Convert length (cm ↔ inches), weight (kg ↔ lbs), temperature (°C ↔ °F), and area (㎡ ↔ pyeong) — type one value and see every unit at once.
Handy anchors: 1 inch = 2.54cm, 1 lb ≈ 453.6g, 1 pyeong (Korean area unit) ≈ 3.3058㎡, °F = °C × 9/5 + 32.
The metric system (SI) is the international standard and essentially every country uses it. The exceptions are the United States, Liberia and Myanmar.
Even the US uses metric in science, medicine and the military. Imperial survives in everyday life — height, weight, distance, cooking.
The UK is mixed: miles for distance, pints for beer, stones for body weight, metric for most else.
Which is why reading American and British material always requires checking the unit.
The Mars Climate Orbiter (1999) was lost because Lockheed Martin supplied thrust data in pound-force seconds while NASA's software expected newton-seconds. A $125 million spacecraft burned up in the Martian atmosphere.
The Gimli Glider (1983) was an Air Canada 767 fuelled using pounds where kilograms were required, loading roughly half what was needed. Both engines stopped at 12,000m and the aircraft glided to a decommissioned airbase.
Medication errors work the same way: recording a weight in pounds and reading it as kilograms multiplies a dose by 2.2. Paediatrics is where this is most dangerous.
The pattern is always the same: the number is carried across and the unit is not checked.
Length and mass share a zero point, so conversion is multiplication. Temperature scales have different zeros.
°C → °F = °C × 9/5 + 32. The offset is the whole difficulty.
Which means "twice as hot" is not a meaningful statement. Twice 10°C is 20°C, but in Fahrenheit that is 50°F and 68°F — nowhere near double.
Kelvin starts at absolute zero, so ratios are genuine there. K = °C + 273.15.
For mental arithmetic, °C × 2 + 30 approximates Fahrenheit well enough for daily use: 20°C → 70°F (actually 68°F).
The weight ounce (28.35g) and the fluid ounce (29.57ml) are unrelated measures that share a name. This is the pair people mix up most.
A US gallon is 3.785L, an imperial gallon 4.546L. A 20% gap that badly distorts fuel economy comparisons.
Ton splits three ways: metric tonne 1,000kg, US short ton 907kg, imperial long ton 1,016kg.
The statute mile is 1.609km, but the nautical mile is 1.852km. Aviation and shipping use the latter.
The dietary "Calorie" is actually a kilocalorie, a thousand times the physics unit.
Both are in use, which is the whole problem. SI notation makes 1KB = 1,000 bytes; binary notation makes 1KiB = 1,024 bytes.
Storage manufacturers use 1,000. So a "1TB" drive shows as 931GB in Windows, which calculates in 1,024s. No capacity is missing — the two are counting differently.
Network speeds are in bits. "100Mbps" is 100 mega*bits* per second, which is 12.5MB/s. That factor of eight is behind a lot of "my internet is slower than advertised".
Conversion does not create precision. "About 5km" converts to 3.10685596 miles, but if the input was approximate then "about 3.1 miles" is the honest output.
Long decimal tails imply an accuracy that isn't there — worth watching in reports and papers.
Everything is computed in the browser and nothing is transmitted.
Exactly 2.54cm. So 6 feet is about 182.9cm.
A Korean area unit used for apartments and land: 1 pyeong ≈ 3.3058㎡, so an 84㎡ apartment is about 25.4 pyeong.
Subtract 32, then multiply by 5/9. 100°F is about 37.8°C.
What this tool bases its numbers on, and how far those numbers go.
Length and mass use the international definitions: 1 inch = 2.54 cm exactly and 1 pound = 453.59237 g exactly, both fixed by the 1959 international yard and pound agreement. Temperature uses °F = °C × 9/5 + 32. Area includes the Korean pyeong at 1 pyeong = 400/121 ≈ 3.3058 m².10 inches is 10 × 2.54 = 25.4 cm. 20°C is 20 × 9/5 + 32 = 68°F.