How to Calculate Bore, Stroke, and Displacement

Engine displacement depends on three factors: the number of cylinders, bore (interior diameter of a cylinder), and stroke (interior length of a cylinder). The formula for displacement is

D = (π/4)B²SN,

where D is displacement, B is bore, S is stroke, and N is the number of cylinders. If you happen to know the amount of displacement but the bore is unknown, you can calculate it with the formula

B = √ 4D/(πSN) .

Likewise, if you know the displacement and bore, but the stroke is unknown, you can compute S with the equation

S = 4D/(πB²N).

These formulas are derived from the equation for cylindrical volume, V = πR²H.

If you need to make engine computations, you can use the calculator on the left to quickly figure the unknown quantity among bore, stroke, and displacement. Simply input the number of cylinders, and fill in exactly two of the other quantities. The calculator will automatically compute the missing third value. Here are some sample computations.

Example 1:

In a 4-cylinder engine, the stroke length is 10 cm and the bore is 4.5 cm. What is the displacement?

Since N = 4, S = 10, and B = 4.5, the displacement in cubic centimeters is

D = (π/4)(4.5²)(10)(4) = 636.1725 cc, or 0.636 L.

Example 2:

A 6-cylinder engine has a displacement of 200 cubic inches and a stroke length of 3.75 inches. What is the bore?

Since N = 6, D = 200, and S = 3.75, the bore length is

B = √ 4(200)/(π(3.75)(6)) = 3.3642 in.

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