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US Customary to Metric: The Conversions You Actually Need

Quick, practical US-to-metric conversions for the situations that come up most often.

Temperature: °F to °C

A fast mental approximation: subtract 32, then divide by 2 (the exact formula divides by 1.8, but dividing by 2 is close enough for a quick read). So 70°F is roughly (70-32)/2 ≈ 19°C, close to the exact 21°C.

Distance: miles to kilometers

Multiply miles by 1.6 to get kilometers (or roughly 8/5). 60 miles ≈ 96 km. For a rough mental check, kilometers are always a bit more than 1.5x the mile number.

Weight: pounds to kilograms

Divide pounds by 2.2 to get kilograms. A 150 lb person is about 68 kg. A quick shortcut: divide by 2, then subtract about 10% of that result.

Volume: cups and gallons to liters

1 US cup ≈ 237 mL, and 1 US gallon ≈ 3.79 liters. For cooking, treating 1 cup as "about 240 mL" is close enough for almost any recipe.

Height: feet/inches to centimeters

Multiply total inches by 2.54. A person who is 5'8" (68 inches) is about 173 cm. As a shortcut, feet-and-inches height in cm is roughly (feet × 30) + (inches × 2.5).

Why the US still uses customary units

The United States is one of the only countries that has not fully adopted the metric system for everyday use, mostly due to the cost and disruption of switching an entire economy's tools, road signs, and manufacturing standards after customary units became deeply embedded. Science, medicine, and international trade in the US already use metric units.

When precision actually matters

For cooking, casual conversation, or general estimates, the approximations above are fine. For medical dosing, engineering, or anything safety-critical, always use an exact calculator rather than a mental shortcut.

Frequently Asked Questions

Is there a single trick that works for all conversions?

No single trick covers every unit, since the conversion factors are all different (1.6 for distance, 2.2 for weight, and so on). It is more reliable to memorize the handful of common ones you actually use.

Why is a US gallon different from a UK (imperial) gallon?

They are genuinely different units with different historical definitions — a US gallon (≈3.79 L) is smaller than an imperial gallon (≈4.55 L) used in the UK and some other countries. Always check which "gallon" a recipe or spec sheet means.