🔧 Engine Compression Ratio Calculator

Compression ratio is the ratio of the total cylinder volume when the piston is at bottom dead center (BDC) to the remaining volume (combustion chamber volume) when the piston is at top dead center (TDC).

CR = (Vswept + Vchamber + Vgasket + Vdeck) ÷ (Vchamber + Vgasket + Vdeck)

Vswept is the per-cylinder displacement swept by the piston moving from BDC to TDC, Vchamber is the combustion chamber volume, Vgasket is the volume occupied by the head gasket, and Vdeck is the volume created by the deck clearance between the piston crown and the top of the cylinder. If there is no gasket or deck clearance, you can leave those at 0.

For example, with a swept volume of 500 cc and a chamber volume of 50 cc (no gasket or deck volume), the compression ratio is (500+50) ÷ 50 = 11.0 : 1.

Under the same conditions, adding 5 cc of head gasket volume increases the denominator (chamber + gasket + deck) to 55 cc, lowering the ratio to (500+55) ÷ 55 ≈ 10.09 : 1. This is why a thicker gasket or larger deck clearance lowers compression ratio.

Using the bore/stroke input mode, a cylinder with an 86 mm bore and 86 mm stroke gives a swept volume of (π/4) × 86² × 86 ÷ 1000 ≈ 499.56 cc, calculated automatically.

This calculator computes static compression ratio, based purely on the cylinder's geometric volumes. The value more closely tied to actual engine behavior and knock is the dynamic compression ratio, which also accounts for intake valve closing timing (cam timing) and is always lower than the static ratio.

A higher static compression ratio generally requires higher-octane fuel, and forced-induction (turbo/supercharged) engines often run a lower static compression ratio than naturally aspirated (NA) engines. Combustion chamber volume is most accurately measured by removing the head and cc-ing it — filling it with liquid to measure the exact volume. This tool computes static CR only.