The metre was first defined in the 1790s as one ten-millionth of the distance from the North Pole to the equator, measured along the meridian through Paris. Over the following two centuries it was redefined several times, using a platinum bar, then a wavelength of light, and since 1983 the distance light travels in a vacuum in 1/299,792,458 of a second. Imperial lengths were tied to the metre in 1959, when the international yard and pound agreement fixed the inch at exactly 25.4 millimetres. Every foot, yard and mile has had an exact metric value ever since.
Where Britain still uses imperial length
Road signs give distances in miles and yards, and speed limits in miles per hour. People commonly give their height in feet and inches. Televisions and monitors are sold by screen diagonal in inches, and so are many tyres and wheels. Meanwhile building, engineering and almost all retail measurements are metric. Most British adults switch between the two without thinking, and that's exactly where mistakes happen.
Precision and pitfalls
Because the inch is exactly 25.4 mm, conversions between inches and millimetres need no rounding until the final step. The trouble usually comes from rounding too early, for example treating an inch as 2.5 cm, which makes a 100-inch measurement 250 cm instead of 254 cm. Another trap is the mile itself: a statute mile is 1,609.344 metres, while a nautical mile, used at sea and in the air, is 1,852 metres, about 15% longer. Check which one a figure uses before converting.
Worked conversions
A 55-inch television has a screen diagonal of 139.7 cm. Because screens are wider than they are tall, the screen itself is about 122 cm wide and 68 cm tall for a standard 16:9 shape, so check the full width with stand before buying a cabinet. A parkrun covers 5 kilometres, which is about 3.11 miles. A marathon is 42.195 kilometres, or 26.2 miles, a distance fixed for the 1908 London Olympics. And a 2.4-metre length of skirting board is about 7 feet 10½ inches, a common size in DIY shops that is sold in metric but often described in feet.
Heights, sizes and clothing
Clothing and shoe sizes add another layer of conversion that doesn't follow simple maths. UK, EU and US shoe sizes use different scales, so a UK size 8 is roughly an EU 42 and a US men's 9, but the exact equivalents vary by brand. Body measurements are more reliable. A 32-inch waist is about 81 centimetres, and a 34-inch inside leg about 86 centimetres. When buying clothes from overseas sites, compare body measurements in centimetres rather than relying on size labels.
Checking your answer
A metre is just over three feet, about 39.4 inches. A kilometre is about five-eighths of a mile. A typical adult stride is roughly three quarters of a metre. If a converted distance would mean walking to the next town in a few dozen steps, a factor of 1,000 has probably gone missing between metres and kilometres. For heights, remember that six feet is about 1.83 metres; most adults fall between about 1.5 and 1.95 metres.
One more worked example
Fencing a rectangular garden 15 metres long and 9 metres wide needs 48 metres of fencing around the perimeter, about 157 feet 6 inches. UK fence panels are commonly sold 6 feet wide, about 1.83 metres, so the run needs 27 panels, because a part panel still means buying a whole one, plus posts. Converting the perimeter to the unit the product is sold in, before dividing, avoids the classic mistake of ordering panels by metre count and coming up short.