Why a conversion goes wrong even when the arithmetic is right
Converting a unit is one multiplication, and nobody gets the multiplication wrong. What goes wrong is the number being multiplied by, and the quiet assumption about which unit the label meant.
Rounded factors are the first trap. A table that stores a pound as 0.4536 kg is already wrong in the fifth digit, and that error is multiplied by whatever you typed. On a bag of coffee it disappears. On a 20 tonne shipment it is 11 kilograms. Every factor on this page is the exact defined value instead: 0.45359237 kg for the pound, 0.0254 m for the inch, 3.785411784 L for the US gallon.
The second trap is that several units share a name. A gallon is 3.785 L in the United States and 4.546 L in the UK. A ton is 1000 kg, 907.18474 kg or 1016.0469088 kg depending on who wrote the document. An ounce is a mass in one tab of this tool and a volume in another. Those are not rounding errors, they are different quantities wearing the same word, and no number of decimal places will rescue a conversion that picked the wrong one.
Temperature is the third trap. Celsius, Fahrenheit and Kelvin have different zero points, so there is no factor at all: a conversion is multiply and then shift. Halving a Fahrenheit reading does not halve the temperature. The temperature tab therefore prints the formula it used, because a factor there would be a lie.
Which tab, and which of the units that share a name
Six tabs: length, weight, temperature, area, volume and speed. Pick the tab first, because the separate unit lists are what stop you converting a fluid ounce into a gram.
Where a name is ambiguous, here is what this page means by it:
- Gallon, pint, quart, cup, fluid ounce: the US customary versions, labelled US. The imperial gallon is listed separately because it is 20 percent larger, and there is no imperial pint here at all.
- Ounce: in the weight tab it is the avoirdupois ounce, a mass. In the volume tab the fluid ounce is a volume. They only look interchangeable for water.
- Tonne: the metric tonne of 1000 kg. Short tons and long tons are absent, so a document that says tons has to be pinned down before you use this page.
- Teaspoon and tablespoon: the US versions. A metric teaspoon is a round 5 ml, and an Australian tablespoon holds four teaspoons rather than three.
The decimals selector is a display setting. The arithmetic always runs at full precision and only the printed answer is rounded, so raising it from 2 to 6 never changes a result, it stops hiding one.
How to convert a value
- Pick the tab for what you are measuring. The unit menus refill and the value resets to 1.
- Choose the two units, or tap one of the common pairs under the boxes, such as cm to in or kg to lb.
- Type into either box. The other one fills in as you type, with no convert button to press and no direction to set first.
- Use the swap button between the boxes to turn the conversion around. It exchanges the units and the values together, so the equation on screen stays true rather than reinterpreting your number.
- Set the decimals selector to the precision you need, then press copy. The copy button hands over plain digits with no thousands separators.
How to check the factor instead of trusting it
- Read the line under the result. It prints the factor actually applied, to twelve significant digits, in the form 1 in = 2.54 cm. On the temperature tab it prints the formula instead.
- Read the note under it, which says where the exact value comes from, then check the table below the tool. That table lists every unit in the tab against the base unit, so you see the whole chain rather than a single pair.
- Sanity check any converter with a known pair. 1 in is 2.54 cm, 1 lb is 0.45359237 kg, minus 40 C is minus 40 F, and 1 gal (US) is 3.785411784 L. A tool that misses those is storing rounded factors.
If two tools disagree, compare the factor lines before the answers. The gap is nearly always visible there, and it is usually a rounded definition rather than a bug.
The handful of definitions everything else hangs off
Imperial and US customary units are not measured against the metric system, they are defined in terms of it. Five exact numbers generate nearly every factor here.
- 1 inch = 25.4 mm exactly, from the 1959 international yard and pound agreement. Foot, yard, mile and the imperial areas all follow from it.
- 1 pound = 0.45359237 kg exactly, from the same agreement. Ounce, stone, short ton and long ton follow.
- 1 US gallon = 231 cubic inches, exactly 3.785411784 L. Quart, pint, cup, fluid ounce, tablespoon and teaspoon are fractions of it.
- 1 imperial gallon = 4.54609 L exactly, a separate definition that never met the US one.
