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Calculator Answers

Compression Ratio Calculator

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

A compression ratio calculator is an automotive engineering utility used to calculate an engine's static compression ratio. It compares total cylinder volume when the piston is at Bottom Dead Center (BDC) with clearance volume at Top Dead Center (TDC). It accounts for cylinder bore, stroke, cylinder head combustion chamber, piston head design, gasket thickness/bore, and deck clearance.

Determine your engine's static compression ratio. Enter cylinder measurements, head gasket dimensions, piston head design volumes, and deck clearances to calculate your engine displacement and CR.

Quick Answer

Calculate static engine compression ratio (CR). Input bore, stroke, combustion chamber, piston head, gasket, and deck specs.

Engine Specs

in
in
cc

Cylinder head combustion volume.

cc

Dish (+ positive) or Dome (− negative).

in
in
in

Piston distance below block deck at TDC.

Compression Ratio Result

Static compression ratio

9.65:1

Total displacement: 716.6 cc

Cylinder displacement716.62 cc
Clearance volume82.8 cc
Total cylinder volume (BDC)799.42 cc
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Examples

350 Chevy: 4.0 in bore, 3.48 in stroke, 64 cc chamber, 5 cc dish, 0.040 in gasket

Compression Ratio: 9.77:1

80 mm bore, 90 mm stroke, 45 cc chamber, 2 cc dome, 1.0 mm gasket

Compression Ratio: 10.51:1

How it works

Compression ratio is calculated by comparing total cylinder volume at Bottom Dead Center (BDC) with clearance volume at Top Dead Center (TDC):

Compression Ratio:

CR = (Cylinder Displacement (Vd) + Clearance Volume (Vc)) ÷ Vc

Displacement Volume (Vd):

Vd = π × (Bore ÷ 2)² × Stroke

Clearance Volume (Vc):

Vc = Chamber Vol + Piston Vol + Gasket Vol + Deck Vol

Dish piston volumes should be entered as positive numbers (adds to clearance volume), while dome piston volumes should be entered as negative numbers (subtracts from clearance volume).

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Frequently asked questions

Compression ratio (CR) is the ratio of the volume of the cylinder and its combustion chamber when the piston is at the bottom of its stroke (maximum volume) to the volume when the piston is at the top of its stroke (minimum volume).

A higher compression ratio extracts more mechanical energy from a given mass of air-fuel mixture, increasing thermal efficiency and power. However, too high of a CR can lead to engine knocking or pre-ignition (detonation) if the fuel octane rating is too low.

Piston heads can have a dish (concave cavity that increases chamber volume and lowers compression ratio) or a dome (convex shape that protrudes into the chamber, reducing volume and increasing compression ratio). A flat-top piston has a head volume of 0 cc (excluding valve relief pockets).

Deck clearance is the distance between the top of the piston and the top of the cylinder block (deck) when the piston is at Top Dead Center (TDC). The volume of this space adds to the total clearance volume.