The United States uses the imperial system; most of the world uses metric. If you travel, follow international recipes, read scientific research, or buy products from abroad, unit conversion is a daily necessity.
This guide covers the conversions that come up most often — with exact formulas and quick mental shortcuts.
Length Conversions
| From | To | Multiply by | Quick trick |
|---|---|---|---|
| Miles | Kilometers | × 1.60934 | ≈ ×1.6 |
| Kilometers | Miles | × 0.62137 | ≈ ×0.6 |
| Feet | Meters | × 0.3048 | ≈ ×0.3 |
| Meters | Feet | × 3.28084 | ≈ ×3.28 |
| Inches | Centimeters | × 2.54 | Exact |
| Centimeters | Inches | × 0.39370 | ÷ 2.54 |
Memory trick for miles → km: multiply by 8, divide by 5. Example: 50 mi × 8 = 400 ÷ 5 = 80 km.
Weight Conversions
| From | To | Multiply by |
|---|---|---|
| Pounds (lbs) | Kilograms (kg) | × 0.453592 |
| Kilograms (kg) | Pounds (lbs) | × 2.20462 |
| Ounces (oz) | Grams (g) | × 28.3495 |
| Grams (g) | Ounces (oz) | × 0.035274 |
| Stone (st) | Pounds (lbs) | × 14 |
| Metric Tons (t) | Pounds (lbs) | × 2204.62 |
Quick mental estimate for lbs → kg: divide by 2, then subtract 10%. Example: 150 lbs ÷ 2 = 75 − 7.5 = 67.5 kg (actual: 68 kg).
Temperature Conversions
Temperature is the trickiest conversion because it's not a simple multiplication — it has an offset.
°F → °C: (°F − 32) × 5/9
°C → °F: (°C × 9/5) + 32
°C → K: °C + 273.15
| Reference point | °F | °C |
|---|---|---|
| Water freezes | 32°F | 0°C |
| Room temperature | 72°F | 22°C |
| Body temperature | 98.6°F | 37°C |
| Water boils | 212°F | 100°C |
Quick trick: double the Celsius and add 30 to get an approximate Fahrenheit. Example: 20°C × 2 = 40 + 30 = 70°F (actual: 68°F). Close enough for weather forecasts.
Volume Conversions
| From | To | Result |
|---|---|---|
| 1 gallon (US) | Liters | 3.785 L |
| 1 liter | Fluid ounces (US) | 33.814 fl oz |
| 1 cup (US) | Milliliters | 236.6 mL |
| 1 tablespoon (US) | Milliliters | 14.787 mL |
Speed Conversions
- 1 mph = 1.60934 km/h
- 1 km/h = 0.62137 mph
- 1 knot = 1.852 km/h = 1.15078 mph
- 1 m/s = 3.6 km/h = 2.237 mph
Why the US Still Uses Imperial
The US is one of three countries (along with Liberia and Myanmar) that haven't officially adopted the metric system. The reasons are largely historical and economic: changing road signs, product labeling, manufacturing equipment, and consumer habits would cost hundreds of billions of dollars and decades of transition.
The result? Americans still need to convert regularly — for science, international trade, cooking with foreign recipes, and travel.