Calculating O-Ring Cut Length for Bonding

When we know: I. D.(Inside Dia.) and C/S (Cross section) we calculate:
[ ( I.D.) + ( one C/S ) ] X 3.14159 = Nominal Cut Length (C/L)

Working backwards: we know the Cut Length and C/S, we calculate:
Cut Length ÷ 3.14159, then subtract one ( - C/S ) = I. D. (Inside Diameter)

Example:
9.147 I.D. + .147 C/S = 9.294 X 3.14159 = 29.20 Cut Length
Backwards:
29.20 C/L / 3.14159 = 9.294 - .147 = 9.147 I. D.