Unit conversions are one of those skills that should be simple but cause more errors than actual math. The problem isn't the arithmetic — it's knowing which conversion factor to use, which direction to multiply, and not confusing similar-sounding units. This guide covers the conversions you'll actually need, with mental math shortcuts and common traps.
All of these conversions are available instantly at CalcHub.
Length & Distance
| Conversion | Factor | Mental Shortcut |
|---|
| Inches → cm | × 2.54 | × 2.5 (close enough) |
| Feet → meters | × 0.3048 | ÷ 3.3 |
| Yards → meters | × 0.9144 | Roughly equal |
| Miles → km | × 1.609 | × 1.6 |
| km → miles | × 0.6214 | × 0.6 |
Quick mental math: For miles to km, multiply by 8 then divide by 5. For km to miles, multiply by 5 then divide by 8. Example: 50 miles × 8 / 5 = 80 km. ✓
Weight & Mass
| Conversion | Factor | Mental Shortcut |
|---|
| Pounds → kg | × 0.4536 | ÷ 2.2 |
| kg → pounds | × 2.2046 | × 2.2 |
| Ounces → grams | × 28.3495 | × 28 |
| Grams → ounces | × 0.03527 | ÷ 28 |
| Stone → kg | × 6.35 | × 6.4 |
Common trap: In the US, body weight is in pounds. In most of the world, it's in kg. A 150-pound person weighs about 68 kg. Quick conversion: divide pounds by 2 and subtract 10%. 150 / 2 = 75, minus 10% = 67.5 kg. Close to the exact 68 kg.
Volume
| Conversion | Factor | Mental Shortcut |
|---|
| Liters → US gallons | × 0.2642 | ÷ 3.8 |
| US gallons → liters | × 3.785 | × 3.8 |
| mL → fluid ounces | × 0.03381 | ÷ 30 |
| Cups (US) → mL | × 236.6 | × 240 |
| Tablespoon → mL | × 14.787 | × 15 |
| Teaspoon → mL | × 4.929 | × 5 |
Common trap: US gallons ≠ Imperial gallons. A US gallon is 3.785 L; an Imperial (UK) gallon is 4.546 L. If you're converting fuel economy, use the right gallon.
Temperature
Celsius to Fahrenheit: F = (C × 9/5) + 32
Fahrenheit to Celsius: C = (F − 32) × 5/9
| °C | °F | Context |
|---|
| 0 | 32 | Water freezes |
| 20 | 68 | Comfortable room |
| 37 | 98.6 | Body temperature |
| 100 | 212 | Water boils |
Mental shortcut for C to F: Double the Celsius, then add 30. Not exact but fast: 25°C → 50 + 30 = 80°F (actual: 77°F). Good enough for weather and cooking estimates.
Speed
| Conversion | Factor | Mental Shortcut |
|---|
| km/h → mph | × 0.6214 | × 0.6 |
| mph → km/h | × 1.609 | × 1.6 |
| m/s → km/h | × 3.6 | × 3.6 |
| knots → km/h | × 1.852 | × 1.85 |
Useful reference: 100 km/h ≈ 62 mph. 60 mph ≈ 97 km/h. Highway speeds are roughly "km/h ÷ 1.6 = mph."
Area
| Conversion | Factor |
|---|
| sq ft → sq m | × 0.0929 |
| sq m → sq ft | × 10.764 |
| Acres → sq m | × 4,046.86 |
| Hectares → acres | × 2.471 |
| Cents → sq ft | × 435.6 |
Common trap: You can't convert length factors for area. 1 foot = 0.3048 meters, but 1 sq ft ≠ 0.3048 sq m. You must square the factor: 0.3048² = 0.0929. This catches people all the time.
Energy & Power
| Conversion | Factor |
|---|
| Calories (food) → kJ | × 4.184 |
| kWh → MJ | × 3.6 |
| BTU → kJ | × 1.055 |
| HP → kW | × 0.7457 |
| kW → HP | × 1.341 |
Data Storage
| Unit | Size |
|---|
| 1 KB | 1,024 bytes |
| 1 MB | 1,024 KB |
| 1 GB | 1,024 MB |
| 1 TB | 1,024 GB |
Common trap: Hard drive manufacturers use 1 GB = 1,000,000,000 bytes (SI), while operating systems use 1 GB = 1,073,741,824 bytes (binary). This is why a "500 GB" hard drive shows as ~465 GB in your OS.
The Three Golden Rules
1. Always include units in your work. Writing "distance = 5" means nothing. "Distance = 5 km" can be checked and converted. Unit tracking prevents errors.
2. When in doubt, do a sanity check. If your conversion gives you a result that seems wrong, it probably is. A 6-foot person converting to 18 meters (instead of 1.8) means you multiplied instead of divided.
3. Use a converter for anything non-trivial. Mental math shortcuts are for estimates. For anything involving money, construction, or dosages, use
CalcHub and get the exact number.
Why doesn't the world just use one system?
Most of the world uses metric (SI). The US, Liberia, and Myanmar are the only countries that haven't officially adopted metric for everyday use. The US tried in the 1970s and it didn't stick. Cultural inertia is powerful.
What's the difference between mass and weight?
Mass is how much matter an object has (measured in kg). Weight is the force of gravity on that mass (measured in Newtons). On Earth, they're proportional, so we use them interchangeably. On the Moon, your mass is the same but your weight is ~1/6th.
Why are there two types of ounces?
Fluid ounces measure volume. Regular ounces (avoirdupois) measure weight. 1 fluid ounce of water happens to weigh about 1 ounce, which is probably why they share a name. But for other liquids (oil, honey, mercury), fluid ounces and weight ounces are very different.
- Length Converter — meters, feet, miles, km
- Weight Converter — kg, pounds, ounces
- Volume Converter — liters, gallons, cups
- Temperature Converter — Celsius, Fahrenheit, Kelvin
- Area Converter — sq ft, sq m, acres, hectares