A centimetre would be appropriate.
The SI (Systeme International) units for mass is the kilogram. For length, it is the Metre, and for volume, the Litre. Note the system is known as the International System, (SI). The International Bureau for Weights and Measures is located near Paris France, the on their web site they have lots of useful information, including for example, the prefixes for the multiples and sub-multiples. Such as Mega, and micro.
An 8 mm drill bit is equivalent to approximately a 5/16 inch drill bit.
Drill bits not only come in fractional number sizes they also come in letter sizes. The F size drill bit is .257 thousandths of an inch in diameter. A 1/4 inch drill bit is .250 thousandths of an inch in diameter, so size F is .007 thousandths of an inch bigger than a 1/4 inch drill bit.
After you measure the width of the door, you can use any unit of length you like to describe it. No unit is 'correct' or incorrect, but there's always one that produces the most convenient number ... the number that's easiest to write down, remember, and tell other people about. For the width of a door, I think inches would be a bit more convenient than feet.
It depends on what you want to measure: its length. width, thickness, strength, elasticity, resistance to abrasion. Since you have not bothered to provide that crucial bit of information, I cannot provide a more useful answer.