What displacement actually measures
Displacement is the total volume that all the pistons sweep through as they move from the bottom of their travel to the top. It is a rough proxy for how much air-fuel mixture the engine can process per cycle, which is why it correlates loosely with power and is used to band engines into classes.
Each cylinder is treated as a circular tube: the bore is its diameter and the stroke is how far the piston travels. The area of the bore circle times the stroke gives one cylinder’s swept volume, and multiplying by the cylinder count gives the engine’s total.
Why millimetres come out as cc
With bore and stroke entered in millimetres, the raw volume is in cubic millimetres. Dividing by 1000 converts that to cubic centimetres (cc), the unit engines are usually quoted in. Litres are simply cc divided by 1000 again, and cubic inches use the fixed conversion of 16.387 cc per cubic inch for the imperial figure.
Bore versus stroke
Two engines can share the same displacement with very different characters depending on how that volume is split between bore and stroke.
- A short-stroke (oversquare) engine — bore larger than stroke — tends to rev higher and make peak power up top.
- A long-stroke (undersquare) engine favours low-end torque and a relaxed redline.
- A square engine (bore equal to stroke) sits between the two and is a common all-round compromise.
What displacement does not tell you
Displacement is only the mechanical size of the engine. Actual power output also depends on compression ratio, valve and port design, forced induction, fuel and ignition tuning, and engine speed. A small turbocharged engine can easily out-power a larger naturally aspirated one, so use displacement as a starting point rather than a power rating.
Formula
displacement = (π/4)·bore²·stroke·cylindersFrequently asked questions
- Why is displacement in cc when I enter millimetres?
- A volume in cubic millimetres divided by 1000 is cubic centimetres. The formula already accounts for this so the result comes out in cc.