Every stage of the hydraulic multiplication
Pedal force is multiplied by the pedal ratio, converted to hydraulic pressure by the master cylinder bore, multiplied again by the caliper pistons into clamp force, and finally converted to torque by the pad friction and the rotor's effective radius.
Comparing this torque against the front axle to check bias? The brake bias calculator takes torque from both axles.
Getting a number you can act on
- 01Enter pedal force and pedal ratio
Pedal ratio is the mechanical advantage between where your foot pushes and where the pushrod connects to the master cylinder — typically 4:1 to 6:1 on a passenger car.
- 02Enter the master cylinder bore
Pushrod force divided by the master cylinder's piston area gives line pressure — a smaller bore produces higher pressure for the same pushrod force, at the cost of more pedal travel.
- 03Enter every caliper piston bore, comma-separated
A caliper's total piston area is the sum of all its pistons on one side — a twin-piston caliper needs both bores entered, a four-piston caliper all four.
- 04Enter pad friction coefficient and effective rotor radius
Friction coefficient is a property of the pad compound, typically 0.35-0.45 for street pads. Effective radius is measured from the rotor's centre to the centre of the pad's contact area, not to the rotor's outer edge.
What the calculator is actually doing
Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.
pressure = (pedal force × pedal ratio) ÷ master cylinder piston areaPascal's law: force applied to a confined fluid produces pressure equal across the whole system.
clamp force = line pressure × total caliper piston areaThe same pressure acting on a larger total piston area at the caliper.
torque = clamp force × pad friction × effective radius × 2Doubled because a caliper squeezes both faces of the rotor — both pads generate friction torque.
Why smaller pistons somewhere in the chain can mean more stopping power
This trips people up because it runs against instinct: a smaller master cylinder bore, for the same pedal effort, produces higher line pressure — not lower — because the same force is now spread across less piston area.
That higher pressure then acts on the caliper pistons, which is where the increased clamp force comes from. The trade is pedal travel: a smaller master cylinder bore needs more fluid displacement to fill the same caliper volume, which means a longer pedal stroke for the same amount of piston movement at the caliper.
This is the entire logic behind master cylinder bore selection in brake system building — going smaller firms up the pedal and increases mechanical advantage, at the cost of needing more pedal travel to move the same amount of fluid. Every stage of the chain trades one of these against another somewhere.
Why effective radius, not rotor diameter
Torque depends on where the friction force is actually applied, which is the centre of the pad's contact patch on the rotor — not the rotor's outer edge, and not its centre bore.
A larger rotor with the same caliper position increases effective radius directly, which is why upgrading to a bigger rotor increases torque even with identical clamp force — the same force is now acting through a longer lever arm.
This is also why caliper position along the rotor matters when comparing brake kits, not just rotor diameter alone — a caliper mounted further out on the same rotor produces more torque for identical clamp force, purely from the longer effective radius.
Brake Pressure Calculator FAQ
How do I calculate brake torque?+
Follow the chain: pedal force times pedal ratio gives pushrod force; divide by master cylinder piston area for line pressure; multiply by total caliper piston area for clamp force; multiply by pad friction coefficient, effective rotor radius and 2 for torque.
Why does a smaller master cylinder increase clamp force?+
The same pushrod force acting on a smaller piston area produces higher hydraulic pressure, which then produces more clamp force at the calipers — at the cost of needing more pedal travel to move the same fluid volume.
What is effective rotor radius?+
The distance from the rotor's centre to the centre of the caliper pad's contact area — not the rotor's outer edge. This is what actually determines the lever arm the friction force works through.
Why is brake torque multiplied by 2?+
Because a disc brake caliper squeezes both faces of the rotor simultaneously, and both pads generate friction torque acting in the same rotational direction.
Standards and references behind these figures
The arithmetic on this page is fixed, but the boundaries and conventions around it come from published standards and manufacturer guidance. These are the documents they come from, so you can check them rather than take them on trust.
01Ford Performance — dynamometer testing and engine performance tech tipsManufacturer guidance on dyno correction and how quoted power figures are arrived at.↗02The Tire and Rim Association — standards filing (NHTSA docket)TRA has been the US standardising body for tire and rim interchangeability since 1903; this filing sets out dimensional practice.↗The chain is exact; the outcome on the road is not. Force and torque calculated through a hydraulic chain are geometry and arithmetic. What that torque actually does — how the car stops — depends on tire grip, road surface, weight transfer, ABS behaviour and brake temperature, none of which this page can see. Treat force and torque figures as design inputs, not a stopping-distance guarantee.