A quick guide to
unit conversions

The multiply-by-factor method

Metric units such as metres, kilograms and litres are used for everyday measurement in India and most of the world. US customary units such as inches, feet, miles and pounds still turn up in product sizes, travel, sport and fitness apps. Moving between the two is easy once you know the method.

Almost every unit conversion works the same way: multiply the value by the number of target units in one source unit. To convert miles to kilometres, multiply by 1.609344, the kilometres in one mile.

1. 1Find the factor: how many of the target unit make up one of the source unit.
2. 2Multiply your value by it.
3. 3Sanity-check the size. Kilometres are shorter than miles, so the number of kilometres should be bigger.
4. 4Round to the precision you need.

When there is no direct factor, chain two conversions. Miles to metres is 1.609344 × 1,000 = 1,609.344 metres per mile, because a kilometre is 1,000 metres.

Common conversion factors

The exact factors are definitions: the inch, foot, mile and pound are defined in metric terms. The reverse factors are rounded reciprocals, so it is usually better to divide by the exact factor than to multiply by a rounded one.

Area and volume need the factor squared or cubed. Since 1 ft = 0.3048 m, one square foot is 0.3048² = 0.09290304 square metres, not 0.3048.

For quick estimates, these approximations are handy: 1 inch ≈ 2.5 cm, 1 mile ≈ 1.6 km, 1 kg ≈ 2.2 lb and 1 metre ≈ 3.3 feet. Use the full factors whenever accuracy matters.

Everyday examples

Here is the method at work on typical sizes:

• Height: 5 ft 9 in is 69 in, and 69 × 2.54 = 175.26 cm, or 1.7526 m.
• Tall height: 6 ft × 30.48 cm per foot = 182.88 cm.
• Room length: 12 ft × 0.3048 = 3.6576 m, about 3.66 m.
• Body weight: 70 kg × 2.20462 = 154.3234 lb, about 154.32 lb.
• Race distance: a 42.195 km marathon ÷ 1.609344 = 26.22 miles.
• Speed: 60 km/h ÷ 1.609344 = 37.28 miles per hour.

Temperature is different

Celsius and Fahrenheit have different zero points as well as different degree sizes, so a single factor will not do. Use these formulas instead:

Two more readings: 25 °C is 77 °F, and 72 °F is 22.22 °C. A difference in temperature converts differently from a reading, because the +32 cancels out. A rise of 10 °C is a rise of 18 °F, found with × 9 ÷ 5 alone.

Converting back, and rounding

To reverse a conversion, divide by the same factor. 5 miles is 5 × 1.609344 = 8.04672 km, and 8.04672 ÷ 1.609344 gives 5 miles again.

Rounding in the middle of a calculation is where errors creep in. Keep full precision through the steps and round once at the end, to the places that suit the job: whole kilograms for body weight, two decimals for a recipe. For example, 75 kg × 2.20462 = 165.3465 lb, which you would write as 165.35 lb.

Rounding too early shows up on the way back: if 8.04672 km is rounded to 8.05 km first, converting back gives 5.002 miles, not 5.

If you convert often, a calculator saves effort and avoids slips. It applies the exact factor and shows as many decimals as you ask for, which helps with anything that matters, such as a form, a parcel weight limit or a fitting measurement.