Why inches and centimetres line up exactly
One inch is exactly 2.54 cm. It 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. The same agreement fixed the pound at 0.45359237 kg. That is why inches, feet, yards, miles, pounds and ounces mean the same thing wherever you are.
Worth memorising: 1 foot = 30.48 cm, 1 yard = 91.44 cm, 1 mile = 1.609 km, 1 pound = 453.6 g, 1 ounce = 28.35 g. And back the other way: 1 cm ≈ 0.394 in, 1 m ≈ 3.28 ft, 1 km ≈ 0.621 mi, 1 kg ≈ 2.205 lb.
The gallon is not one thing
Length and weight are settled. Volume is not. A US gallon is 3.785 L, an imperial gallon is 4.546 L — more than 20% apart. The US gallon comes from the old English wine gallon, while the imperial gallon was redefined in 1824 as the volume of ten pounds of water. Two different ancestors, two different sizes.
This bites hardest on fuel economy. 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: 29.57 mL in the US, 28.41 mL in the UK.
Getting a feel for °C and °F
The exact formula is °F = °C × 9/5 + 32, but for a rough read, double it and add 30. At 20 °C that gives 70 °F against an actual 68 °F — close enough to know how to dress.
A few anchors help more than the formula. Freezing is 0 °C / 32 °F. Room temperature 20 °C / 68 °F. Body heat 37 °C / 98.6 °F. Boiling 100 °C / 212 °F. A hot summer day at 90 °F is 32 °C, and 100 °F is 38 °C. One quirk worth knowing: −40 °C and −40 °F are the same temperature.
Stone, and why UK weights read oddly
A stone is 14 pounds, about 6.35 kg. In Britain and Ireland body weight is normally spoken as stone plus pounds — "twelve stone four" rather than 172 pounds or 78 kg. Converting to a single decimal loses the form people actually use, so this page gives you both.
The unit survives in everyday speech even though official contexts went metric decades ago, which is why a British scale often shows all three at once.
TV inches are the diagonal
Screen sizes measure corner to corner. A 55 inch set has a 139.7 cm diagonal, and on a 16:9 panel that comes out around 121.8 cm wide by 68.5 cm tall. Estimating the width from the diagonal leaves you roughly 20 cm short.
Bezels and the stand foot add further width, so before buying a wall mount or a media unit check the outer dimensions the manufacturer publishes. Ultrawide monitors stretch this gap further — same diagonal, far more width.
Tyre pressure: psi, kPa or bar
The number on your door-jamb sticker is usually psi in the US and kPa almost everywhere else. Same pressure, but 32 and 220 look nothing alike — so putting one number into the other scale is how people end up badly under- or over-inflated.
bar shows up on European equipment and espresso machines (9 bar). MPa is used for concrete strength and hydraulics. hPa is the storm pressure on a weather map and mmHg is blood pressure. They all measure the same thing, so they all convert.
Common pressure values
The values people look up most, worked out in advance.
| psi | kPa | bar | kgf/cm² |
|---|---|---|---|
| 20 psi | 137.9 | 1.379 | 1.406 |
| 24 psi | 165.5 | 1.655 | 1.687 |
| 28 psi | 193.1 | 1.931 | 1.969 |
| 30 psi | 206.8 | 2.068 | 2.109 |
| 32 psi | 220.6 | 2.206 | 2.25 |
Most passenger cars sit between 32 and 36 psi. Check the sticker inside the driver's door rather than the tyre wall — the number on the tyre is its maximum, not the recommendation for your car.
Torque: lbf·ft, N·m and kgf·m
US specs quote lbf·ft, European and SI specs quote N·m, and Korean and Japanese specs quote kgf·m. The same engine reads as 144, 196 and 20 depending on which sheet you are holding, which makes cross-shopping harder than it should be.
One number does most of the work: 1 lbf·ft = 1.3558 N·m, and 1 kgf·m = 7.233 lbf·ft. Crane and construction data sometimes uses tonf·m, which is 1,000 times kgf·m.
Common torque values
| lbf·ft | N·m | kgf·m | lbf·in |
|---|---|---|---|
| 50 lbf·ft | 67.8 | 6.91 | 600 |
| 75 lbf·ft | 101.7 | 10.37 | 900 |
| 100 lbf·ft | 135.6 | 13.83 | 1,200 |
| 125 lbf·ft | 169.5 | 17.28 | 1,500 |
| 150 lbf·ft | 203.4 | 20.74 | 1,800 |
Force: kgf, tonne-force and the two different tons
kgf (kilogram-force, sometimes written kp for kilopond) is the force one kilogram presses down with on Earth. It is not a mass, even though the number looks like one — that is why a load cell reading 5,000 kgf and a scale reading 5,000 kg are saying different things about the same object.
Converting kgf to newtons is a defined value: 1 kgf = 9.80665 N exactly. Going to tons is where it gets slippery, because "ton" is not one unit. The metric tonne-force (tf) is simply 1,000 kgf, so 8,228 kgf is 8.228 tf and you can move the decimal point in your head. The US short ton-force is 2,000 lbf, which is a different quantity — the same 2,000 kgf reads as 2 tf but 2.205 short tonf. On crane charts, rigging tables and load-cell specs that 10% gap is the whole margin.
