Bore in sq", just of the circle, but only one front caliper, and only one side of it (in the case of opposed pistons).
It's just a general guide that way, and wont always get you right the first time, but if you use a little common sense of what you are using, what it used stock, etc... you will get it right or very close the first time.
It's not really technically the right way to figure it, but it is alot simpler, and usually works out.
If you check out the bore sizes of a lot of cars, you'll find the 12-14% in most with either manual 4wl disc or disc/drum, and some with power 4wldisc.
A lot of vehicles with power 4wl disc will go up to 18% on master size, and use a high pedal, combined with either hydroboost or a dual diaphragm booster. That's only a 1/8" upstep in bore diameter in most cases.
The choice of master bore and stroke has to do also with the size of the rear brakes, which normally are 50-75% the size of the front in the case of 4wldisc... usually expressed as a percentage of front and rear added together... A 70/30 4wldisc system has rear brakes which are about 1/2 the size of the front.
Front 4piston, 1 7/8" diameter (5.52sq")
Rear 4piston 1 3/8" diameter (2.9sq") 52% of front
master, manual, 1" bore (.7854sq") 14% of front
master, power, 1 1/8" bore (.994sq") 18% of front
You'll find others to be very similar. The smallest master I've found on 4wldisc came stock on the mid 70s Ford/Lincolns, and used only 13% with power brakes. It had a long stroke. Ford and Mopar tend to use smaller masters than GM in many cases. I guess that's why they often have softer brakes. Vehicles made post 1994 often have much larger master cyl bores, and discontinued the step-bore master for low-drag brakes.
piston diameter 1 7/8= 1.875
to find area use pi(rxr) or I like [.7854(dxd)] instead
(1.875x1.875)(.7854)=2.76sq" for one piston
5.52sq"= a 1 7/8 diameter 4 piston caliper.
single piston 2 5/8 diameter
5.4sq" =a single piston caliper with a 2 5/8" piston
ARI Vehicle List, brake system bore sizes:
(Do not take the sizes listed here as gospel. Some are not correct, or not accurate)