- 1 nautical mile = 1852 m exactly, which is where the knot comes from.
That is why the factors here run to twelve digits rather than three. They are definitions, not measurements a better experiment could improve.
Getting the number out of the page
- Copy button: copies the converted value alone, with no unit and no thousands separators, so it lands in a spreadsheet cell as a number rather than text.
- The displayed result is grouped with commas for reading. That grouping never reaches your clipboard, which is the usual reason a pasted figure refuses to add up.
- Select by hand to take the whole sentence including the units and the factor line.
- Keep the factor with the number when you send a converted figure on. A weight in pounds with no note of which pound is how a project ends up converting twice.
The same conversion in a spreadsheet, in code, and in a kitchen
Excel, Google Sheets and LibreOffice Calc all ship a CONVERT function that takes unit codes as text:
=CONVERT(100, "cm", "in")
=CONVERT(1, "lbm", "kg")
=CONVERT(20, "C", "F")
The codes are case sensitive, mass is lbm rather than lb, and the gallon code means the US gallon, so an imperial gallon has to be done by hand. An unknown code returns an error rather than a guess.
In code
There is nothing to install. Keep the exact factor as a named constant and multiply, the same way this page does: metres = inches * 0.0254. The rule worth enforcing in review is that the constant holds the full defined value, because a 0.4536 that slips into a shared module is very hard to find later.
In a kitchen
Cup measures are the least standard thing in cooking. This page uses the US customary cup of 236.5882365 ml, while US nutrition labelling uses 240 ml, Japanese recipes 200 ml and older Commonwealth books 250 ml. A cup of flour in grams depends on the ingredient and the packing, so no unit converter can answer it.
Country by country
The UK is the awkward case: road signs in miles, pints of draught beer and body weight in stones, but metric shop scales, fuel and school science. Aviation mixes them too, with altitudes in feet and speeds in knots.
What this tool will not do
- It will not convert between different quantities. Mass to volume needs a density, and fuel economy between litres per 100 km and miles per gallon is a reciprocal rather than a factor.
- It does not know how precise your measurement was. Ask for six decimals on a figure read off a tape measure and you get six decimals of false confidence. Round back to the precision of the input before quoting it.
- Negative values are allowed everywhere, which makes sense for temperature and for differences but not for a length or a volume. Below absolute zero the tool says so and still answers.
- Currency, data size, fuel economy and time are not included. Currency cannot be a fixed factor at all.
For anything with legal or safety weight, such as a customs declaration or a structural load, quote the unit named in the governing document and keep the original beside the conversion so a reviewer can check it. At the time of writing the factors here match the exact definitions described above, unchanged since 1959 for length and mass.
Frequently asked questions
How many centimetres are in an inch?
Exactly 2.54, because the inch has been defined as exactly 25.4 millimetres since the 1959 international yard and pound agreement. It is not an approximation that gets better with more decimals, it is the definition, so 1 in is 2.54 cm and 1 cm is 0.393700787402 in to twelve digits. Every other imperial length follows from it: a foot is 12 inches or 0.3048 m, a yard is 0.9144 m, and a mile is 5280 feet or 1609.344 m.
Is a US gallon the same as a UK gallon?
No, and the gap is large enough to ruin a recipe or a fuel economy figure. The US liquid gallon is defined as 231 cubic inches, which is 3.785411784 litres. The imperial gallon used in the UK is 4.54609 litres, about 20 percent bigger. The same split runs through the smaller units: a US fluid ounce is 29.5735295625 ml and an imperial fluid ounce is 28.4130625 ml. This page labels every US unit as US and offers the imperial gallon separately so you never pick one by accident.
How do I convert Celsius to Fahrenheit in my head?
Double the Celsius figure and add 30. It is close enough for weather: 20 C gives an estimate of 70 against a true 68, and 30 C gives 90 against a true 86. The error grows as you move away from room temperature, so use the exact formula when it matters. That formula is F = C multiplied by 9/5 plus 32, and the reverse is C = F minus 32, multiplied by 5/9. The one value where the two scales agree is minus 40.
Why do two unit converters give slightly different answers?
