Compression Ratio Calculator
Bore, stroke, chamber, gasket and deck. Enter what you have and get the static compression ratio and total displacement.
Engine dimensions
- This is static compression ratio. Dynamic ratio depends on camshaft timing and is always lower.
- Deck clearance is negative when the piston rises above the block deck at top dead centre.
- Measure your gasket compressed thickness, not the thickness in the box.
What goes into the ratio
Compression ratio is the cylinder volume at bottom dead centre divided by the volume at top dead centre. The swept volume comes from bore and stroke. Everything else is clearance volume, and there are four separate contributions to it that people routinely forget.
- The combustion chamber in the head.
- The head gasket, using its bore and its compressed thickness.
- Deck clearance, the gap between the piston crown and the block deck.
- The piston itself, a dish adding volume or a dome subtracting it.
Leave out the gasket and the deck and you will calculate half a point higher than the engine actually runs.
How much ratio can you run
On pump premium with an iron head, around 9.5:1 is comfortable. Aluminium heads shed heat better and let you run roughly a point higher for the same fuel. Modern chamber design and direct injection push that further again.
Past about 11:1 on 91 octane you are relying on the knock sensor pulling timing to save the engine, which means you paid for compression you never get to use. Above 12:1 plan on race fuel or E85.
Static is not dynamic
The number this calculator gives you is static compression, which assumes the intake valve shuts at bottom dead centre. It does not. A long duration camshaft holds it open well past that, so some of the charge gets pushed back out and the cylinder never sees its full swept volume.
That is why a big cam lets you run more static compression on the same fuel. The dynamic ratio, the one that decides whether it detonates, is lower than the number on the spec sheet.
Frequently asked questions
How do I measure combustion chamber volume?
Seal the chamber with a plexiglass plate and grease, then fill it through a small hole with a graduated burette of light oil or alcohol. The volume that goes in is your chamber cc. Casting numbers give you the nominal figure, but any head that has been milled or ported no longer matches it.
How much does milling the head raise compression?
Roughly, taking 0.010 inch off a small block V8 head removes about 1.5 to 2cc from the chamber, which is worth around a tenth to two tenths of a point. Chamber shape decides the exact figure, so measure rather than assume. Remember that milling also changes the intake manifold alignment.
Can I raise compression with a thinner head gasket?
A little. Going from 0.040 to 0.028 inch on a 4 inch bore removes about 2.5cc, worth roughly a tenth of a point. It also brings the piston closer to the head and to the valves, so check both clearances before you commit.
What compression ratio for a turbo build?
Lower than naturally aspirated, because boost adds its own cylinder pressure. Traditional builds sat at 8.0 to 8.5:1. Modern engines with direct injection and good knock control run 9.5 to 10.5:1 boosted quite happily. How much boost, what fuel and what the knock control can do all move the answer.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.