Engine Compression Ratio Calculator
Calculate static compression ratio and swept displacement from measured cylinder, chamber, gasket, deck and signed piston volumes.
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Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Find swept cylinder volume and combine all top-dead-center clearance components with explicit signs before calculating their ratio.
CR=(swept+clearance)/clearance; clearance=chamber+gasket+signed deck+signed piston
Worked example
An 86 mm bore and 86 mm stroke sweep about 499.557 cm³ per cylinder. A 50 cm³ chamber, 5 cm³ dish, zero deck and a 1 mm gasket with 87 mm opening produce about 9.197 ratio.
Assumptions and input details
Static geometric calculation only. Enter measured compressed gasket thickness; gasket opening must be at least bore diameter. A dome or piston protrusion contributes negative volume under this page’s explicit convention; combined clearance must remain positive. Dynamic valve timing, knock, fuel choice, mechanical clearance and engine safety are not inferred.
References: JE Pistons: compression ratio and displacement geometry
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
The 36 volume definitions distinguish US liquid, US dry and Imperial measures. Cups carry an explicit definition. Japanese shō is 2401/1331 L and a gō is one tenth of it; the explicitly labelled 180 mL rice cup is separate.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · UK Imperial definitions · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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