Compression Ratio Calculator
Calculate static compression ratio from swept and clearance volume.
How this calculator works
CR = (swept volume + clearance volume) ÷ clearance volume
This is static compression ratio. Dynamic compression, which accounts for when the intake valve actually closes, is what determines whether an engine will detonate — a long-duration cam lowers it considerably.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- SAE J1312 (Engine Power Test Code) 2020 — static compression ratio as swept plus clearance volume over clearance volume
Cited to a specific edition on purpose — table numbers and values move between cycles. Confirm against whichever edition is enforced locally.
Worked example
With bore 4 in, stroke 3.48 in, combustion chamber volume 64 , deck and gasket volume 10 , this calculator returns 10.68 .
Compression ratio for familiar engine builds
Each row is a link that fills in the calculator above. Send one and the other end sees the same figures you do.
| Job | Size | Compression ratio |
|---|---|---|
| Small-block 350, stock | 4 in × 3.48 in × 76 × 12 | 9.14 |
| Same block, smaller chambers | 4 in × 3.48 in × 64 × 10 | 10.68 |
| Bored 0.030 over | 4.03 in × 3.48 in × 64 × 10 | 10.83 |
| Stroker build | 4.03 in × 3.75 in × 64 × 10 | 11.59 |
| Boost-friendly, low compression | 4 in × 3.48 in × 96 × 14 | 7.51 |
This is static compression. What decides whether it detonates on pump fuel is dynamic compression, which the cam sets.
Frequently asked questions
What compression ratio can I run on pump gas?
Around 10.5:1 on 91 octane with an iron head, and up to 11.5:1 with aluminium heads, which shed heat faster. Modern engines with knock sensors and direct injection go higher still.
What is the difference between static and dynamic compression?
Static uses the full stroke. Dynamic starts from where the intake valve actually closes, which on a long-duration cam is well past bottom dead centre. Dynamic compression is what predicts detonation.
How does milling the head change compression?
It reduces chamber volume, raising the ratio. Removing 0.010 inch from a small-block head cuts roughly 1.5 cc, which lifts compression by about a tenth of a point.
Does higher compression always make more power?
Up to a point. Thermal efficiency rises with compression, but gains diminish above about 12:1 while detonation risk climbs steeply. Beyond that you need race fuel or careful knock control.