Why metric and imperial units coexist

How the two systems differ

In the metric system every unit connects by powers of ten, so conversion is a matter of moving the decimal point. Imperial multipliers vary: twelve inches to a foot, three feet to a yard, sixteen ounces to a pound, all of which must be memorised. In exchange, common everyday sizes land on numbers that divide conveniently. Metric base units are organised as the International System of Units (SI), built on seven quantities: length, mass, time, electric current, temperature, amount of substance and luminous intensity. Conversely, a common explanation for twelve inches to the foot is that twelve divides evenly by 2, 3, 4 and 6.

What remains in metric countries

Most countries use the metric system, yet imperial units persist in specific fields because industry standards settled that way. Since an international agreement in 1959 the inch has been defined as exactly 2.54 cm, so values in these fields convert to metric exactly.

• Screen size: diagonal inches
• Tyres: wheel diameter in inches, width in millimetres
• Aviation altitude: feet
• Plumbing and thread standards: largely inch-based

Same name, different value

A gallon differs in value by region, and so does an ounce. Converting a figure from foreign material by unit name alone therefore goes wrong. Confirming which region's definition applies matters more than memorising the formula. The US gallon is about 3.785 litres and the imperial gallon about 4.546 litres, a gap of nearly 20 percent. Fluid ounces differ too: about 29.57 mL in the US and about 28.41 mL in the UK.

Separating weight from volume

An ounce measures weight in one usage and volume in another. The shared name causes confusion, but they measure different things and their relationship depends on the substance. Confusing the two in a recipe changes the result considerably. A weight ounce is about 28.35 g, and one US fluid ounce of water weighs about 29.6 g, so for water the two look similar, but for oil or flour they drift far apart.

Traditional Korean units

Units such as pyeong, geun, don and gwan remain in use. Property area is officially in square metres but often spoken in pyeong, gold trades in don and meat in geun. These have defined values and can be converted, though conventions vary by region and by item. For reference, one pyeong is about 3.3058 square metres, one don is 3.75 g and one gwan is 3.75 kg, while a geun of meat is usually counted as 600 g. For vegetables and fruit a geun is sometimes counted as 400 g, depending on the region.

When unit mix-ups caused real accidents

Serious accidents caused by mixed units have been recorded more than once. In 1999 NASA's Mars Climate Orbiter was lost because one team worked in imperial-based units and another in metric, and the values were passed across unchanged. In 1983 a Canadian airliner ran out of fuel in flight after fuel quantities were confused between kilograms and pounds. In both cases the arithmetic was correct; the step of confirming which unit was in use was missing.

Checklist for converting foreign material

When converting material that uses other units, such as overseas recipes, product specifications or travel information, checking in the order below sharply reduces mistakes.

• First confirm which country's convention the source follows (US, UK or elsewhere)
• Separate weight units from volume units
• Round the result to match the precision of the original
• Always write the unit next to the number, and never leave a bare figure

A feel for rough conversion

When precision is not needed, approximate ratios are enough. Remember an inch as about 2.5 cm, roughly one finger joint, a foot as about 30 cm, a mile as 1.6 km, a pound as 0.45 kg and a US gallon as about 3.8 litres. Tying them to familiar examples helps them stick: six feet tall is about 183 cm, and the marathon's 26.2 miles is about 42 km.