What is the formula for compression ratio?

Swept volume plus clearance volume, divided by clearance volume. Swept volume is π/4 × bore² × stroke for one cylinder; clearance volume is the chamber plus the gasket bore, the deck gap and the piston dish, with a dome subtracted.

What compression ratio can run on pump gas?

Roughly 10:1 on iron heads and up to about 11.5:1 on aluminium heads with careful timing, though cam duration, quench and altitude all shift the line. Above that, premium fuel becomes mandatory and race fuel or E85 eventually does.

How does boost change compression ratio?

It raises the pressure the piston starts with. Multiplying the static ratio by absolute manifold pressure over atmospheric gives a corrected figure — 9:1 at 10 psi behaves roughly like 15:1 naturally aspirated as far as detonation is concerned.

What is dynamic compression ratio?

The ratio measured from the point where the intake valve actually closes rather than from bottom dead centre. It is the number that predicts cranking pressure and detonation on the street, and it needs rod length and the intake closing angle to compute.

How much does milling the head change compression?

It depends on chamber shape, but a rough guide on a small-block V8 is about 0.15 to 0.2 points of compression per 0.010 inch removed. Enter your target ratio here and the tool gives the exact chamber volume you need.

Compression Ratio Calculator

Work out static, dynamic and boosted compression from bore, stroke, chamber, gasket, deck and piston.

The Compression Ratio Calculator runs entirely in your browser. Engine measurements you enter are computed on your device and never uploaded.

Open the Engine Displacement Calculator

About Compression Ratio Calculator

Compression ratio is the total cylinder volume at the bottom of the stroke divided by whatever is left at the top. The swept part is easy; the clearance volume is where builds go wrong, because it is the sum of four separate measurements — the combustion chamber, the head-gasket bore, the deck clearance between piston crown and block deck, and the piston's own dish or dome. Get one of them wrong by a couple of cubic centimetres and a supposedly pump-fuel engine turns into a detonation problem. This calculator breaks the clearance volume into those four contributions so you can see which one is driving the number, then works backwards to the chamber volume a target ratio would need.

Features

Static compression ratio from the full measurement stack
Clearance volume broken into chamber, gasket, deck and piston
Handles domed pistons and negative deck clearance
Total displacement in cc and cubic inches from the cylinder count
Effective ratio under boost, for choosing a safe static figure
Dynamic compression ratio from rod length and intake closing point
Solves backwards for the chamber volume a target ratio needs
Fuel guidance for the ratio you land on

How to use the Compression Ratio Calculator

Enter bore and stroke in millimetres
Add the chamber volume, gasket bore and thickness, deck clearance and piston dish
Read the static ratio and the clearance breakdown underneath
Add boost pressure or cam timing to see the effective and dynamic ratios
Set a target ratio to find the chamber volume that reaches it

Example

Input

Bore 101.6 mm · Stroke 88.4 mm · Chamber 64 cc
Gasket 104.14 × 1.016 mm · Deck 0.635 mm · Flat top

Output

Static compression: 10.21:1
Swept 716.7 cc · Clearance 77.8 cc

A stock 5.7-litre small block. Milling 6 cc from the chamber takes it past 11:1 — premium fuel territory.

Common errors & troubleshooting

The ratio comes out far higher than expected. — Check the deck clearance and gasket thickness. Both are small numbers over a large bore area, and leaving one out removes several cc of clearance volume, which moves the ratio by a full point.
A domed piston produces an impossible result. — Enter a dome as a negative piston volume, since it takes space away from the chamber. If the dome is larger than everything else combined the clearance volume goes negative and no ratio exists.
Dynamic compression looks much lower than static. — It should be. Dynamic compression measures from where the intake valve closes, not from bottom dead centre, so a long-duration cam bleeds cylinder pressure back into the intake and lowers the effective ratio without changing a single measured volume.

Frequently asked questions

What is the formula for compression ratio?: Swept volume plus clearance volume, divided by clearance volume. Swept volume is π/4 × bore² × stroke for one cylinder; clearance volume is the chamber plus the gasket bore, the deck gap and the piston dish, with a dome subtracted.
What compression ratio can run on pump gas?: Roughly 10:1 on iron heads and up to about 11.5:1 on aluminium heads with careful timing, though cam duration, quench and altitude all shift the line. Above that, premium fuel becomes mandatory and race fuel or E85 eventually does.
How does boost change compression ratio?: It raises the pressure the piston starts with. Multiplying the static ratio by absolute manifold pressure over atmospheric gives a corrected figure — 9:1 at 10 psi behaves roughly like 15:1 naturally aspirated as far as detonation is concerned.
What is dynamic compression ratio?: The ratio measured from the point where the intake valve actually closes rather than from bottom dead centre. It is the number that predicts cranking pressure and detonation on the street, and it needs rod length and the intake closing angle to compute.
How much does milling the head change compression?: It depends on chamber shape, but a rough guide on a small-block V8 is about 0.15 to 0.2 points of compression per 0.010 inch removed. Enter your target ratio here and the tool gives the exact chamber volume you need.

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