This blog sticks to continental European units of measure. For example, lengths are given in cm (centimetre) or mm (millimetre, which is one tenth of a cm, i. e. 10 mm = 1 cm).

If you are used to describe lengths in inches, just divide the cm value by 2.54 to get the length in inches. If the length was given in mm, just move the decimal point one step to the left after dividing by 2.54, which corresponds to division by 10.

You don’t have a calculator at hand and hate long division on paper? To divide by 2.54 to a good approximate value just multiply by 2 twice (= multiply by 4), then move the decimal point one step to the left (if there was no decimal point in your number, this means inserting one before the last digit).

Here is a cheat-sheet for converting cm to inch:

Why this works: 2.54 is approximately equal to 2.5 = 10/4, and 4 = 2*2, so dividing by 2.54 is approximately equal to multiplying by 2 and once more by 2, and finally dividing by 10.

Example. A recipe says you need a circle with ca. 30 cm diameter. You calculate: 30, times 2 is 60, times 2 is 120, divided by 10 is 12. So, 30 cm is approximately 12 inch. If it were 30 mm instead, then one more division by 10 would give 30 mm to be approximately equal to 1.2 inch.