Convert length units between units, using exact SI factors.
There are exactly 2.54 centimetres in an inch. That is not an approximation but a definition: in 1959 the inch was fixed internationally at precisely 25.4 millimetres, and every other inch-to-metre relation follows from it, including those used for SI conversion in the BIPM brochure. A 55-inch television diagonal is therefore 139.7 centimetres.
A nautical mile is 1,852 metres and corresponds to one minute of arc along a meridian, which is why navigation uses it. A statute mile is 1,609.344 metres. The difference is about 15%, and confusing them at sea would mean a serious course error. The BIPM SI Brochure accepts the nautical mile for use alongside SI.
The metre. Since 1983 it has been defined through the speed of light: the distance light travels in a vacuum in 1/299,792,458 of a second. That definition is set out in the BIPM's SI Brochure, which is why the metre no longer depends on any physical artefact.
Because the result is rounded. The inch-to-centimetre ratio is exact, but a foot-to-metre figure rounded to two decimals already carries an error, which compounds if you convert in several steps. This calculator works at full precision using the factors fixed in the BIPM SI Brochure, and rounds only once, at the very end.