Inputs
Results
Bore (in) = √(4 × pedal-force × pedal-ratio) / (π × √(PSI)).
What this calculator actually solves
Master cylinder bore is the single biggest factor in how a brake system responds. Too small a bore and the pedal feels hard and the pads don't move. Too large a bore and the pedal feels soft, the pads drag, and you run out of travel before lockup.
This calculator reads the caliper piston volume, the pedal ratio, and the maximum pedal force you can realistically apply, and gives you the bore that achieves your target line pressure with reasonable pedal feel.
Who this is for
Hot-rodders upgrading from a single-piston caliper to a 4-piston caliper and trying to figure out whether the stock master cylinder will still be enough.
Race mechanics picking a bore for a specific pedal feel (firm-but-modulate vs soft-and-easy).
How to read the results
Pick the standard master cylinder bore closest to the calculated value. Common sizes are 0.625", 0.700", 0.750", 0.812", 0.875", 0.937", 1.000", 1.062". Going one size larger gives a softer pedal; one size smaller gives a harder pedal.
If the calculated bore is wildly different from your stock bore, you may need to upgrade calipers or re-spec the pedal ratio.
Limitations
Master cylinder bore doesn't tell you about volume reserve. After filling the calipers, the MC still needs 1–2 cc of reserve volume to prevent the pedal from going to the floor under heavy braking. We compute bore only.
This calculator doesn't account for brake booster assist. With a vacuum booster (most production cars), pedal force needed is much lower; the numbers above are for manual / non-boosted reference.
Frequently Asked Questions
Larger or smaller bore — which gives a firmer pedal?
Smaller bore gives a firmer pedal because the same pedal force produces higher pressure. But it also reduces fluid volume, so the calipers won't move as far — that's why big calipers need bigger bores, not smaller ones.
Should I round up or down?
Pick the closest standard size. Going larger means softer pedal and slightly more travel; going smaller means firmer pedal and less travel but risk of running out of volume reserve.
My car has a brake booster — does the math change?
Yes — a vacuum booster typically multiplies pedal force by 3–5x, so the effective pedal force at the master is much higher than your foot pressure. Use the boosted force, not your foot pressure, as the input.
About this tool
What this tool is for: Master Cylinder Bore Size Calculator is a workbench reference for working mechanics, machinists, restoration shops, and serious hobbyists. It is built because the usual online calculators skip the friction loss, the sign convention, the unit conversion, or the engineering code that actually matters on the job.
Purpose and scope
Master cylinder bore size from caliper piston volume, pedal ratio, and stroke. It ignores the proportioning valve; for cars with one, set the rear bias separately.
How to use the body of this page
The sections above this footer — “What this calculator solves,” “Who this is for,” “How to read the results,” and “Limitations” — describe in detail what the formula does, why a tech would use it, how to interpret the number, and where the model is wrong. The FAQ block answers the three or four questions most asked about this specific tool. Together they make up the “About” content for this page; you do not need to look elsewhere.
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