Unit Converter

// Start with the pair you actually mean

A unit converter should open on the comparison people recognise: kilometres and miles, kilograms and pounds, Celsius and Fahrenheit. Each category starts with that familiar pair instead of making neighbouring units look related merely because they happen to share a row.

Both value fields are editable and update live. Choose different units from either list, type on either side, or swap the pair in one click. Common conversions below load the value and both units together, so a row for 26.2 miles never leaves you to reconstruct what it was meant to answer.

When you do want the wider context, Show every unit reveals the complete category as a read-only table. That keeps nanometres and nautical miles available without making eleven equal-looking inputs compete to be the main question.

// Is a kilobyte 1,000 bytes or 1,024?

Both, depending on who is counting, and this is the one category where the popular converters are actively unhelpful. The SI prefixes are decimal by definition: kilo means 1,000, mega means 1,000,000, and a kilobyte is 1,000 bytes. The powers of two got the same names by convention in early computing, because 1,024 is close enough to 1,000 that nobody minded, and the ambiguity has been with us ever since.

The unambiguous names for the binary sizes were standardised in 1998: kibibyte, mebibyte, gibibyte, written KiB, MiB and GiB. A kibibyte is exactly 1,024 bytes. They are not a different amount of data from what people mean when they say 'kilobyte' in a binary context, they are just the name that says which one you meant.

The Decimal and Binary toggle on the data category switches between the two sets rather than fudging one into the other, and switching it keeps the quantity fixed while changing how it is counted. One decimal terabyte is about 931 GiB. Some operating systems historically labelled that binary result GB, but the quantity did not change and no bytes went missing.

// Where these numbers come from, and which ones are exact

Most of the conversions people reach for are exact by definition rather than measured. The international yard and pound agreement of 1959 fixed the inch at exactly 0.0254 metres, which makes the foot exactly 0.3048 metres and the mile exactly 1,609.344 metres. The same agreement fixed the pound at exactly 0.45359237 kilograms, so an ounce is exactly one sixteenth of that and a stone is exactly fourteen of them. The US gallon is exactly 231 cubic inches, the imperial gallon is exactly 4.54609 litres, and an acre is exactly 4,046.8564224 square metres. None of those are approximations that got refined over time. They are agreed values, and they will not change.

A few of the factors here are derived rather than declared, and this page stores the arithmetic instead of a copied decimal. One pound-force per square inch is one pound of force, which is the exact pound times standard gravity of exactly 9.80665 metres per second squared, spread over an exact square inch. Writing that multiplication out means a mistyped digit is a visible error rather than a plausible-looking constant nobody checks.

The catalogue keeps the published definitions separate from presentation. Browser arithmetic uses IEEE-754 double precision. Integer results in the ordinary display range, such as byte counts, are shown whole; non-integers are rounded to eight significant figures. Ordinary large values use grouped decimals, so a mile reads 1,609,344,000,000 nanometres rather than programmer-style exponent notation. Only genuinely unwieldy magnitudes use rounded typographic notation such as × 10²⁵.

// The gallon problem, and other units that share a name

A US gallon is about 3.79 litres and an imperial gallon is about 4.55 litres, a difference of roughly twenty percent. Both are called a gallon, both are still in daily use, and a recipe or a fuel figure that does not say which one it means is genuinely ambiguous. The volume category lists them separately for that reason, along with the US pint and the imperial pint, which differ by a similar margin.

The same trap is set in the weight category, where a US ton is 2,000 pounds and an imperial or long ton is 2,240. In the temperature category the trap is different in kind: Celsius and Fahrenheit do not share a zero, so converting between them is not a multiplication. Twenty degrees Celsius is not twice ten degrees Celsius in Fahrenheit, and a converter that treats temperature as a simple ratio gets every value except one wrong.

// fair questions

How many kilometres are in a mile?

Exactly 1.609344, by definition rather than by measurement. The 1959 international yard and pound agreement fixed the inch at exactly 0.0254 metres, and a mile is 63,360 inches, which works out to exactly 1,609.344 metres. Going the other way, one kilometre is about 0.62137119 miles.

Is a kilobyte 1,000 bytes or 1,024 bytes?

By the SI definition it is 1,000. The 1,024-byte unit is properly called a kibibyte (KiB), and 1,024 kibibytes make a mebibyte (MiB). Both conventions are in wide use, which is why this page ships them as separate unit sets behind the Decimal and Binary toggle instead of picking one and hoping you meant that one.

Why does my 1 TB drive show up as 931 GB?

Because two different units are both being called gigabytes. The manufacturer counted 1,000,000,000,000 bytes, which is one terabyte in decimal units. Your operating system divided by 1,024 three times, which gives about 931 of the binary units it labels GB but which are really gibibytes. Every byte you paid for is there.

What is the difference between a US gallon and a UK gallon?

A US gallon is exactly 231 cubic inches, or about 3.785 litres. An imperial gallon, still used in the UK, is exactly 4.54609 litres. The imperial gallon is roughly twenty percent larger, so a fuel economy figure in miles per gallon means noticeably different things on either side of the Atlantic. Both are listed separately in the volume category.

How do I convert Celsius to Fahrenheit by hand?

Multiply by 9, divide by 5, then add 32. So 20 degrees Celsius becomes 68 Fahrenheit, and 100 becomes 212. The addition is the part that matters: the two scales have different zero points, so you cannot convert temperatures by multiplying alone. The one value that reads the same on both scales is minus 40.

Are the conversions on this page exact?

The source factors are exact wherever the standard defines them: that includes the inch, foot, mile, pound, acre, US gallon and standard atmosphere. The browser calculates with IEEE-754 double precision. Integer results in the ordinary display range are shown whole; non-integers and genuinely extreme magnitudes are rounded to eight significant figures, with the defining values stated above where the extra digits matter.