They chop you in half (through your waistline) using dental floss and use a piece of string with knots tied in at 25.4mm intervals. They count the knots and perform some sort of calulatory jiggery pokery using an abacus and runestones.
Then multiply by 100.
*Note* If you squirm during the chopping process you have to join the RAF...Likewise if you're fat.
Because, I know exactly what they use hospital/gym wise and even tho i dont know the Army's preferred method its seemed worth posting. Considering nobody else bothered to try and answer in a helpful manner