Free compression ratio calculator

Compression ratio calculator static, from measured volumes

Enter the bore, stroke, combustion chamber, piston dome or dish, head gasket and deck clearance to get the static compression ratio and every volume that goes into it.

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Lengths are in

Cylinder

Volume at top dead center

Piston top
Piston at TDC is

Dish, valve reliefs and a piston below the deck add volume; a dome or a piston above the deck takes it away. Use the piston maker's dome or dish figure, and the gasket maker's bore and compressed thickness.

Static compression ratio-
-

Per cylindercc

In short

Static compression ratio = (swept volume + clearance volume) ÷ clearance volume, per cylinder. Clearance is the chamber cc plus the piston dish (or minus the dome), the gasket volume (π ÷ 4 × gasket bore² × compressed thickness) and the deck volume. A 4.030 × 3.48 in cylinder with a 64 cc chamber, 5 cc reliefs, a 0.041 in gasket and 0.025 in deck is about 9.75:1.

How it works

How to use the Compression Ratio Calculator

1

Enter bore and stroke

In inches or millimeters; volumes are always in cc.

2

Add the TDC volumes

Chamber cc, piston dish or dome cc, gasket bore and compressed thickness, and deck clearance.

3

Read the ratio

The static ratio, a table of each volume, and the chamber cc for a target ratio.

What it does

What the compression ratio calculator works out

Every volume listed

Swept, chamber, piston, gasket and deck volumes per cylinder, in cc.

Dome or dish

A dish or valve reliefs add volume; a dome takes it away.

Deck either way

A piston below the deck adds volume; one above the deck subtracts it.

Target ratio

The chamber volume you'd need for a ratio, with the same piston, gasket and deck.

Inches or millimeters

Lengths in either unit.

Ratio
(swept + clearance) ÷ clearance, per cylinder
Clearance
chamber + piston (dish +, dome −) + gasket + deck
Gasket volume
π ÷ 4 × gasket bore² × compressed thickness
Deck volume
π ÷ 4 × bore² × deck clearance (piston above deck: negative)
Units
1 in³ = 16.387064 cc
FAQ

Questions about the compression ratio calculator

How do you calculate compression ratio?
Add the volume swept by the piston to the volume left at top dead center, then divide by the volume at top dead center. With 727.4 cc swept and 83.1 cc at TDC: (727.4 + 83.1) ÷ 83.1 = 9.75:1.
Does a domed piston raise compression?
Yes. A dome takes up space at top dead center, so you subtract its volume from the clearance volume. A dish or valve reliefs add volume and lower compression.
How does a thinner head gasket change compression?
It shrinks the gasket volume. On a 4.030 in bore engine with a 4.100 in gasket bore, going from 0.041 in to 0.027 in removes about 3 cc per cylinder, roughly 0.3 points of compression here.
What is the difference between static and dynamic compression?
Static compression comes from the volumes alone. Dynamic compression also allows for when the intake valve closes, so it's lower and depends on the cam.
How do I measure chamber and dome volume?
Fill the chamber from a burette through a clear plate and read the cc used. For a dome or dish, set the piston a measured distance down the bore, fill the space the same way, and compare the fluid used with the calculated cylinder volume for that distance.
Keep going

Guides and features for this

Engine displacement calculatorBayRobinGeneral auto repair shopsBayRobinDigital vehicle inspection guideGuideHow to explain repairs to customersGuideAll guides on running an auto repair shopTopic guides
Sources

Where the formulas and rules come from

Checked October 2026. A calculation from the volumes you enter. Use measured chamber and piston volumes and the gasket maker's figures.

Link to this tool

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