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ConversionThe world runs on two measurement systems: metric (meters, kilograms, liters) used almost everywhere, and imperial/US customary (feet, pounds, gallons) used in the United States, Liberia, and Myanmar. If you cook, travel, shop internationally, or read foreign specs, you bump into both constantly.
This guide collects the exact conversion factors for length, weight, volume, and temperature — plus the mental shortcuts people actually use day to day.
Try it yourself: Convert any length, weight, or volume instantly. Length Converter →
Length: inches, feet, miles
| Imperial | Metric | Exact factor |
|---|---|---|
| 1 inch | 2.54 cm | exact by definition |
| 1 foot | 30.48 cm | 12 × 2.54 |
| 1 yard | 0.9144 m | 36 inches |
| 1 mile | 1.60934 km | 5,280 feet |
Mental trick: a mile is about 1.6 km — so 60 mph ≈ 100 km/h. A foot is roughly 30 cm, an inch about 2.5 cm.
Weight: ounces and pounds
| Imperial | Metric | Exact factor |
|---|---|---|
| 1 ounce | 28.3495 g | avoirdupois ounce |
| 1 pound | 0.45359237 kg | 16 ounces |
| 1 stone | 6.35029 kg | 14 pounds (UK) |
| 1 ton (US) | 907.185 kg | 2,000 pounds |
Mental trick: a kilogram is about 2.2 pounds. To convert pounds to kilos in your head, halve the number and subtract 10% (200 lb → 100 − 10 = ~90 kg; exact is 90.7).
Volume: cups, pints, gallons
| Imperial | Metric | Note |
|---|---|---|
| 1 fluid ounce | 29.5735 ml | US |
| 1 cup | 236.588 ml | 8 fl oz |
| 1 pint | 473.176 ml | US — UK pint is 568 ml |
| 1 gallon | 3.78541 L | US — UK gallon is 4.546 L |
Watch out: the UK and US pints and gallons are different sizes. A UK pint (568 ml) is about 20% bigger than a US pint (473 ml) — this matters in recipes and fuel economy alike.
Temperature
°F = °C × 9/5 + 32 · °C = (°F − 32) × 5/9
Mental trick: double the Celsius and add 30 for a rough Fahrenheit (20°C → 70°F; exact is 68°F). Good enough for weather, not for ovens.
Why does the split persist?
The US actually legalized metric in 1866 and tried to convert in the 1970s, but the effort was voluntary and stalled — retooling factories, road signs, and everyday habits costs billions. Meanwhile, US science, medicine, the military, and most manufacturing run on metric behind the scenes. The result: Americans live in both systems, often without noticing.
Cooking: where the systems collide daily
Recipes are the most common conversion battlefield. Volume conversions are exact (1 cup = 236.6 ml), but weight conversions depend on the ingredient's density — a cup of flour weighs ~120–125 g while a cup of sugar weighs ~200 g. That's why serious bakers prefer scales and grams: weight is precise, volume is approximate. Key kitchen anchors: 1 tablespoon = 15 ml, 1 teaspoon = 5 ml, 1 stick of butter = 113 g (4 oz).
Area: square feet to square meters
| Imperial | Metric |
|---|---|
| 1 sq ft | 0.0929 m² |
| 1 sq yard | 0.8361 m² |
| 1 acre | 0.4047 hectares |
| 1 sq mile | 2.590 km² |
Mental trick: divide square feet by 10 for a rough m² (1,000 sq ft ≈ 93 m², and ÷10 gives 100 — close enough for listings). An acre is just under half a hectare — about 4,047 m², or roughly 70% of a soccer field.
Speed and distance on the road
Speed limits translate cleanly: 30 mph ≈ 50 km/h, 60 mph ≈ 100 km/h, 70 mph ≈ 113 km/h. For fuel and trip planning, remember that distances in kilometers look ~60% bigger than the same trip in miles — a "300 km drive" is under 190 miles, not the marathon it sounds like to mile-brained drivers.
Frequently asked questions
Path dependence and cost: switching every road sign, factory, and textbook is enormously expensive, and past voluntary metrication efforts stalled. US science, medicine, and the military already use metric — everyday life just never followed.
Exactly 1.609344 km. For mental math, 1.6 is close enough — 60 mph is roughly 100 km/h.
No. A UK pint is 568 ml while a US pint is 473 ml — the British one is about 20% larger. The same goes for gallons: 4.546 L (UK) vs 3.785 L (US).
68°F — comfortable room temperature. The quick mental trick is to double and add 30 (20 × 2 + 30 = 70), which lands within a couple of degrees.
Disclaimer: Conversion factors shown are exact definitions. Rounded mental-math tricks are approximations — use a converter for engineering, medical, or commercial measurements.