Complete Guide to Metric and Imperial Conversions
Reference tables and mental math tricks for converting between metric and imperial units. Covers length, weight, temperature, volume, and area.
Whether you're reading a recipe from another country, buying furniture from an international retailer, or interpreting weather forecasts while traveling, you'll inevitably need to convert between metric and imperial units. This guide covers the most common conversions, gives you quick reference tables, and shares mental math tricks so you can estimate on the fly without reaching for a calculator.
Length and Distance
Length is the most commonly converted measurement type. Here are the key relationships between metric and imperial length units.
| Conversion | Exact Factor | Quick Estimate |
|---|---|---|
| 1 inch | 2.54 cm | About 2.5 cm |
| 1 foot | 30.48 cm | About 30 cm |
| 1 yard | 0.9144 m | Slightly less than 1 meter |
| 1 mile | 1.60934 km | About 1.6 km |
| 1 cm | 0.3937 in | About 0.4 inches |
| 1 meter | 3.2808 ft | About 3 ft 3 in |
| 1 km | 0.6214 mi | About 0.6 miles |
Mental Math Tricks for Distance
Inches to cm: multiply by 2.5 (or multiply by 5, then halve) 6 inches → 6 × 2.5 = 15 cm (exact: 15.24 cm) cm to inches: divide by 2.5 (or multiply by 0.4) 20 cm → 20 × 0.4 = 8 inches (exact: 7.87 inches) Miles to km: multiply by 1.6 (or multiply by 8, then divide by 5) 10 miles → 10 × 1.6 = 16 km (exact: 16.09 km) km to miles: multiply by 0.6 (or multiply by 3, then divide by 5) 100 km → 100 × 0.6 = 60 miles (exact: 62.14 miles) Fun fact: The Fibonacci sequence approximates mile-to-km conversion. 5 miles ≈ 8 km, 8 miles ≈ 13 km, 13 miles ≈ 21 km
For precise conversions, use the Inches to CM Converter or the Kilometers to Miles Converter.
Weight and Mass
Weight conversions come up constantly in cooking, fitness tracking, shipping, and travel. The most common conversions involve pounds, kilograms, and ounces.
| Conversion | Exact Factor | Quick Estimate |
|---|---|---|
| 1 pound (lb) | 0.4536 kg | About 0.45 kg |
| 1 kilogram | 2.2046 lbs | About 2.2 lbs |
| 1 ounce (oz) | 28.3495 g | About 28 grams |
| 1 stone | 6.3503 kg | About 6.35 kg or 14 lbs |
| 100 grams | 3.5274 oz | About 3.5 oz |
Mental Math Tricks for Weight
Pounds to kg: divide by 2, then subtract 10% of the result 150 lbs → 150/2 = 75, minus 7.5 = 67.5 kg (exact: 68.0 kg) kg to pounds: double it, then add 10% of the result 70 kg → 70 × 2 = 140, plus 14 = 154 lbs (exact: 154.3 lbs) Quick pound-to-kg reference points: 100 lbs = 45.4 kg 150 lbs = 68.0 kg 200 lbs = 90.7 kg
Convert weights precisely with the Kilograms to Pounds Converter.
Temperature
Temperature is the trickiest conversion because Fahrenheit and Celsius use different zero points, not just different scale sizes. You can't simply multiply by a single number.
Formulas:
°F to °C: (°F - 32) × 5/9
°C to °F: (°C × 9/5) + 32
Key reference points:
-40° = -40° (the one temperature where they match!)
0°C = 32°F (water freezes)
10°C = 50°F (cool day)
20°C = 68°F (room temperature)
25°C = 77°F (warm day)
30°C = 86°F (hot day)
37°C = 98.6°F (body temperature)
100°C = 212°F (water boils)
Mental math shortcut for °C to °F:
Double the Celsius, then add 30.
25°C → 50 + 30 = 80°F (actual: 77°F, close enough for weather)
Mental math shortcut for °F to °C:
Subtract 30, then halve it.
80°F → (80-30)/2 = 25°C (actual: 26.7°C)The "double and add 30" trick is surprisingly accurate in the range of everyday temperatures (50 to 100°F / 10 to 38°C). Outside that range, it drifts, but for checking weather forecasts it works well enough.
For exact temperature conversion, use the Celsius to Fahrenheit Converter.
Volume and Cooking Measurements
Cooking is where metric and imperial collide most often. International recipes use milliliters and grams, while American recipes use cups, tablespoons, and ounces.
| Imperial | Metric | Notes |
|---|---|---|
| 1 teaspoon (tsp) | 4.93 mL | Round to 5 mL |
| 1 tablespoon (tbsp) | 14.79 mL | Round to 15 mL |
| 1 fluid ounce | 29.57 mL | Round to 30 mL |
| 1 cup | 236.59 mL | Round to 240 mL |
| 1 pint | 473.18 mL | About 475 mL |
| 1 quart | 946.35 mL | Slightly less than 1 liter |
| 1 gallon | 3,785.41 mL | About 3.8 liters |
Important note: UK/Imperial pints and gallons are larger than US versions. A UK pint is 568 mL (vs. 473 mL in the US), and a UK gallon is 4,546 mL (vs. 3,785 mL). This catches people off guard when following British recipes in an American kitchen.
Area and Common Real-World Conversions
Area conversions come up in real estate, gardening, construction, and land measurement.
| Conversion | Factor |
|---|---|
| 1 square foot | 0.0929 m² (about 929 cm²) |
| 1 square meter | 10.764 sq ft |
| 1 acre | 4,047 m² (about 0.4 hectares) |
| 1 hectare | 2.471 acres (about 2.5 acres) |
| 1 square mile | 2.59 km² |
Practical Reference Points
Sometimes the most useful thing isn't a formula but a reference point you can visualize.
- A standard door is about 80 inches (203 cm) tall and 36 inches (91 cm) wide.
- A standard parking space is roughly 8.5 ft by 18 ft (2.6 m by 5.5 m).
- A football field (American) is 100 yards, which is about 91.4 meters.
- A basketball court is 94 ft (28.7 m) long.
- A marathon is 26.2 miles, or 42.195 km.
- Room temperature is about 68-72°F, or 20-22°C.
- A liter of water weighs 1 kg (2.2 lbs). A gallon weighs about 8.3 lbs (3.8 kg).
Quick Conversion Cheat Sheet
For the conversions you need most often, these approximate ratios are easy to remember and accurate enough for everyday use.
Length: 1 inch ≈ 2.5 cm 1 cm ≈ 0.4 inches 1 foot ≈ 30 cm 1 meter ≈ 3 ft 3 in 1 mile ≈ 1.6 km 1 km ≈ 0.6 miles Weight: 1 lb ≈ 0.45 kg 1 kg ≈ 2.2 lbs 1 oz ≈ 28 grams 100 g ≈ 3.5 oz Volume: 1 cup ≈ 240 mL 1 liter ≈ 4.2 cups 1 gallon ≈ 3.8 liters 1 liter ≈ 0.26 gallons Temperature: °C to °F: double it, add 30 °F to °C: subtract 30, halve it
For precise results, use CalcHive's conversion tools: Inches to CM, KG to LBS, Celsius to Fahrenheit, and KM to Miles. Each converter handles bidirectional conversion with full precision.