About this tool
Compression Ratio Calculator is a free, in-browser tool that computes an engine's static compression ratio from its build specifications. It adds up all the clearance volumes above the piston at top dead centre and compares them to the swept volume of the cylinder. Every value stays on your device — nothing is uploaded.
Swept volume is (π ÷ 4) × bore² × stroke ÷ 1000 (cc). The head-gasket volume is (π ÷ 4) × gasket bore² × thickness ÷ 1000, and the deck-clearance volume is (π ÷ 4) × bore² × deck height ÷ 1000. The total clearance volume is the combustion-chamber cc plus the gasket and deck volumes plus the piston dish (a positive dish adds volume; a dome is entered as a negative number and subtracts). The compression ratio is (swept + clearance) ÷ clearance, shown as X.X:1.
Use it to plan a build: change the gasket thickness, mill the head (smaller chamber) or swap pistons and watch the ratio move. Enter zero in any optional field — gasket, deck or piston — that does not apply to your engine.
Frequently asked questions
How is compression ratio defined?
It is the total cylinder volume at bottom dead centre divided by the volume at top dead centre: (swept volume + clearance volume) ÷ clearance volume. The clearance volume is everything left above the piston at the top of its stroke.
How do I enter a domed versus dished piston?
A dish (a bowl in the piston crown) adds clearance volume, so enter it as a positive number of cc. A dome fills part of the chamber, so enter it as a negative number — it subtracts from the clearance volume and raises the ratio.
What if I have no head gasket or zero deck clearance?
Leave those fields at zero. The gasket and deck terms become zero and drop out of the clearance volume, so only the combustion chamber and piston volumes remain.
Does this give static or dynamic compression?
Static compression — the geometric ratio from the build volumes. Dynamic compression, which also depends on camshaft timing and intake-valve closing, is lower and is not calculated here. Everything is computed locally in your browser.
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