Half-off sales |
| 1/3 | 0.333… | 33.33% | Splitting bills 3 ways |
| 2/3 | 0.666… | 66.67% | Supermajority votes |
| 1/4 | 0.25 | 25% | Quarterly reports, recipes |
| 3/4 | 0.75 | 75% | Battery indicators |
| 1/5 | 0.2 | 20% | Standard tip |
| 1/8 | 0.125 | 12.5% | Recipe measurements |
| 1/10 | 0.1 | 10% | Mental math foundation |
| 1/12 | 0.0833… | 8.33% | Monthly portion of annual |
Sale Price Calculator
What you actually pay after a discount. Multiply by the “pay” factor for instant calculation.
| Discount | You Pay | $25 | $50 | $100 | $200 |
|---|---|---|---|---|---|
| 10% off | × 0.90 | $22.50 | $45 | $90 | $180 |
| 15% off | × 0.85 | $21.25 | $42.50 | $85 | $170 |
| 20% off | × 0.80 | $20 | $40 | $80 | $160 |
| 25% off | × 0.75 | $18.75 | $37.50 | $75 | $150 |
| 30% off | × 0.70 | $17.50 | $35 | $70 | $140 |
| 40% off | × 0.60 | $15 | $30 | $60 | $120 |
| 50% off | × 0.50 | $12.50 | $25 | $50 | $100 |
Percentage Errors That Cost Money
These mistakes are easy to make and can lead to significant miscalculations in financial situations.
A 20% discount plus a 10% discount is not 30% off. The second discount applies to the already-reduced price.
$100 × 0.80 = $80
$80 × 0.90 = $72 (not $70)
If something increases by 25%, it does not take a 25% decrease to return to the original. The percentages are calculated from different bases.
To return to $100: $125 × 0.80 = $100
That’s a 20% decrease, not 25%
Losses require disproportionately larger gains to recover. A 50% loss requires a 100% gain to break even.
$50 + 50% = $75 (still down $25)
$50 + 100% = $100 (break even)
Moving from 10% to 15% is a 5 percentage point increase, but a 50% relative increase. News headlines often conflate these.
Absolute: +5 percentage points
Relative: (15-10)/10 = +50%
The Formulas
Each formula corresponds to one of the calculator functions above.