Defined values, and the ones that only look defined
The unit converter updates every unit in a family from a single number — length, weight, area, volume, temperature, tyre pressure and torque.
One inch is exactly 2.54 cm. That is not a convenient rounding: the 1959 international yard and pound agreement, signed by the US, UK, Canada, Australia, New Zealand and South Africa, defined the inch as 25.4 mm and fixed the pound at 0.45359237 kg. Because those are definitions, inches, feet, yards, miles, pounds and ounces mean exactly the same thing wherever you are, and extra decimal places never add error.
Volume never got the same treatment
A US gallon is 3.785 L and an imperial gallon is 4.546 L — more than 20% apart. The US gallon descends from the old English wine gallon; the imperial gallon was redefined in 1824 as the volume of ten pounds of water. Two ancestors, two sizes. It bites hardest on fuel economy, where a car rated at 30 mpg does about 12.75 km/L on the US gallon but only 10.6 km/L on the imperial one. Fluid ounces differ too, at 29.57 mL against 28.41 mL.
The numbers people actually come looking for
A stone is 14 pounds, about 6.35 kg, and British scales still speak in stone and pounds. Screen sizes are diagonals, so a 55-inch set is 139.7 cm corner to corner and about 121.8 cm wide by 68.5 cm tall on a 16:9 panel — estimating width from the diagonal leaves you roughly 20 cm short before bezels and the stand are added. Tyre pressure appears as psi in the US and kPa nearly everywhere else, so 32 and 220 are the same pressure. Torque specs read as lbf·ft in the US, N·m in Europe and kgf·m in Korea and Japan, at 1 lbf·ft = 1.3558 N·m. And one quirk worth keeping: −40 °C and −40 °F are the same temperature.
Clothing and shoes: measure first, convert second
The shoe and clothing size converter works from foot length because only one system says out loud what it is describing. Japanese shoe sizes are the foot length in centimetres — a shoe marked 26.0 is built for a 26.0 cm foot. US, UK and EU numbers are conventions layered on top, which is why they never line up neatly. US men's is roughly the centimetre length minus 18, US women's runs about 1.5 higher than men's for the same foot, and EU is roughly the length plus 15.
That men's-to-women's gap catches people out constantly: a women's US 9.5 and a men's US 8 are close to the same physical shoe, which matters when the style you want is only stocked in the other department.
Then there is the part no conversion table can fix. Internal length varies 2 to 5 mm between brands at the same marked size — more than a half size in some ranges. So "I'm a US 9" carries less information than it sounds like. Measure your foot in the evening, when feet are at their largest, use the longer foot, and match the millimetres against the brand's own size guide.
Clothing has no equivalent anchor, so its conversions are looser still — except in Japan, where 号 (gō) is tied to a measurement. JIS L 4005 sets 7号 at an 80 cm bust and adds about 3 cm every two sizes, making 9号 an 83 cm bust. If a listing gives you a bust measurement, that number is worth more than the size label.
Ring sizes: three systems measuring the same millimetres
The ring size converter works from finger circumference or a ring's inner diameter. Each national system is just a different way of writing the same physical measurement.
The EU system (ISO 8653) is the most transparent: the size number is the inner circumference in millimetres, so EU 52 means 52 mm around. The US system is diameter-based, following diameter (mm) = 11.63 + 0.8128 × size. Japan and Korea share one scale in the JIS S 4700 lineage, where size 1 is a 13.0 mm inner diameter and each size adds exactly 1/3 mm.
The steps are different sizes — about 1.28 mm of circumference per US half size against about 1.05 mm per Japanese or Korean size — so the systems never align perfectly, and two adjacent Asian sizes sometimes round to the same US half size. Measure with a real ruler rather than an on-screen sizer, because screen scaling varies between devices by enough to throw the result a full size. Fingers are widest at the end of the day and in warm weather, so measure then, and when you are between sizes, take the larger one.
Percentages: the base is what people get wrong
The percentage calculator covers percent of a number, one number as a percent of another, change between two values, and discounts in both directions.
The trap is that change is always measured against the old value. Going from $1,200 to $1,500 is a 25% increase; going back from $1,500 to $1,200 is a 20% decrease. Same $300, different base, different answer. The same asymmetry is why a 20% discount followed by a further 20% is not 40% off — it is 36%, because the second cut applies to the already-reduced price.