Everyday Unit Conversion: The Numbers You Actually Need

Most of the world uses metric. The US (mostly), UK (partly) and a handful of other countries don't. If you travel, cook, do fitness, work in engineering, or just read recipes from anywhere on the internet — you bump into unit conversions weekly. This is the practical cheat sheet for the conversions you'll actually use, plus mental shortcuts that work without a phone.

The everyday conversions

Length (travel and DIY)

• 1 inch = 2.54 cm. Multiply by 2.5 for rough estimates.
• 1 foot ≈ 30 cm. Easy.
• 1 mile ≈ 1.6 km. Or 5/8 = 0.625, so multiply km by 0.6 and add 10% for miles.
• 1 km = 0.62 mile. Or just slightly more than half — 60% rule.
• 1 yard ≈ 0.91 m. Roughly the same as a meter.

Mental shortcut: "km is 0.6 miles, miles is 1.6 km." Halving distance + adding 10% gets you within 4% — close enough for everything except engineering.

Weight (luggage, groceries, fitness)

• 1 kg = 2.2 lb. Exact for most purposes is 2.20462.
• 1 lb = 454 g. Roughly half a kilo.
• 1 oz = 28 g. Useful in cooking.
• 1 stone = 6.35 kg. UK uses stones for body weight.

Mental shortcut: "kg × 2, then add 10% = lb." 70 kg = 140 + 14 = 154 lb. Actual: 154.3 lb. Close enough.

Temperature

Temperature has the trickiest conversion because the zero points differ:

• F → C: (F - 32) × 5/9. Or: subtract 30, halve. Loses accuracy at extremes but fine for weather.
• C → F: C × 9/5 + 32. Or: double, add 30. Same caveat.

Useful anchor points:

• 0°C = 32°F (water freezes)
• 10°C = 50°F (cool)
• 20°C = 68°F (room temperature)
• 30°C = 86°F (hot)
• 37°C = 98.6°F (body temperature)
• 100°C = 212°F (water boils)

Memorise the table — for daily use you rarely need to compute precisely.

Volume (cooking, drinks, fuel)

• 1 cup (US) = 240 ml. UK cup is 250 ml — close enough.
• 1 tablespoon = 15 ml.
• 1 teaspoon = 5 ml.
• 1 US gallon = 3.79 liter.
• 1 UK gallon = 4.55 liter. ~20% larger than US.
• 1 fluid ounce = 30 ml. Close enough — actual is 29.57 ml US.

Watch out: a US cup, an Australian cup and a UK cup are slightly different. For baking precision, weigh ingredients in grams instead of measuring by volume.

Speed (driving abroad)

• 50 mph = 80 km/h.
• 60 mph = 100 km/h. Almost exactly. Useful anchor.
• 70 mph = 113 km/h.
• 100 km/h = 62 mph.

Mental shortcut for mph → km/h: multiply by 1.6 (or by 8, then divide by 5).

Fuel economy (the confusing one)

The US uses miles per gallon (mpg). Most of the world uses liters per 100 km (l/100km). The relationship is inverse — higher mpg = lower l/100km.

• 30 mpg (US) ≈ 7.8 l/100km. Average sedan.
• 40 mpg (US) ≈ 5.9 l/100km. Efficient.
• 50 mpg (US) ≈ 4.7 l/100km. Hybrid.

To convert: 235.2 / mpg = l/100km. Or 235.2 / l/100km = mpg.

Watch out: UK gallon is bigger than US gallon, so a UK car at "40 mpg" is more efficient than a US car at "40 mpg".

Area (real estate)

• 1 sq foot = 0.093 sq meter. Roughly 10 sq ft per sq m.
• 1 sq meter = 10.76 sq feet.
• 1 acre = 4047 sq meter. Roughly the size of a football pitch (4828 sq m).
• 1 hectare = 10,000 sq meter = 2.47 acres.

Cooking conversions worth memorising

• 1 oz ≈ 30 g (or 28 g for precision baking)
• 1 cup = 240 ml = 8 fl oz
• 1 tablespoon = 15 ml = 3 teaspoons
• Oven 350°F ≈ 180°C (most baking)
• Oven 400°F ≈ 200°C (roasting)
• Oven 450°F ≈ 230°C (high heat)

Data sizes (downloads, storage)

The annoying thing: "kilobyte" means two different things depending on context.

• 1 KB = 1024 bytes (binary, used by RAM, file sizes in some OSes).
• 1 kB = 1000 bytes (decimal, used by hard drive marketing, network speeds).

The difference compounds: a "1 TB" hard drive is 10^12 bytes = 909 binary GB. This is why a new 1 TB drive shows up as 931 GB in your OS — same number of bytes, different naming convention.

Pressure (tires, weather)

• 1 bar ≈ 14.5 psi.
• 30 psi ≈ 2.07 bar. Typical car tire.
• 1013 millibar = 1 atm (standard atmospheric pressure).
• 1 inHg = 33.86 millibar. US weather reports use inches of mercury.

The 5-second sanity check

When you do a conversion in your head, sanity check the order of magnitude:

• 1 km is about half a mile (closer to 0.6).
• 1 kg is about 2 lbs (closer to 2.2).
• 1 liter is about 1 quart (close enough).
• 1 m is about 1 yard (close enough).

Get the order of magnitude right and small errors don't matter for everyday use. For precise needs (engineering, navigation, medicine) — use a calculator or the tool below.