Imperial units of length

The imperial system is still common in the United States and the United Kingdom. It uses different conversion factors than the metric system — they are not all powers of ten.

The four common units

Symbol	Name	Reference
in	inch	the width of a thumb
ft	foot	the length of a typical adult shoe
yd	yard	the length of a long step
mi	mile	a 15-minute walk

The conversion factors

$1 ft = 12 in$
$1 yd = 3 ft = 36 in$
$1 mi = 1760 yd = 5280 ft = 63360 in$

    mi
    ↕ ×1760
    yd
    ↕ ×3
    ft
    ↕ ×12
    in
  

Unlike the metric system, the steps are not all the same. You really do have to remember three different numbers: 12, 3 and 1760.

How to convert

The rule is the same as in the metric system:

Smaller → larger : divide.
Larger → smaller : multiply.

Examples

$5 ft = ? in$ — multiply by 12: $5 \times 12 = 60 in$ .
$36 ft = ? yd$ — divide by 3: $36/3 = 12 yd$ .
$2 mi = ? ft$ — multiply by 5 280: $2 \times 5280 = 10560 ft$ .
$3520 yd = ? mi$ — divide by 1 760: $3 520/1 760 = 2 mi$ .

A useful comparison with the metric system

A quick rule of thumb (these are not exact, but handy for everyday estimating):

$1 in \approx 2.5 cm$
$1 ft \approx 30 cm$
$1 yd \approx 1 m$ (a metre is slightly longer)
$1 mi \approx 1.6 km$