Metric vs Imperial: A Complete Guide to Unit Conversion (With Cheat Sheet)
Why We Have Two Systems
Most of the world uses metric (SI). Only the US, Myanmar, and Liberia officially use imperial. Globally, scientists, doctors, aviation, and shipping use metric even in the US.
Length Conversions
| From | To | Multiply By | Memory Trick |
|---|---|---|---|
| Inches | Centimeters | 2.54 | ~2.5 cm per inch |
| Feet | Meters | 0.3048 | 1 foot ≈ 30 cm |
| Miles | Kilometers | 1.60934 | 1 mile ≈ 1.6 km |
| Kilometers | Miles | 0.62137 | Divide km by 1.6 |
Weight Conversions
| From | To | Multiply By |
|---|---|---|
| Pounds (lb) | Kilograms (kg) | 0.4536 |
| Kilograms (kg) | Pounds (lb) | 2.2046 |
| Stones | Kilograms | 6.35 |
Temperature
°F to °C: (F − 32) × 5/9 | °C to °F: (C × 9/5) + 32
Quick memory: 0°C = 32°F, 100°C = 212°F, 37°C = 98.6°F (body temp)
Volume Conversions
| From | To | Multiply By |
|---|---|---|
| US Gallons | Liters | 3.785 |
| Liters | US Gallons | 0.2642 |
| Fluid oz (US) | Milliliters | 29.57 |
Quick mental math: Speed limits — multiply mph by 1.6 for km/h. 60 mph = 96 km/h. 100 km/h = 62 mph. Weight: double kg and add 10% for rough lb conversion. 70 kg ≈ 154 lb (exact: 154.3).
Frequently Asked Questions
How do I convert Celsius to Fahrenheit mentally?
Quick approximation: double the Celsius, then add 30. 20°C → 20×2+30 = 70°F (exact: 68°F). For exact: (C × 9/5) + 32. Body temp: 37°C = 98.6°F.
Why doesn't the US use the metric system?
Historical inertia, cost of conversion, and political resistance. Multiple attempts to switch failed. The US does use metric in science, medicine, military, and international trade — the everyday use of imperial is mostly consumer habit.
What is the difference between US fluid ounces and UK fluid ounces?
They're different! US fluid oz = 29.57 mL. UK (imperial) fluid oz = 28.41 mL. So 1 US cup (8 fl oz) = 236.6 mL vs 1 UK cup (10 fl oz) = 284.1 mL. This matters in cooking.