Compression Ratio Calculator
Static CR from bore, stroke, and chamber volume.
About this calculator
Static compression ratio compares the total cylinder volume at the bottom of the stroke to the volume remaining at the top -- CR = (swept volume + clearance volume) / clearance volume. Swept volume is what the piston physically displaces as it travels from bottom dead center to top dead center, calculated from Bore and Stroke using the cylinder volume formula (pi/4 x bore^2 x stroke). Clearance volume is everything left over at top dead center: the combustion Chamber volume cast into the head, the volume added by the compressed Head Gasket thickness and bore, and the Piston dome/dish -- a dished piston adds clearance volume (lowering CR), while a domed piston displaces into the chamber and reduces it (raising CR), which is why this calculator treats a negative Piston dome/dish value as a dish and positive as a dome.
The Effective CR (est.) is a rougher, separate estimate of dynamic compression -- what the cylinder actually compresses to once the intake valve closing point is accounted for, since a late-closing camshaft lets some of the intake charge push back out before the compression stroke effectively begins, so dynamic CR always reads lower than static CR for the same hardware. This calculator assumes a typical street-cam intake valve closing point rather than reading your actual camshaft specification, so treat Effective CR as a ballpark, not a substitute for a real dynamic-compression calculation using your cam card's actual numbers. Recommended Octane is a rule-of-thumb guide based on static CR alone and does not account for forced induction, ignition timing, cylinder head design, or fuel quality variation -- always defer to your engine builder's actual tuning requirements.
Inputs
Results
Static compression ratio
10.23
Effective CR (est.)
7.92
How to Use This Calculator
- Enter Bore (inches), Stroke (inches), and Number of cylinders.
- Set Chamber volume (cc), Gasket bore (inches), and Gasket thickness (inches).
- Adjust Piston dome/dish (cc) as needed.
- Review Static compression ratio and Effective CR (est.).
- Use Swept vol/cyl (cc) and Clearance volume (cc) to inform your decision.
How the result changes with Bore (inches)
| Bore (inches) | Static compression ratio | Effective CR (est.) |
|---|---|---|
| 2 | 3.31 | 2.73 |
| 3 | 6.19 | 4.89 |
| 6 | 21.76 | 16.57 |
What each input means
- Bore (inches)
- Cylinder bore diameter in inches.
- Stroke (inches)
- Piston stroke length in inches.
- Number of cylinders
- Total cylinder count.
- Chamber volume (cc)
- Combustion chamber volume from head casting (measured via burette method).
- Gasket bore (inches)
- Head gasket bore opening diameter (slightly larger than cylinder bore).
- Gasket thickness (inches)
- Compressed head gasket thickness.
- Piston dome/dish (cc)
- Negative = dished piston (adds clearance), positive = domed piston (reduces clearance).
What each result means
- Static compression ratio
- Geometric compression ratio: (swept + clearance) / clearance.
- Effective CR (est.)
- Dynamic CR estimate based on typical intake valve closing at 60° ABDC.
- Swept vol/cyl (cc)
- Displacement per cylinder in cubic centimeters.
- Clearance volume (cc)
- Total clearance volume per cylinder (chamber + gasket - dome + dish).
- Gasket volume (cc)
- Volume contributed by the head gasket.
- Total displacement (cc)
- Total engine displacement across all cylinders.
- Total displacement (ci)
- Total engine displacement in cubic inches.
- Total displacement (L)
- Total engine displacement in liters.
How this is calculated
Worked example, using the default values
- Identify Input Parameters7 parametersBore (inches) = 4, Stroke (inches) = 3.48, Number of cylinders = 8, Chamber volume (cc) = 64, Gasket bore (inches) = 4.1, Gasket thickness (inches) = 0.04, Piston dome/dish (cc) = -5 = 7 input(s) provided
- Calculate Static compression ratioStatic compression ratio = (sweptVolumePerCylCc + clearanceVolumeCc) / max(1, clearanceVolumeCc)10.23 = 10.23
- Calculate Effective CREffective CR = (effectiveSwept + clearanceVolumeCc) / max(1, clearanceVolumeCc)7.92 = 7.92
- Calculate Swept vol/cylSwept vol/cyl = (π / 4) * pow(boreIn, 2) * strokeIn * 16.387716.6 = 716.6
- Calculate Clearance volumeClearance volume = chamberCc + gasketVolumeCc - pistonCc77.7 = 77.7
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does a dished piston lower compression ratio while a domed piston raises it?
Clearance volume is the space the compressed air/fuel mixture occupies at the top of the stroke, and compression ratio gets smaller as that space gets bigger. A dished piston has a bowl cut into its crown, which adds extra volume to the clearance space at top dead center -- more clearance volume means a lower ratio. A domed piston does the opposite: it physically displaces into the combustion chamber, shrinking clearance volume and raising the ratio for the same head and block.
Is Effective CR the number I should use to pick my fuel octane?
Static CR (the geometric ratio calculated from your hardware dimensions) is what this calculator's Recommended Octane guide is based on, but Effective CR -- the dynamic compression achieved once intake valve closing timing is factored in -- is generally the better predictor of detonation risk in practice, since a late-closing performance camshaft can meaningfully reduce effective cylinder pressure even with a high static ratio. This calculator's Effective CR uses a typical street-cam assumption rather than your actual cam specs, so read it as a rough directional figure rather than a tuning-grade number.
Does head gasket thickness meaningfully change compression ratio?
Yes -- Gasket thickness (inches) directly sets the gap between the deck surface and the cylinder head, and that gap adds volume to the clearance space the same way a dished piston does. A thicker gasket increases clearance volume and lowers static CR; a thinner gasket (or removing material via head milling) reduces clearance volume and raises it, which is why engine builders treat gasket selection as a real compression-ratio tuning variable, not just a sealing choice.
How much does increasing bore diameter change the compression ratio?
Bore has an outsized effect because swept volume scales with the SQUARE of bore diameter (pi/4 x bore^2 x stroke), while a bigger bore also slightly increases gasket volume through Gasket bore. In practice, boring an engine oversize during a rebuild raises both total displacement and static compression ratio somewhat, which is one reason clearance volume (chamber, gasket, and piston dish/dome together) needs to be re-verified after any overbore rather than assumed unchanged.
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