About UnitChart

How this converter works, where its numbers come from, and what it will not do.

Where the conversion factors come from

Metric definitions follow the SI brochure published by the BIPM. Imperial and US customary definitions follow the 1959 international yard and pound agreement, under which the yard is exactly 0.9144 m and the pound is exactly 0.45359237 kg. Every inch, foot, mile, ounce and ton on this site is derived from those two exact definitions rather than from a rounded decimal, so conversions stay consistent no matter which pair of units you pick.

How conversions are calculated

Each category has a single base unit. Converting from one unit to another means converting into the base unit and back out again. Doing it this way means that converting inches to feet and then feet to yards gives exactly the same answer as converting inches to yards directly — a property that is easy to lose if every pair carries its own hand-entered factor.

Three kinds of relationship are supported: plain ratios (the vast majority), affine scales where the zero points differ (temperature), and inverse relationships where one value rises as the other falls (L/100km against MPG).

Precision and rounding

Results are computed in IEEE 754 double precision and displayed to 10 significant digits, with trailing zeros removed. Displaying fewer digits than the arithmetic carries is what keeps floating-point artefacts such as 0.30000000000000004 off the page. Where a value is too large or too small for readable decimal notation it is shown in exponential form.

Time, months and years

Months and years have no fixed length. This site uses the mean Gregorian year of 365.2425 days and defines a month as one twelfth of it. Calendar arithmetic between two specific dates will give a slightly different answer, and should.

Digital storage

Storage sizes use the binary convention — 1 KB = 1024 bytes — because that is what operating systems report. Drive manufacturers use decimal SI units instead, which is why a drive sold as 1 TB appears as roughly 931 GB. Network transfer rates use decimal units, matching how they are advertised.

What is on the site

17 categories, 1,256 conversion pages and 620 pre-computed specific-value pages.

Limits

These figures are for reference. Verify independently before relying on a conversion for engineering, medical, navigational or any other safety-critical purpose. Currency is deliberately not offered, because a static page cannot hold an exchange rate that is honest for more than a few minutes.