Coefficient of friction (µ) of brake pads. Ask your supplier or look on the internet for specific data. Numbers can range from as low as 0.2 for some street pads, all the way up to 0.7 or even higher for use on some very fast race cars with lots of down force. Manufacturers have more exact numbers for you, usually in the form of a µ vs. temp. graph, but as an indication, most street pads range from 0.3 - 0.5 and race pads from 0.4 - 0.6.

When looking at the "µ vs. temp. graph" of the brake pads of your choice, estimate or calculate the average value of µ for the temperature range your brake pads will be operating in and use that number in the calculator.

Note that brake pad size does not affect Friction Force (experienced as "Stopping Power"). A larger brake pad applies the same force (from the piston(s) behind it) as a smaller pad, but it does so against a larger area of the rotor. This reduces the pressure (pressure = force / area). The only factors that determine the Friction Force between pads and rotor are µ (= coefficient of friction) and Normal Force (= force perpendicular to back of brake pads).

Braking with bigger pads will spread the heat (kinetic energy of the moving vehicle-mass, converted into heat by means of friction during braking) over a larger surface area of friction material and therefore bigger brake pads will "run cooler" and will usually show less wear as a result (everything else being equal).

Street Pads have a 2-letter Friction Code printed on the back or edge as part of a larger manufacturer code. The first letter indicates friction at low operating temperatures (200 - 400F) and the second at high(er) operating temperatures (300 - 650F).

D = 0.15 - 0.25
E = 0.25 - 0.35
F = 0.35 - 0.45
G = 0.45 - 0.55
H = 0.55 and up

Example: GF
This pad shows a high initial bite with µ between 0.45 and 0.55 and a slightly reduced bite when warming up, with µ between 0.35 and 0.45
How this pad exactly behaves is impossible to tell from just the Friction Code, but it does give an indication. A look at the "µ vs temp. graph." is much more informative.