Whether you're cooking from a European recipe, traveling abroad, or working in science or engineering, converting between metric and imperial units is an everyday skill. This guide covers every major conversion category with exact formulas, quick-reference tables, and practical tips.
Why Two Systems Exist
The metric system (SI) was developed in France in the 1790s and is now used by every country in the world except the United States, Myanmar, and Liberia. The imperial system evolved from Roman and British measurements over centuries. The US uses a variant called "US customary units" that's similar but not identical to imperial.
Length Conversions
Length is one of the most common conversions you'll need. Here's your complete reference:
| Metric | Imperial | Exact Conversion |
|---|---|---|
| 1 millimeter (mm) | 0.0394 inches | 1 inch = 25.4 mm |
| 1 centimeter (cm) | 0.3937 inches | 1 inch = 2.54 cm |
| 1 meter (m) | 3.281 feet | 1 foot = 0.3048 m |
| 1 meter (m) | 1.094 yards | 1 yard = 0.9144 m |
| 1 kilometer (km) | 0.6214 miles | 1 mile = 1.609 km |
Quick Mental Math for Length
- cm to inches: Divide by 2.5 (exact: 2.54)
- meters to feet: Multiply by 3.3 (exact: 3.281)
- km to miles: Multiply by 0.6 (exact: 0.621)
- inches to cm: Multiply by 2.5 (exact: 2.54)
- feet to meters: Multiply by 0.3 (exact: 0.305)
- miles to km: Multiply by 1.6 (exact: 1.609)
Weight Conversions
Weight conversions come up constantly in cooking, shipping, fitness, and international commerce.
| Metric | Imperial | Exact Conversion |
|---|---|---|
| 1 gram (g) | 0.0353 ounces | 1 oz = 28.35 g |
| 1 kilogram (kg) | 2.205 pounds | 1 lb = 0.4536 kg |
| 1 kilogram (kg) | 35.27 ounces | 1 oz = 28.35 g |
| 1 metric ton | 2,205 pounds | 1 short ton = 907.2 kg |
Temperature Conversions
Temperature is where the metric-imperial difference is most noticeable in daily life.
Formulas
- °C to °F: Multiply by 9/5, then add 32 (or: °F = °C × 1.8 + 32)
- °F to °C: Subtract 32, then multiply by 5/9 (or: °C = (°F - 32) ÷ 1.8)
- °C to K: Add 273.15
| Description | °C | °F |
|---|---|---|
| Water freezes | 0°C | 32°F |
| Room temperature | 20-22°C | 68-72°F |
| Body temperature | 37°C | 98.6°F |
| Water boils | 100°C | 212°F |
| Oven: Low | 150°C | 300°F |
| Oven: Medium | 180°C | 350°F |
| Oven: High | 220°C | 425°F |
Volume Conversions
Volume conversions are critical for cooking, fuel, and scientific measurements.
| Metric | Imperial (US) | Exact Conversion |
|---|---|---|
| 1 milliliter (mL) | 0.0338 fl oz | 1 fl oz = 29.57 mL |
| 1 liter (L) | 33.81 fl oz | 1 fl oz = 29.57 mL |
| 1 liter (L) | 0.2642 gallons | 1 gallon = 3.785 L |
| 1 liter (L) | 4.227 cups | 1 cup = 236.6 mL |
| 5 mL | 1 teaspoon | 1 tsp = 4.93 mL |
| 15 mL | 1 tablespoon | 1 tbsp = 14.79 mL |
Area Conversions
| Metric | Imperial | Exact Conversion |
|---|---|---|
| 1 square meter (m²) | 10.76 sq ft | 1 sq ft = 0.0929 m² |
| 1 hectare | 2.471 acres | 1 acre = 0.4047 hectares |
| 1 square km | 0.3861 sq miles | 1 sq mile = 2.59 km² |
Speed Conversions
| From | To | Multiply By |
|---|---|---|
| km/h | mph | 0.6214 |
| mph | km/h | 1.609 |
| m/s | mph | 2.237 |
| knots | km/h | 1.852 |
Recommended Measuring Tools
Accurate conversions start with reliable measuring instruments. These tools are highly rated on Amazon:
Common Conversion Mistakes to Avoid
- Confusing US and Imperial gallons: A US gallon is 3.785L; a UK imperial gallon is 4.546L. Always check which system your source uses.
- Rounding too early: In precise work (science, engineering), keep extra decimal places until your final answer.
- Forgetting to convert squared/cubed units: If 1 foot = 0.3048 meters, then 1 square foot = 0.0929 square meters (not 0.3048). You must square the conversion factor for area and cube it for volume.
- Mixing weight and mass: In everyday use, kilograms and pounds are treated as weights. In physics, kilogram is mass and Newton is force.