Unit conversion basics

By Haris Asif · Published October 4, 2026

Converting units is multiplying by a ratio that equals one. Once you see it that way, any conversion becomes a short, checkable calculation.

Think in ratios

A conversion factor is a fraction whose top and bottom describe the same amount in different units. Because 1 inch and 2.54 centimetres are the same length, 2.54 cm ÷ 1 in equals one. Multiplying by it changes the units without changing the amount.

Example: convert 5.5 feet to centimetres. 1 ft is 30.48 cm, so 5.5 × 30.48 = 167.64 cm. Writing the units next to the numbers (ft × cm/ft) lets the feet cancel and leaves centimetres.

Exact and approximate factors

The international yard and pound agreement of 1959 defined the imperial units exactly in terms of metric ones:

So 2.54 cm per inch is exact, not rounded. Factors such as “1 kg ≈ 2.2 lb” are rounded versions. For anything where precision matters, use the exact factor and round at the end.

Converting through a base unit

When you convert between two units that are not directly related, go through a base unit. SizeKit’s converters turn your value into metres (or kilograms), then into the target unit. For miles to kilometres: miles × 1,609.344 gives metres, and dividing by 1,000 gives kilometres.

Common mistakes

• Dividing when you should multiply. Check direction with a sanity test. There are more centimetres than inches in any length, so inches to centimetres must make the number bigger.
• Mixing mass and volume. Ounces for weight and fluid ounces for volume are different units. So are US and UK gallons.
• Rounding too early. Round only the final answer, especially in multi-step conversions.
• Forgetting that squares and cubes scale differently. 1 m is 100 cm, but 1 m² is 10,000 cm², not 100 cm².
• Treating feet and inches as decimals. 5 ft 6 in is 5.5 ft, not 5.6 ft. Convert the inches by dividing by 12.

Quick mental estimates

• 1 inch is about 2.5 cm.
• 1 kilometre is about 0.6 miles; 1 mile is about 1.6 km.
• 1 kilogram is a little over 2.2 pounds.
• A foot is about 30 cm.

These are good for a sense of scale. Use the converters for exact figures.