Unit Conversion Made Simple

The world runs on two measurement systems, and life keeps making you translate between them. Here are the conversions that actually come up — with tricks to remember them.

Length: the travel conversions

1 mile = 1.609 km and 1 km = 0.621 miles. Quick trick: km ≈ miles × 1.6, or use the "half plus a bit" rule the other way — 100 km is about 62 miles. For height forms: 1 foot = 30.48 cm, 1 inch = 2.54 cm, so 5 ft 7 in = 152.4 + 17.8 = 170.2 cm. Screen sizes are measured diagonally in inches everywhere — a "15.6-inch" laptop is a 39.6 cm diagonal.

Weight: luggage, gym and gold

1 kg = 2.205 lb — so a 23 kg airline allowance is about 50 lb, and a "45 lb" gym plate is 20.4 kg. Cooking swaps: 1 ounce = 28.35 g. And the one South Asia relies on for gold: 1 tola = 11.664 g, so a 5-tola bangle set is 58.32 g — plug that straight into our Zakat Calculator alongside today's gold rate.

Temperature: the odd one out

Temperature scales don't share a zero, so you can't just multiply. Exact: °F = °C × 9⁄5 + 32 (and back: °C = (°F − 32) × 5⁄9). Quick mental estimate: double the Celsius and add 30 — 25 °C ≈ 80 °F (exact 77). Handy anchors: 0 °C = 32 °F (freezing), 37 °C = 98.6 °F (body temperature), 100 °C = 212 °F (boiling), and −40° is the same in both scales.

Why conversions go wrong

Most conversion errors come from rounding too early or converting twice. Convert once, at the end of a calculation, using the full factor. The stakes can be spectacular: NASA lost the 125-million-dollar Mars Climate Orbiter in 1999 because one team worked in pound-force units and another in newtons. Your luggage fee is a smaller tragedy, but the lesson is the same — let a calculator hold the precise factors.

Frequently asked questions