Metric or imperial measure
Until the 50's/60's, drum makers made their own drumheads from real animal skins (as we still find them today on some percussion instruments like djembes for example...). Depending on the country of manufacture, the unit of measurement was then variable according to local standards, in inches in the USA ('imperial' unit) and in centimeters in France and Europe ('metric' unit).
At ASBA, drumheads were for a long time manufactured in 'metric'. The switch to the 'inch' standard dates back to the 1960s, when the words "made to international dimensions" appeared on the inside of the drums.
The inch became the international standard since the invention of the synthetic drumhead by Evans, an American drummer, who naturally offered his products in imperial units. Evans, along with Remo, quickly became the leader in the manufacture of synthetic drumheads, and this standard was imposed on all drum manufacturers around the world.
How to measure the size of your drum?
It is the diameter of the drum that must be taken into account to know the size of your drumhead. But, it is difficult to find a French "meter" in inches... Except for the seamstresses! Indeed, the tape measure for sewing uses the inch measure. Otherwise, measure in cm and divide the centimeters obtained by 2.54 to obtain the equivalent in inch. (Same thing to calculate the depth of your drums...)
30.48cm / 2.54 = 12 inch drumhead
35,56 centimeters / 2,54 = 14 inches drumhead
50,8cm / 2,54 = 20 inch drumhead