Free Unit Converters: Length, Weight, Temperature, Data

Converters

A converter answers one question: what does this quantity look like in the unit the other side asked for? Right now this category holds one tool, a unit converter covering length, mass, volume, area, speed, temperature, data sizes and time. Everything in it is arithmetic over definition values, which is the only kind of conversion this site will do.

Units and temperature

Definition values, not market data

Every number in a unit converter is either a definition or a measurement, and the difference decides whether a tool can be honest. A definition is fixed by agreement: the 1959 international yard and pound agreement made 1 inch exactly 25.4 millimetres and 1 pound exactly 0.45359237 kilograms; the US gallon has been 231 cubic inches by statute since 1836; IEC 80000-13 defines 1 KiB as exactly 1024 bytes. None of those can change, so a page can hold them without ever going stale.

A price is the opposite. A currency conversion rate, a fuel price, a tax threshold and a pay benchmark are all measurements or prices, and they move. This site does not publish any of them, because we have not found a source we are willing to cite and a confidently wrong number is worse than a missing feature. That is why the converters category holds units of measure, and will never hold currencies — the two look alike on a page and are not alike at all underneath.

The three traps unit conversion actually sets

Decimal against binary. "500 GB" on a drive label and "465 GB" in the file manager are the same number of bytes read through two prefix systems. One steps by 1000, the other by 1024, and both are correct in their own convention. A tool that shows one reading and hides the other creates the argument instead of settling it, so the unit converter always prints both.

Temperature is not a ratio. Doubling 20 °C does not give 40 °C of anything, because the zero point is a convention, not an absence of heat. Any converter that treats degrees like metres will be wrong in a way that looks plausible. This one represents temperature as an offset plus a scale factor written out explicitly, so the round trip through the four physical anchors — freezing, boiling, −40 and absolute zero — comes back to exactly the value you typed.

Significant digits, not decimal places. A single page has to span 1 nanometre and 1 kilometre. Rounding to two decimal places destroys the small end; rounding to ten destroys nothing but makes the large end unreadable. So the precision control is in significant digits, the integer part is never truncated, and the unrounded machine result is printed next to the rounded one whenever the two differ.

What this converter refuses, and why

Four things are absent from the unit list, and each absence is a decision rather than an oversight.

• Historical and regional units. The Chinese chi, cun, jin and mu, the old European ell and league, and the many regional pounds were all different lengths and masses in different places and centuries. There is no basis for calling one of them the correct one, so picking one from memory would hand the reader a confident wrong answer.
• Calendar months and years. A month is 28 to 31 days and a year is 365 or 366; converting either to seconds requires an extra assumption about which one you meant. The time units here are the fixed ones — second through week, plus the Julian year of exactly 365.25 days, which is the astronomy convention and is stated as such.
• Mach number. The speed of sound depends on air temperature and pressure, so Mach 1 is a condition rather than a fixed speed.
• Currencies. As above: a price is not a definition.

What it does instead is refuse the input shapes that quietly change the answer. Thousands separators are rejected because a comma is a decimal point in much of the world. Hex, binary and octal literals are rejected because they are alternative notations for a number rather than another quantity. Values that overflow to infinity produce a message rather than the word "Infinity" in an output field.

Which one do you need?

• I need a length, weight or volume in different units. Use the unit converter, pick the category, then the two units. Every factor is a definition value and the page prints the one it used.
• My drive says 500 GB and the system says 465 GB — which is right? Both. The same tool prints the decimal and binary readings of the same byte count at the same prefix level, which is the whole explanation in two numbers.
• I need degrees Celsius as Fahrenheit and I need to trust it. The converter handles temperature as an affine scale, states the formula, and round-trips the result back to your input so you can see it is exact rather than nearly right.
• I need a percentage, not a unit. That is a different question in a different category: the percentage calculator covers X% of Y, X as a percentage of Y, and percentage change.
• I need a date or a timestamp in the right format. The Unix timestamp converter in the date and time category converts in both directions and says which unit it read.
• I need a colour or a number base converted. Not published yet. This category is where those tools will live, and they will appear here the day they exist rather than before.

Other tools and guides on this site

Conversions rarely appear on their own; they are usually one step inside something else.

• All tools — the full index, grouped by category, with the one tool that runs on our server marked as such.
• Percentage calculator — when the question is a proportion rather than a unit, including the percentage change between two values.
• Unix timestamp converter — the other everyday conversion: between a number and a moment, in your time zone and in UTC.
• Base64 encoder and decoder — when the value has to change its shape to survive a channel rather than change its unit.

Common questions

Where do the conversion factors come from?

From definitions, not from measurements. 1 inch is exactly 25.4 millimetres because the 1959 international yard and pound agreement says so; 1 pound is exactly 0.45359237 kilograms by the same agreement; 1 US gallon is 231 cubic inches by statute; 1 KiB is exactly 1024 bytes by IEC 80000-13. Numbers like those cannot drift, so writing them into the page is the same act as writing them into a standard. Temperature is the exception that proves the rule: it is affine, not proportional, so it is represented by a pair of formulas with the offset written out in full.

Is anything I type uploaded, logged or stored?

No. The page is a static file that imports one module of pure functions; the arithmetic, the formatting and every unit list run in the tab you already have open. There is no request to our server, no analytics event carrying your value, no cookie and no storage. That is also why there is no usage limit: a conversion costs us nothing to run.

Why does 1 GB show up as 0.93 GiB?

Because the two prefixes mean different things and both are legal. SI prefixes step by 1000, so 1 GB is 1,000,000,000 bytes. IEC binary prefixes step by 1024, so 1 GiB is 1,073,741,824 bytes. Divide and you get 0.93132257. Drive manufacturers quote the decimal figure because it is the larger number; most operating systems report the binary one. This converter prints both readings at the same prefix level instead of quietly choosing one for you.

Why is there no currency conversion?

Because a currency conversion rate is not a definition — it is a price, and it changes every second. Publishing one would mean either shipping a stale number or depending on a live feed we have no citable source for, and a confidently wrong rate is worse than no rate at all. The same reasoning removes historical and regional variants of units (the Chinese chi, cun, jin and mu differ by dynasty and by region), calendar months and years, and Mach number, whose value depends on air temperature.

Will this category get number bases and colour formats?

That is the plan in the toolbox map, and they belong here. They are not on the page yet because this site only lists tools that exist: number base conversion (binary, octal, hex), colour format conversion between hex, RGB and HSL, and a byte-level view of a value are the next candidates. Until one of them ships, it is not listed — a link to a tool that does not exist is not a roadmap, it is a broken promise.

Why is there only one tool in this category?

Because the category page appears the day its first tool does, and that day is today. An empty category page is exactly the thin page this site refuses to publish, so a category is never declared in advance — the hub and its first tool are always created together. Every tool on this site appears in exactly one category, and this one is the first of the converters.