Almost every country outside the US, Liberia, and Myanmar uses the metric system — which means anyone traveling, following a recipe from a different country, reading a fitness plan, or measuring for a DIY project regularly runs into the same handful of conversions. You do not need to memorise conversion factors for units you'll use once a year. You need the ones that come up constantly, and a reliable way to check the rest.
Length — travel, DIY, and everyday distances
| Metric | Imperial | Exact factor |
|---|---|---|
| 1 kilometer | 0.621371 miles | km → miles |
| 1 meter | 3.28084 feet | m → ft |
| 1 centimeter | 0.393701 inches | cm → in |
| 30.48 centimeters | 1 foot | ft → m |
Quick mental shortcut: km × 0.62 ≈ miles (a 10K run is about 6.2 miles), and cm ÷ 2.5 ≈ inches (a 180cm person is roughly 71 inches, or 5'11").
Weight — fitness, groceries, and luggage
| Metric | Imperial | Link |
|---|---|---|
| 1 kilogram | 2.20462 pounds | kg → lbs |
| 1 gram | 0.035274 ounces | g → oz |
| 1 kilogram | 0.157473 stone | kg → stone |
Airlines almost always publish baggage limits in kilograms outside the US — a 23kg checked bag allowance is about 50.7 lbs. Gym plates and protein servings follow the same pattern: a "20kg" barbell plate pair is roughly 44 lbs.
Temperature — weather, cooking, and health
| Celsius | Fahrenheit | Context |
|---|---|---|
| 0°C | 32°F | Water freezes |
| 21°C | 70°F | Room temperature |
| 37°C | 98.6°F | Body temperature |
| 100°C | 212°F | Water boils |
| 180°C | 356°F | Common oven bake temp |
The exact formula is °F = (°C × 9/5) + 32 — see the full Celsius to Fahrenheit converter for any value, including oven temperatures for recipes written in either system.
Volume — cooking and recipes
| Metric | US Customary | Link |
|---|---|---|
| 1 liter | 0.264172 gallons | L → gal |
| 250 milliliters | ≈ 1.06 cups | mL → cups |
| 1 tablespoon | 3 teaspoons | tbsp → tsp |
| 29.5735 milliliters | 1 fluid ounce | fl oz → mL |
Recipe conversion is where small errors compound fastest — baking especially is sensitive to ratios. When a recipe lists "1 cup" (US) it means 236.6 mL, not the 250 mL "metric cup" used in Australia — close enough for most cooking, but worth knowing if you bake often.
Worked example: converting a recipe
A UK recipe calls for 180°C oven, 500g flour, 250mL milk. Converted for a US kitchen: 356°F oven (round to 350°F), 17.6 oz flour (about 4 cups, since 1 cup flour ≈ 120g), and ≈ 1 cup milk. Small rounding is fine in cooking — it rarely is in engineering, medicine dosages, or construction, where you should always use exact values from the converter rather than mental shortcuts.
Frequently asked questions
Why does the US still use imperial units? Historical and economic inertia — switching an entire country's infrastructure, road signs, and manufacturing tooling is expensive. The US officially recognises metric for science and much of manufacturing, but everyday imperial use persists.
Are UK and US imperial units identical? Mostly, but not always — a UK (imperial) gallon is about 20% larger than a US gallon, and UK/US fluid ounces differ slightly too. Always check which system a recipe or spec is using.
What's the fastest way to convert something not on this chart? The unit converter covers 14 categories (length, weight, temperature, area, volume, speed, time, data, energy, pressure, power, angle, frequency, fuel economy) with over 100 units, live conversion tables, and the exact formula for every pair.
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