Compression Ratio Calculator
Calculate static compression ratio from bore, stroke, chamber volume, head gasket, deck clearance, and piston dome or dish volume.
Combustion chamber volume (cc'd or from specs)
Compressed thickness
Leave blank to use cylinder bore
Distance from piston top to deck surface (positive = below deck)
Positive = dome (raises CR), Negative = dish (lowers CR)
What is Compression Ratio?
Compression ratio (CR) is the ratio of the total cylinder volume when the piston is at the bottom of its stroke (BDC) to the volume remaining when the piston is at the top (TDC). It's expressed as a ratio like 10.5:1.
Why It Matters
CR directly affects an engine's thermal efficiency, power output, and fuel requirements. Higher compression extracts more energy from each combustion event, but also increases the risk of detonation (knock) if the fuel can't handle it.
- 8.5:1 – 10.5:1 — typical for pump gas street engines
- 10.5:1 – 12:1 — usually requires premium fuel (93 octane)
- 12:1+ — race fuel or very careful tuning with direct injection
- 7.5:1 – 9:1 — common in boosted builds to leave room for boost pressure
Static vs Dynamic Compression
This calculator gives static compression ratio — based purely on geometry. Dynamic (effective) compression ratio accounts for cam timing, specifically intake valve closing point. A cam with late intake closing reduces the effective compression because air escapes back out the intake before the valve closes.
The Formula
CR = (Swept Volume + Clearance Volume) / Clearance Volume
Where clearance volume = chamber volume + gasket volume + deck volume − dome volume (or + dish volume). Swept volume = π/4 × bore² × stroke.