Metric vs Imperial: A Practical Conversion Guide for Travelers and Cooks
Only three countries have not officially adopted the metric system, yet imperial units remain everywhere — American recipes, aviation, TV screen sizes, and body-weight talk in the gym. Fluency in both systems is a small skill with daily payoffs.
Length: The Benchmarks
One inch is 2.54 cm — memorize this one exactly, since screen sizes and paper formats depend on it. One foot is about 30 cm, one metre is roughly 3.3 feet, and one mile is 1.6 km. For quick km-to-miles conversion, multiply by 0.6: a 10 km run is 6.2 miles.
Useful anchors: a doorway is about 2 m tall, a credit card is 8.5 cm wide, and a marathon is 42.2 km. Anchors let you sanity-check conversions instantly — if a calculation says a person is 25 feet tall, you know a decimal slipped.
Weight: Kilograms and Pounds
One kilogram is 2.2 pounds. To convert kg to lb, double it and add 10%: 70 kg → 140 + 14 = 154 lb. Going the other way, halve the pounds and shave a little: 180 lb is about 82 kg. One ounce is 28 grams — critical for American recipes, where a "stick of butter" is 4 oz or 113 g.
Temperature: The One Non-Linear Conversion
Celsius to Fahrenheit is °F = °C × 9/5 + 32 — the only common conversion with an offset, because the scales cross at −40°. For mental math, double the Celsius and add 30: 20 °C ≈ 70 °F (exact: 68). This approximation is within 2-3 degrees across the everyday weather range.
Key anchors: 0 °C / 32 °F is freezing, 37 °C / 98.6 °F is body temperature, 100 °C / 212 °F is boiling, and a 180 °C oven is 356 °F — which is why recipes say 350.