Metric vs. imperial: a practical conversion cheat sheet

Most people don't need a comprehensive metric-imperial reference. They need the same five or six conversions, over and over: distance on a road sign, a recipe in the wrong system, a body weight, a room measurement. Here's the short list, with rough mental tricks for the ones worth memorizing.

The conversions people actually look up

Distance:   1 mile = 1.609 km        1 km = 0.621 miles
Weight:     1 lb = 0.454 kg          1 kg = 2.205 lb
Length:     1 inch = 2.54 cm         1 cm = 0.394 inches
Volume:     1 US gallon = 3.785 L    1 L = 0.264 US gallons

Temperature belongs on this list too (°F to °C comes up constantly, in weather and cooking especially), but it doesn't reduce to a clean multiplier the way these do; it needs a separate formula. See why converting Celsius to Fahrenheit isn't just multiply-and-add for the details.

Kilometers to miles: multiply by 0.62

This is the one worth having automatic, since road signs and speed limits outside the US use km. The exact factor is 0.621371, but 0.62 is close enough for a mental estimate, and for a rougher version, 5/8 (0.625) is easy to compute in your head: 80 km × 5/8 = 50 miles, which happens to be exact. Going the other way, miles to km, multiply by 1.6.

There's a neat coincidence that makes this even easier: 1 mile is very close to 1.609 km, and the golden ratio is about 1.618. Because of that, consecutive Fibonacci numbers approximate the conversion well: 5 miles ≈ 8 km, 8 miles ≈ 13 km, 13 miles ≈ 21 km. It's not exact, but it's accurate to within a couple percent across the range most people actually need, and it requires no arithmetic at all if you've got the sequence memorized.

Kilograms to pounds: multiply by 2.2

2.2 is close enough for almost any everyday use (the exact figure is 2.20462). Going from pounds to kilograms, the easiest mental move is to halve the pound figure and then subtract about 10%: 150 lb → half is 75, minus 10% (7.5) → about 68 kg, which matches the precise conversion (68.04 kg) closely.

Inches and centimeters: 2.5, then correct

1 inch = 2.54 cm almost exactly. For a fast estimate, multiply inches by 2.5 and nudge up slightly for larger numbers, since the true factor is a bit more than 2.5. A 12-inch ruler is 30.48 cm; the quick estimate (12 × 2.5 = 30) undershoots by less than 2%, which is fine for most non-precision uses.

Why the US still primarily uses imperial units

The US officially recognized the metric system as early as 1866 and made it the preferred system for trade in the Metric Conversion Act of 1975, but that act was voluntary, with no deadline and no mandate. By the time it passed, road signage, tools, construction standards, school curricula, and consumer expectations were already built around imperial units, and retooling all of that (manufacturing equipment, building codes, generations of habit) carries a real cost with no clear forcing function to justify it. Science, medicine, the military, and much of US manufacturing that trades internationally already operate in metric; it's primarily everyday consumer contexts, road distances, recipes, body weight, that stayed imperial, largely because switching those over would require coordinated change across millions of individual habits and systems at once, not just a government mandate.

The short version

Km to miles: ×0.62 (or ×5/8, or ride the Fibonacci sequence). Miles to km: ×1.6. Kg to lb: ×2.2. Lb to kg: halve, then subtract 10%. Inches to cm: ×2.5, then nudge up slightly. None of these mental shortcuts are exact, but they're close enough for the situations where you're converting in your head instead of reaching for a calculator, and this site's converters are there for whenever precision actually matters.