Compression Ratio Calculator

Calculate static compression ratio from bore, stroke, head chamber, head gasket, deck clearance and piston dish or dome. Check what a thicker gasket or new pistons would do before you buy.

Static compression ratio
8.85 : 1
Swept volume per cylinder
399.5 cc
Clearance volume
50.9 cc
chamber + gasket + deck + piston

Example values are illustrative, not a specific engine's factory specs. Measure your own chamber and deck height before ordering parts.

The formula

CR = (swept volume + clearance volume) ÷ clearance volume. Swept volume is one cylinder's displacement. Clearance volume is everything left when the piston is at the top: the combustion chamber in the head, the head gasket's thickness times its bore area, the gap between the piston and the top of the block, and any dish in the piston (a dome subtracts).

Compression and boost

Higher compression makes more power and better economy, but brings the engine closer to knock. Turbo builds often lower compression with a thicker head gasket or dished pistons so they can run more boost on pump fuel. On a 1.6 NA Miata, a head gasket 0.5 mm thicker takes compression from 9.4 to about 9.0. Try it above.

Static vs dynamic compression

This calculator gives static compression, the geometric number. Dynamic compression also accounts for when the intake valve closes, which depends on the camshaft. Two engines with the same static ratio can behave very differently with different cams.