What is the difference between the metric and imperial systems?

The metric system (SI) is decimal: every unit is a power of ten times a base unit, so converting means moving the decimal point. Imperial units use historical ratios — 12 inches to a foot, 16 ounces to a pound, 3 feet to a yard — so each conversion needs its own factor.

Why do US and UK units with the same name differ?

They were standardised separately. A US gallon is 3.785 L and an imperial gallon 4.546 L; a US ton is 2,000 lb (907 kg) and a UK long ton is 2,240 lb (1,016 kg). Always check which system a figure comes from.

How precise are the conversions shown?

The factors are exact at full precision (1 inch = 25.4 mm, 1 lb = 453.59237 g). Displayed values are rounded, so avoid re-converting a rounded result to get back to the original unit.

Unit Converter

Convert various units including length, weight, and temperature easily.

How Unit Conversion Works

A unit conversion multiplies a value by the ratio between two units. When both units measure the same quantity, that ratio is a fixed number: 1 inch is 2.54 cm, 1 pound is 453.59237 g and 1 litre is 1,000 mL. Temperature is the exception — its scales have different zero points, so converting between °C and °F needs an offset as well as a factor. SI prefixes keep the metric system consistent: kilo- always means 1,000 and milli- always means 1/1,000, whatever the base unit is.

Unit Conversion Guide

Supports conversion between various units including length (cm, m, km, inch, ft), weight (g, kg, lb, oz) and more.

Worked Examples

1 inch = 2.54 cm, so 12 in = 30.48 cm and 1 ft = 0.3048 m.

1 lb = 453.59237 g, so 5 lb = 2.268 kg.

Precision and Units with the Same Name

Rounding is the main source of error: convert once from the original unit instead of chaining rounded results. Watch for units that share a name but not a size — a US gallon is 3.785 L against 4.546 L for the imperial gallon, and a US short ton is 907 kg against 1,016 kg for a UK long ton. Metric prefixes are exact powers of ten; imperial factors are fixed by international agreement and are exact too, but only at full precision.

Frequently Asked Questions

What is the difference between the metric and imperial systems?: The metric system (SI) is decimal: every unit is a power of ten times a base unit, so converting means moving the decimal point. Imperial units use historical ratios — 12 inches to a foot, 16 ounces to a pound, 3 feet to a yard — so each conversion needs its own factor.
Why do US and UK units with the same name differ?: They were standardised separately. A US gallon is 3.785 L and an imperial gallon 4.546 L; a US ton is 2,000 lb (907 kg) and a UK long ton is 2,240 lb (1,016 kg). Always check which system a figure comes from.
How precise are the conversions shown?: The factors are exact at full precision (1 inch = 25.4 mm, 1 lb = 453.59237 g). Displayed values are rounded, so avoid re-converting a rounded result to get back to the original unit.