Which one a sheet means usually follows where it was drawn: Korean, Japanese and European engineering documents mean the metric tonne-force, and US documents mean the short ton unless they write "metric". Structural work in newtons uses kN, and US structural work uses kip (1,000 lbf).
Common force values
From kgf — the scale most Asian and European load specs are written in.
| kgf | tf (metric) | kN | lbf |
|---|---|---|---|
| 1 kgf | 0.001 | 0.01 | 2 |
| 10 kgf | 0.01 | 0.1 | 22 |
| 100 kgf | 0.1 | 0.98 | 220 |
| 500 kgf | 0.5 | 4.9 | 1,102 |
| 1,000 kgf | 1 | 9.81 | 2,205 |
The tf column is just the kgf figure divided by 1,000 — that one is safe to do in your head. The lbf column is not, and neither is the US short ton.
Common length and weight values
| Inches | cm | Pounds | kg |
|---|---|---|---|
| 1 in | 2.54 | 1 lb | 0.454 |
| 2 in | 5.08 | 2 lb | 0.907 |
| 3 in | 7.62 | 3 lb | 1.361 |
| 5 in | 12.7 | 5 lb | 2.268 |
| 10 in | 25.4 | 10 lb | 4.536 |
The figures behind this page
- Length — 1 in = 25.4 mm · 1 ft = 0.3048 m · 1 yd = 0.9144 m · 1 mi = 1,609.344 m (all defined values)
- Weight — 1 lb = 453.59237 g · 1 oz = 28.349523125 g · 1 stone = 14 lb
- Volume — 1 US gal = 3.785411784 L · 1 imperial gal = 4.54609 L
- Area — 1 sq ft = 0.09290304 m² · 1 acre = 4,046.8564224 m²
- Temperature — °F = °C × 9/5 + 32 · K = °C + 273.15
- Force — 1 kgf = 9.80665 N (defined) · 1 tf = 1,000 kgf · 1 lbf = 0.45359237 × 9.80665 N · 1 kip = 1,000 lbf · 1 US short tonf = 2,000 lbf
Working with precious metals instead? Troy ounces are a different unit again — see the gold price calculator.
Frequently asked questions
How many centimetres is one inch?
Exactly 2.54 cm. That is not a rounded figure — the 1959 international yard and pound agreement defined an inch as precisely 25.4 mm, so the value is identical in every country that signed it. Going the other way, 1 cm is about 0.3937 inches, 10 cm is about 3.94 inches and 30 cm is about 11.81 inches.
How many centimetres is one foot?
One foot is 30.48 cm. Twelve inches make a foot and three feet make a yard, which is 91.44 cm. Because heights in the US and UK are given as feet and inches together, you cannot convert the inches alone — 5 foot 9 works out as (5 × 12 + 9) × 2.54 = 175.26 cm.
How many kilograms is a pound?
One pound is 0.45359237 kg, roughly 453.6 grams. That figure is also fixed by international agreement rather than approximated. For quick mental maths, a pound is about 0.45 kg and a kilogram is about 2.2 pounds. Sixteen ounces make a pound, and an ounce is about 28.35 grams.
Is a US gallon the same as a UK gallon?
No, and the gap is over 20%. A US gallon is 3.785 litres while an imperial gallon is 4.546 litres. The US gallon descends from the old wine gallon, whereas the imperial gallon was redefined in 1824 as the volume of ten pounds of water. It matters for fuel economy: 30 mpg is about 12.75 km/L on the US measure but only 10.6 km/L on the imperial one.
What is a stone in weight?
A stone is 14 pounds, about 6.35 kg. It is still the everyday way of giving body weight in the UK and Ireland, usually written as something like 12 stone 4 pounds rather than as a single decimal number. This page converts kilograms into stone and pounds together so you get the form people actually use.
How do I convert kgf to tons?
Divide by 1,000 — but only if the sheet means metric tons. A tonne-force (tf) is exactly 1,000 kgf, so 8,228 kgf is 8.228 tf and 54,200 kgf is 54.2 tf. The US short ton-force is a different unit: it is 2,000 lbf, so the same 2,000 kgf reads as 2 tf but 2.205 short tonf. Korean, Japanese and European engineering documents mean the metric tonne-force; US documents mean the short ton unless they write metric.
Is kgf the same as kg?
No. A kilogram is a mass and a kilogram-force is the force that mass presses down with on Earth — 1 kgf = 9.80665 N by definition. The numbers look identical on Earth, which is exactly why load-cell and crane specs write kgf rather than kg. In newtons the difference is obvious: 100 kg of mass exerts 980.665 N, which is 100 kgf.
How wide is a 55 inch TV really?
TV sizes describe the diagonal, not the width. A 55 inch screen has a 139.7 cm diagonal, which on a 16:9 panel works out as roughly 121.8 cm wide and 68.5 cm tall. Guessing the width from the diagonal leaves you about 20 cm out. Bezels and the stand add more on top, so check the manufacturer's outer dimensions before buying a mount or a cabinet.