About the Compression Ratio Calculator
This compression ratio calculator works out an engine’s static compression ratio — the cylinder volume with the piston at bottom dead centre divided by the volume left when it is at top dead centre. Enter bore and stroke, the combustion chamber volume of the head, head-gasket bore and compressed thickness, deck clearance and the piston’s dish or dome volume.
Engine builders use it when choosing pistons, heads and gaskets so the finished motor suits the fuel it will run on and whether it will be boosted. Small changes matter: a thinner gasket or a few cc smaller chamber can add half a point of compression, which may be the difference between running on regular and needing premium.
Switching between inches and millimetres shows a separate set of length fields, pre-filled with the same engine in each unit, so the result stays consistent. Enter dish and valve-relief volume as a positive number and dome volume as a negative number. Deck clearance is positive when the piston sits below the deck at TDC. The result is static compression; dynamic compression also depends on camshaft intake-valve closing.
With the default inputs, the static compression ratio is 9.63 : 1. Change any value above to recalculate instantly.
How to use the compression ratio calculator
- 1Choose inches or millimetres for the length measurements.
- 2Enter bore, stroke, gasket bore, gasket thickness and deck clearance.
- 3Enter the head’s chamber volume in cc (from the spec sheet or by cc-ing the head).
- 4Enter piston volume: positive for dish and valve reliefs, negative for a dome.
- 5Read the compression ratio and use the chart to see how different heads would change it.
Formula and method
Swept volume Vs is the cylinder volume the piston travels through: π ÷ 4 × bore² × stroke. Clearance volume Vc is everything left above the piston at top dead centre: the head’s combustion chamber, the cylinder of space formed by the compressed head gasket (π ÷ 4 × gasket bore² × thickness), the gap between the piston crown and the deck (π ÷ 4 × bore² × deck clearance), plus the dish volume of the piston (or minus a dome).
Volumes computed in cubic inches are converted with 1 in³ = 16.387064 cc; in millimetres, 1,000 mm³ = 1 cc. The static ratio ignores camshaft timing; dynamic compression, which determines detonation risk more directly, is always lower.
- CR
- Static compression ratio
- Vs
- Swept volume of one cylinder
- Vc
- Clearance volume at TDC
Worked examples
Small-block 350 with 64 cc heads
A 4.00 × 3.48 in cylinder sweeps 716.6 cc. Clearance is 64 cc chamber + 8.87 cc gasket + 5.15 cc deck + 5 cc piston reliefs = 83.0 cc. (716.6 + 83.0) ÷ 83.0 gives about 9.63:1.
.030 over, zero deck, domed pistons, 58 cc heads
Boring to 4.03 in raises swept volume to 727.4 cc. With a 58 cc chamber, zero deck clearance and a 5 cc dome, clearance falls to 61.9 cc, pushing compression to about 12.76:1 — race-fuel territory.
Small-block 350 entered in millimetres
Switching to millimetres fills in the same engine: 101.6 × 88.392 mm with a 104.14 mm gasket bore, 1.041 mm gasket and 0.635 mm deck clearance. Because 1 in = 25.4 mm exactly, the swept volume (716.6 cc), clearance (83.0 cc) and ratio (9.63:1) match the inch version to within rounding (the gasket thickness is rounded to 1.041 mm).
Metric 2.0 L four-cylinder
An 86 × 86 mm cylinder sweeps 499.6 cc, for 1,998 cc across four cylinders. Clearance is 40 + 4.16 + 0.58 − 2 = 42.74 cc, giving (499.6 + 42.7) ÷ 42.7 ≈ 12.7:1, typical of a modern direct-injection engine.
Frequently asked questions
What is a good compression ratio for pump gas?+
Most naturally aspirated engines on 91–93 octane pump fuel run roughly 9.5:1 to 11:1 with iron heads, and a little higher with aluminium heads and modern chamber designs. Boosted engines usually run 8:1 to 10:1.
How does head gasket thickness change compression?+
A thicker gasket adds clearance volume and lowers compression. On a 4-inch bore, each 0.010 in of gasket thickness is about 2.1 cc, which typically changes compression by around 0.2 to 0.25 points.
Should piston dome volume be positive or negative?+
In this calculator, a dish or valve reliefs add clearance volume, so enter them as positive cc. A dome takes up space in the chamber, so enter it as a negative number.
What is the difference between static and dynamic compression?+
Static compression uses the full stroke. Dynamic compression starts counting only after the intake valve closes, so a long-duration camshaft lowers dynamic compression and lets you run a higher static ratio without detonation.
How do I measure combustion chamber volume?+
Seal a clear plate over the chamber with valves and spark plug installed, then fill it from a graduated burette with a thin liquid. The volume dispensed in cc is the chamber volume.