mmilbury:
You failed to say what type or use of "orifice" you mean. As a result, Mogens has assumed you mean orifice plates. I think you may be referring to the orifice size of Safety Relief Valves, where the nomenclature is as follows:
Orifice Letter /Area, in2
D / 0.110
E / 0.196
F / 0.307
G / 0.503
H / 0.785
J / 1.287
K / 1.838
L / 2.853
M / 3.600
N / 4.340
P / 6.380
Q / 11.05
R or S / 16.00*
T / 19.64*
T or U / 26.00*
* = Code Letters and orifice areas not consistent for these large orifices between various manufacturers.
I hope this is what you needed.
Art Montemayor
Spring, TX