Almost always because one of them stores a rounded factor. A converter that holds 1 lb as 0.4536 kg rather than the exact 0.45359237 kg is already wrong in the fifth digit, and the error is multiplied by whatever you typed. The second cause is display rounding: two tools can hold the same value and print it to a different number of decimals. Compare the factor each tool used, not just the answer, which is why this page prints the factor it applied under every result.
Is an ounce a weight or a volume?
Both words exist and they measure different things. The avoirdupois ounce in the weight tab is a mass of exactly 28.349523125 g. The US fluid ounce in the volume tab is a volume of 29.5735295625 ml. They are close in number for water by coincidence of history and not by definition, and for anything denser or lighter than water the two diverge immediately. A fluid ounce of honey and an ounce of honey are not the same quantity.
How many kilograms is a stone?
One stone is 14 pounds, which is exactly 6.35029318 kg. Body weight in the UK and Ireland is usually quoted in stones and pounds, so 11 st 4 lb is 11 times 14 plus 4, or 158 pounds, which is about 71.7 kg. Convert the whole figure to pounds first and then to kilograms in one step, because converting the stones and the pounds separately and adding is where the arithmetic usually goes wrong.
What is the difference between a tonne, a short ton and a long ton?
A tonne, sometimes written metric ton, is 1000 kg and is the only one of the three on this page. A US short ton is 2000 pounds, or 907.18474 kg. A UK long ton is 2240 pounds, or 1016.0469088 kg. The three differ by up to 12 percent, so a shipping weight or an emissions figure quoted as tons is ambiguous until you know which one is meant. Freight documents and climate reporting normally mean the metric tonne.
Why does the result show something like 6.2e-7 instead of a small number?
Because at the precision you selected the answer would round to zero, and zero would be a lie. One millimetre in miles is 0.00000062 and change, so at two decimals a fixed rendering gives 0.00, which tells you nothing. When that happens the tool switches to scientific notation and spends the same digit budget on significant digits instead. Raise the decimals selector and the result goes back to ordinary notation as soon as there is something to show.
Is a knot the same as a mile per hour?
No. A knot is one nautical mile per hour, and the nautical mile is exactly 1852 metres, roughly 15 percent longer than the statute mile of 1609.344 metres. So 10 knots is about 11.5 mph or 18.5 km/h. Aviation and shipping use knots because a nautical mile is one minute of latitude, which makes chart work easier. Road speed limits never use them, so a speed quoted in knots is a boat or an aircraft.
Does this page send my numbers anywhere?
No. The whole conversion table is part of the page, the arithmetic runs in your browser, and nothing you type is uploaded, logged or written into the address bar. That also means the page works with the network switched off once it has loaded, and that a link you copy carries the tool rather than your figures. The only event the site records is an anonymous counter when you press the copy button, which notes the unit pair and not the value.
Sources
- The metric definitions and the structure of the SI come from the International Bureau of Weights and Measures, which publishes the SI Brochure as the defining reference for the metre, the kilogram and the kelvin.
- The inch of exactly 25.4 mm and the pound of exactly 0.45359237 kg come from the 1959 international yard and pound agreement, signed by the national standards bodies of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa.
- The US customary volume units are defined from the gallon of 231 cubic inches, and the conversion tables published by the US National Institute of Standards and Technology in its guide to the SI carry the same exact values used here.
- The imperial gallon of exactly 4.54609 litres is fixed by UK weights and measures legislation, and the nautical mile of exactly 1852 metres by international maritime and aviation convention.
- Quantity names and symbols follow the ISO 80000 series. Where a symbol on this page is ambiguous in ordinary writing, such as the ton or the gallon, the unit list spells out which one is meant.
More tools
For a change expressed as a proportion rather than a unit, the percentage calculator handles increases, decreases and the difference between two figures. For the other kind of conversion, between binary, decimal and hexadecimal, use the number base converter. If the reason you are converting kilograms to pounds is a health figure, the BMI calculator takes metric or imperial input directly and skips the step.
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Privacy: every conversion runs in your browser. The unit tables are part of the page, nothing you type is uploaded or logged, and no value is written into the URL, so a copied link carries the tool and not your numbers. The only event the site records is an anonymous copy counter that notes the unit pair, never the value.