Compression Ratio Calculator

How it works

Swept = π/4 × (bore/10)² × (stroke/10) · Clearance = chamber + gasket + deck volumes · CR = (swept + clearance) ÷ clearance

Compression ratio compares the cylinder volume when the piston is at bottom dead centre (swept + clearance) to the volume at top dead centre (clearance alone). The swept volume is the short cylinder the piston carves out — bore area times stroke — while the clearance volume is everything left above the piston at TDC: the combustion chamber, the space the head gasket adds, and any deck clearance where the piston stops below the block deck. Working the bore and stroke in centimetres makes every volume come out in cc directly. A higher ratio extracts more work per cycle and wants higher-octane fuel to avoid knock.

Worked example

An 86 mm bore × 86 mm stroke gives a 58.1 cm² bore area and a 499.6 cc swept volume. A 50 cc chamber, a 1.0 mm gasket (5.8 cc) and 0.5 mm of deck (2.9 cc) total 58.7 cc of clearance, so CR = (499.6 + 58.7) ÷ 58.7 ≈ 9.5:1.

Frequently asked questions