Compression Ratio Calculator: Static CR from Bore, Stroke, Chamber, Head Gasket and Piston

October 20, 2026

Compression ratio calculator: enter bore, stroke, chamber volume, head gasket, deck clearance and piston dish or dome to get static compression ratio, displacement, and what a different head gasket would do.

Compression ratio calculator

Engine

Start from

Bore and stroke in

Bore (mm)

Stroke (mm)

Cylinders

Clearance volume

Combustion chamber (cc)

measured volume of the chamber in the head

Head gasket bore (mm)

the gasket's own bore, which can differ from the cylinder

Gasket and deck in

Gasket compressed thickness (in)

Deck clearance (in)

piston below the deck at TDC = positive, above = negative

Piston top

dish and reliefs add volume, a dome takes it up

Dish / dome volume (cc)

from the piston maker - enter as a positive number

Result

Example inputs shown - pick your engine above or enter your own measurements.

- static compression ratio

Clearance volume, per cylinder

Part	cc

Same engine, different head gasket thickness

Gasket thickness	mm	Compression ratio

How compression ratio is calculated

Static compression ratio compares the volume above the piston at bottom dead center with the volume left when it reaches top dead center:

Compression ratio = (swept volume + clearance volume) ÷ clearance volume Swept volume = π ÷ 4 × bore² × stroke Clearance volume = chamber + gasket + deck + piston dish (or - dome) Gasket volume = π ÷ 4 × gasket bore² × compressed thickness Deck volume = π ÷ 4 × bore² × deck clearance

This is the standard formula published by JE Pistons, HP Academy and Grassroots Motorsports. Everything is per cylinder; displacement is swept volume times the cylinder count.

Static vs. dynamic compression

This calculator gives the static compression ratio. JE Pistons points out that dynamic compression also depends on cam timing: a high-compression engine loses some compression pressure if the intake valve stays open after the piston starts the compression stroke (the intake valve closing point). If you go to a thinner head gasket or deck the block or head, re-check piston-to-valve clearance - JE says to check it whenever an engine is built or the heads or valvetrain are changed, and measures it with the head gasket that will be used.

Getting the signs right

A dish or valve reliefs add clearance volume and lower compression; a dome takes up clearance volume and raises it. Piston makers don't all print it the same way - JE treats a dome as negative and a dish as positive, while MAHLE lists a dished piston as a negative crown volume - so the calculator asks which type you have and takes the volume as a positive number. Deck clearance follows HP Academy's convention: positive when the piston sits below the deck at top dead center.

Use the gasket's bore, not the cylinder's. HP Academy defines gasket volume by the gasket's own bore and its compressed thickness, and notes that the gasket bore is frequently different from the cylinder bore - Cometic's MLS gasket for the BMW M30B34/B35, for example, has a 93 mm bore on a 92 mm cylinder.

Factory specs for the presets

Engine	Bore x stroke (mm)	Factory CR

Bore and stroke come from BMW technical data (including the M60, M70 and S70 V8 and V12 engines), BMW training manuals and press kits, FIA homologation forms, GM and Acura/Honda specifications. The Toyota 2JZ-GTE figures are from HP Academy. The M20B25 compression ratio is from BMW's 1985 E30 brochure (325i: 9.7:1, 9.0:1 with catalyst); FIA Group N form N-5292 lists the same 9.7:1 as the maximum. Where no factory compression ratio could be verified, the table leaves it blank rather than guess. Factory combustion chamber volumes are only filled in where the manufacturer publishes them (GM LS3 68 cc, LS7 70 cc) - for everything else, measure your heads.

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