The metric system
Metric is decimal and coherent: everything scales by powers of ten, and units relate to each other physically (one liter is a cubic decimeter; one kilogram was originally the mass of a liter of water). Length builds from the meter: millimeter (1/1000), centimeter (1/100), kilometer (1000). Weight builds from the gram with the same prefixes. Learn the prefixes once — milli, centi, kilo — and you can convert anything mentally by moving a decimal point.
Adopted during the French Revolution and refined ever since, metric is now the official system of every country except the United States, Liberia, and Myanmar — and the language of all science and most industry worldwide.
The imperial system
Imperial (and its close cousin, US customary) grew organically from body parts and trade: the inch (thumb width), foot, yard, and mile (1,760 yards — blame Roman marching paces via British statute). Weight runs ounce → pound (16 oz) → ton. The conversion factors are arbitrary — 12 inches per foot, 3 feet per yard, 5,280 feet per mile — which is exactly why the system frustrates learners.
One saving grace: since 1959, imperial units are legally defined in metric terms. An inch is exactly 2.54 cm; a pound exactly 0.45359237 kg. Conversions are therefore exact, not approximate.
Where each is used
- United States: imperial dominates daily life (miles, pounds, Fahrenheit), though science, medicine, and the military use metric.
- United Kingdom: famously mixed — miles on road signs, pints in pubs, but metric in shops, schools, and industry.
- Everywhere else: metric throughout, with a few cultural holdovers (screen sizes in inches, aviation altitudes in feet).
- Science, medicine, Olympics: metric universally.
Key conversions to memorize
- Length: 1 in = 2.54 cm · 1 ft = 30.48 cm · 1 yd = 0.9144 m · 1 mi = 1.609 km
- Weight: 1 oz = 28.35 g · 1 lb = 0.454 kg · 1 kg = 2.205 lb
- Everyday anchors: a mile ≈ 1.6 km · a yard ≈ a meter (0.91) · a pound ≈ half a kilo (0.45) · an inch ≈ two-and-a-half cm
Cooking: where the systems collide
Nowhere do metric and imperial clash more confusingly than the kitchen. US recipes use cups, tablespoons, and Fahrenheit; most of the world uses grams, milliliters, and Celsius — and volume-to-weight conversion depends on the ingredient (a cup of flour ≈ 120 g; a cup of sugar ≈ 200 g), so no single factor works.
- 1 cup = 236.6 ml (US) — but Commonwealth "cups" are 250 ml. Check the recipe's origin.
- 1 tbsp = 15 ml; 1 tsp = 5 ml — thankfully consistent.
- Oven: 350°F = 175°C (often rounded to 180°C); 400°F = 200°C; 425°F = 220°C.
- Butter: 1 US stick = 113 g = 8 tbsp — the rare ingredient sold in convertible units.
The professional move: cook by weight. Grams are precise, scalable (doubling 137 g is trivial; doubling "⅔ cup" is not), and immune to how tightly you pack the flour. A $15 kitchen scale eliminates an entire category of conversion error — the rare case where the best converter is hardware. For everything else, our unit converter covers weight, volume-adjacent, and temperature instantly.
For exact figures, use our unit converter — it handles length, weight, temperature, and data with one-tap swap. Temperature deserves its own treatment: see Celsius to Fahrenheit, explained. And if "missing" hard-drive space puzzles you, read kilobytes vs. kibibytes.