Tricks for doing maths in your head, plus quick-estimate tables you can reach for fast.
| To convert | Quick rule | Example |
|---|---|---|
| km → miles | × 5 ÷ 8 (≈ × 0.62) | 100 km ≈ 62 mi |
| miles → km | × 8 ÷ 5 (≈ × 1.6) | 50 mi ≈ 80 km |
| kg → lb | × 2.2 | 10 kg ≈ 22 lb |
| lb → kg | ÷ 2.2 (≈ × 0.45) | 150 lb ≈ 68 kg |
| °C → °F | × 2 + 30 (rough) | 20°C ≈ 68°F |
| litres → US gallons | ÷ 4 (≈ × 0.26) | 20 L ≈ 5.3 gal |
| cm → inches | ÷ 2.5 | 30 cm ≈ 12 in |
| Value | Approx |
|---|---|
| π (pi) | 3.14159 |
| √2 | 1.414 |
| √3 | 1.732 |
| e | 2.718 |
| 1 mile | 1.609 km |
| 1 inch | 2.54 cm |
| 1 foot | 30.48 cm |
| 1 kg | 2.205 lb |
| Fraction | Decimal | Percent |
|---|---|---|
| 1/2 | 0.5 | 50% |
| 1/3 | 0.333 | 33.3% |
| 2/3 | 0.667 | 66.7% |
| 1/4 | 0.25 | 25% |
| 3/4 | 0.75 | 75% |
| 1/5 | 0.2 | 20% |
| 1/6 | 0.167 | 16.7% |
| 1/8 | 0.125 | 12.5% |
| 1/10 | 0.1 | 10% |
| Bill | 15% | 18% | 20% |
|---|---|---|---|
| $20 | $3.00 | $3.60 | $4.00 |
| $40 | $6.00 | $7.20 | $8.00 |
| $60 | $9.00 | $10.80 | $12.00 |
| $80 | $12.00 | $14.40 | $16.00 |
| $100 | $15.00 | $18.00 | $20.00 |
Percentages flip. x% of y always equals y% of x, and one side is often easier — 8% of 25 is awkward, but 25% of 8 is just 2. Build the rest from 10% (move the decimal one place): 5% is half of that, 20% is double, and 15% is 10% plus half of 10%.
Multiplying by 11 (two digits): add the two digits and drop the sum in the middle. 35 × 11 → 3 (3+5) 5 = 385. If the middle adds past 9, carry the 1.
Squaring a number ending in 5: take the tens digit n, work out n × (n+1), and stick 25 on the end. 35² → 3 × 4 = 12 → 1225.
Doubling and halving turns hard multiplications into easy ones: double one number and halve the other until it's simple. 16 × 25 → 8 × 50 → 4 × 100 = 400.
Quick multipliers: × 5 is × 10 then halve; × 9 is × 10 minus the number itself; × 4 is double, then double again.
Divisibility checks: a number divides by 3 (or 9) if its digit sum does; by 4 if its last two digits do; by 6 if it passes both the 2 and 3 tests. For everyday sums, the real skill is estimating — round each number to something friendly, do the easy maths, then nudge the answer in the direction you rounded. For exact conversions use the unit converter.