Feet vs Mil vs Mile: Three Units People Confuse

The foot, the mil and the mile all belong to the imperial system, and two of them are separated by a single letter. That is unfortunate, because they operate at wildly different scales and appear in wildly different documents.

The mil is a manufacturing unit. It measures the thickness of plastic sheeting, paint films, PCB copper, wire insulation and foil. A specification calling for '6 mil poly' means 0.006 inches — about 0.15 millimetres.

The mile is a travel unit. Nobody has ever specified a coating in miles, and nobody has ever driven a mil. The confusion is purely orthographic, but it appears often enough in purchase orders to be worth stating plainly.

The second trap is more expensive. A mil and a millimetre are both abbreviated informally as 'mil' in speech, and they differ by a factor of about 39.

One mil is 0.0254 mm. One millimetre is 39.37 mil. A specification for 10 mil sheeting is 0.254 mm; reading it as 10 mm would give you material forty times too thick and roughly forty times too expensive.

The rule of thumb: if a spec is American and describes a thickness, 'mil' almost certainly means thousandths of an inch. If the document is otherwise metric, it means millimetres. When it genuinely matters, ask.

Going the other way, miles to feet does the reverse conversion. If you want a figure already worked out, conversion chart lists the common ones, and about & methodology documents every constant used here and where it comes from.

In military and ballistics contexts, 'mil' means a milliradian — an angular unit, not a length. There are 6,283 true milliradians in a circle, though several armies round to 6,000 or 6,400 for ease of arithmetic.

The useful property is that one milliradian subtends approximately one metre at a thousand metres, which makes range estimation straightforward. It has nothing to do with either the thickness mil or the mile.

Look at the magnitude. Thickness specs are single or double digits — 4 mil, 6 mil, 30 mil. Distances are hundreds or thousands. If a number looks implausible for its unit, it probably is.

Look at the context. Coating, film, gauge, sheeting, plating, trace width — all thickness. Route, altitude, distance, radius, frontage — all length.

And when converting, use a tool built for the unit you actually have. Feet to mil and feet to miles are different pages here for exactly this reason.

Spec sheets are often doubled up: mil and millimetres side by side, or mil only in an American catalogue and millimetres only in a European one. Knowing which is which saves a wrong order.

The quick check is the size of the number. Sheeting thickness in mil is usually between 1 and 60. The same thickness in millimetres is between 0.025 and 1.5. If the number is below one, it is probably millimetres; if it is above one and whole, probably mil.

Gauge is a third system — used for sheet metal and wire — where the number goes down as the material gets thicker, the opposite of mil. 16-gauge steel is thicker than 20-gauge. So when gauge appears in the same document, do not assume everything increases in the same direction.

Mil is not the only small thickness unit you will meet. The micron — or micrometre — is a millionth of a metre, which is 0.001 millimetres. One mil is 25.4 microns.

Microns usually turn up in filtration, coatings and film specifications. A "1 micron" air filter stops particles larger than 0.0000394 inches — about a quarter of a mil.

The practical rule is to write the unit out in full whenever the number could be read two ways. "6 mil (0.006 in / 0.15 mm)" removes every ambiguity at the cost of a few characters, and it has never caused a wrong order.

Related conversions: ft/s to mph for speed, sq ft to sq miles for land area, and nautical miles for the unit people most often confuse with the mile.

There is a good reason a site about feet and miles carries a feet-to-mil page: people search for "feet to mil" thinking it means miles, and the reverse.

If there were only one calculator and it assumed every query meant miles, the person looking for a thickness would get an answer wrong by a factor of 63 million — and nothing would tell them.

So the two are kept apart and the difference is stated plainly on both pages. The goal is not only correct arithmetic but the correct question before the arithmetic starts. A calculator that answers confidently in the wrong unit is worse than no calculator at all.