Brake Pedal Force Calculator works back from target line pressure through master cylinder bore, pedal ratio and booster assist to the force your foot must apply to the brake pedal.
How the Brake Pedal Force Calculator Finds Foot Effort
The Brake Pedal Force Calculator works backward from the line pressure your brakes need to the force your foot must put on the pedal. Hot rod builders, manual brake converters and Formula SAE teams use it to size a master cylinder and pedal before the car is on the road.
Enter the line pressure you want, the master cylinder bore and stroke, and the pedal ratio. You can type the ratio or measure the pedal, and you can add a vacuum booster.
US units use psi, inches and pounds-force. Metric uses bar, millimeters and newtons.
The Brake Pedal Force Calculator Formula
The master cylinder turns pushrod force into line pressure. So the pushrod force you need is the line pressure times the bore area.
$$F_{pushrod} = P \times \frac{\pi d_{mc}^2}{4}$$
The pedal is a lever that multiplies your foot force by the pedal ratio. Without a booster, foot force is the pushrod force divided by that ratio.
$$F_{foot} = \frac{F_{pushrod}}{\text{Pedal ratio}}$$
With the defaults, 800 psi through a 1.0 in bore takes 628 lbf at the pushrod. On a 6.2:1 pedal with no booster, that is 101 lbf at the pedal, or about 451 N.
A common mistake is entering the master cylinder’s part number size in the wrong unit. A “1-inch” bore is 25.4 mm, so check which unit the tool is set to before typing.
Measuring Your Pedal Ratio
Pedal ratio is the distance from the pivot to the center of the pedal pad, divided by the distance from the pivot to the pushrod. Wilwood calls these A and B.
$$\text{Pedal ratio} = \frac{A}{B}$$
Choose Measure the Pedal, and the Brake Pedal Force Calculator works out the ratio for you. A 12 in A and a 2 in B give 6:1.
Wilwood gives typical ratios of 6:1 to 7:1 for pedals without a booster, and 4.5:1 to 5:1 for boosted pedals. Motion Raceworks puts most factory power-brake pedals at about 4:1.
The alert flags a manual setup under 5.5:1 as likely to feel hard. It also flags a booster paired with a ratio above 5:1, since that gives long pedal travel.
How Much Force Is Comfortable
Brake & Front End magazine puts the comfortable push of an average seated driver at about 70 lbf on the pedal pad. The second card of the Brake Pedal Force Calculator compares your result with that figure.
At the defaults, 101 lbf is 45% more than a comfortable push. The same card shows that 70 lbf would only make about 553 psi on this setup, and that you would need an 8.98:1 pedal to reach 800 psi at 70 lbf.
The alert turns yellow above 70 lbf. It suggests a smaller bore, a booster or a higher pedal ratio to lighten the pedal.
The pressure you target depends on your calipers and car. Builders on the Pro-Touring forum commonly work between 800 and 1,200 psi for street disc brakes, and the tool’s alert uses the same range.
Pedal Travel and Bore Size
Leverage always costs travel. The third card of the Brake Pedal Force Calculator multiplies the master cylinder stroke by the pedal ratio.
$$\text{Pedal travel} = \text{MC stroke} \times \text{Pedal ratio}$$
A 1.10 in stroke on a 6.2:1 pedal needs 6.82 in of pedal travel. Summit Racing notes that the master cylinder should bottom out before the pedal reaches the floor.
The fourth card shows the trade-off in bore size. Dropping from a 1.0 in bore to 0.875 in cuts the foot force from 101 to about 78 lbf, but it moves less fluid per stroke, 0.66 in³ instead of 0.86.
A larger 1.125 in bore firms up the pedal but raises the force to about 128 lbf. Large calipers need more fluid, so too small a bore can leave the pedal low or near the floor.
Adding a Vacuum Booster
A vacuum booster uses engine vacuum on a diaphragm to push the master cylinder along with your foot. Its extra force is the diaphragm area times the vacuum.
$$F_{assist,max} = \frac{\pi d_{diaphragm}^2}{4} \times \text{Vacuum}$$
Brake & Front End notes most engines make around 8 psi of vacuum. At that level, a 7 in diaphragm adds more than 300 lbf, and the default 9 in single diaphragm tops out at about 509 lbf.
Below that limit, the booster multiplies your input by its boost ratio. With a 3:1 booster and the same 800 psi target, the pedal drops to about 34 lbf at 6.2:1, or about 52 lbf on a 4:1 power-brake pedal.
A tandem diaphragm doubles the available assist. The first card of the Brake Pedal Force Calculator shows how much of the pushrod force the booster supplies.
Booster Runout
Every booster has a runout point where the vacuum can push no harder. Past it, the booster adds nothing more, so any extra force has to come from your foot.
The default 9 in booster at 8 psi runs out at about 972 psi with a 1.0 in bore. Ask for more pressure than that, and the alert turns yellow and explains that the pedal will suddenly feel much harder.
Real pedal feel also depends on hose expansion, caliper flex and pad grip, so treat the result as a starting point.
Limits and Setup Mistakes
Every size and pressure must be greater than zero, and the pedal ratio must be at least 1:1. Engine vacuum cannot exceed 14.7 psi, since a vacuum can never beat atmospheric pressure.
When measuring the pedal, B must be shorter than A, or the Brake Pedal Force Calculator flags it. The booster ratio must be above 1:1.
Entering engine vacuum in inches of mercury instead of psi overstates the assist about twofold. A gauge reading of 16 inHg is only about 7.9 psi.
Measuring A to the top edge of the pedal pad instead of its center inflates the ratio. Measure to the middle of the pad, where your foot pushes.
Keeping a boosted pedal ratio after removing the booster leaves a very hard pedal. A manual conversion usually needs a new pedal ratio, a smaller bore, or both.
Brake Pedal Force Questions
How do you calculate brake pedal force?
Multiply the line pressure you need by the master cylinder bore area to get pushrod force. Then divide by the pedal ratio. For 800 psi with a 1.0 in bore, the pushrod needs 628 lbf. On a 6.2:1 pedal, your foot supplies about 101 lbf. A booster divides that further by its boost ratio.
What pedal ratio do I need for manual brakes?
Wilwood puts typical manual ratios at 6:1 to 7:1. A higher ratio means less foot force but more pedal travel. With a 1.0 in bore, 70 lbf of foot force would take an 8.98:1 pedal to reach 800 psi, which is why manual setups usually pair a 6:1 to 7:1 pedal with a smaller bore.
Does a smaller master cylinder bore make braking easier?
Yes, for the same pedal force it makes more line pressure. Going from a 1.0 in bore to 0.875 in drops the foot force for 800 psi from 101 to about 78 lbf. The trade-off is more pedal travel, since the smaller bore moves less fluid each stroke.
How much force does a brake booster add?
Up to its diaphragm area times the engine vacuum. A 9 in single diaphragm at 8 psi can add about 509 lbf, and a tandem unit of the same size about twice that. Below runout, a 3:1 booster cuts the foot force needed to about a third.
What line pressure do disc brakes need?
It depends on the caliper piston area and the car. Builders commonly target 800 to 1,200 psi for street disc brakes. To see how much clamping force that pressure makes at your calipers, the brake caliper clamping force calculator takes it from there. For the force the whole car needs to stop, the braking force calculator works from weight and speed.