Compression Ratio Calculator for Engine Builds
How do you calculate compression ratio? Static compression ratio is the cylinder volume at bottom dead centre divided by the volume left at top dead centre. Enter the bore, stroke and every volume that makes up the combustion space: chamber, head gasket, deck clearance and piston dish. Use a positive piston volume for a dish and a negative one for a dome.
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Formula
swept = (pi / 4) x bore^2 x stroke
gasket = (pi / 4) x gasket bore^2 x compressed thickness
deck = (pi / 4) x bore^2 x deck clearance
clearance = chamber + gasket + deck + piston dish (a dome is negative)
CR = (swept + clearance) / clearance
Lengths in mm convert to cc by dividing the mm3 result by 1000. Inches are converted at 25.4 mm. The static ratio ignores the intake valve closing point.
Worked example
Bore 86 mm, stroke 86 mm, chamber 50 cc, gasket bore 88 mm and 1.0 mm thick, deck clearance 0.5 mm, piston dish 5 cc:
- Swept volume: 0.7854 x 86 x 86 x 86 / 1000 = 499.56 cc
- Gasket: 0.7854 x 88 x 88 x 1.0 / 1000 = 6.08 cc
- Deck: 0.7854 x 86 x 86 x 0.5 / 1000 = 2.90 cc
- Clearance: 50 + 6.08 + 2.90 + 5 = 63.99 cc
- CR = (499.56 + 63.99) / 63.99 = 8.81 : 1
Swapping the dish for a 5 cc dome gives 53.99 cc of clearance and 10.25 : 1.
Use this when
You are planning a rebuild, a head or piston swap, or a decking cut, and want to see how each part changes the ratio. Chamber volume is the most sensitive number, so measure it.
Related calculators
The engine displacement calculator gives the swept volume of the whole engine, which this page uses per cylinder. The fuel injector size calculator is the next step once power targets change.
Common mistakes
- Entering the piston dome as a positive number. A dome is negative.
- Using the uncompressed gasket thickness instead of the compressed one.
- Mixing cc with cubic inches or mm with inches.
- Treating the static ratio as the effective ratio of a long-cam engine.
Frequently asked questions
How is static compression ratio calculated?
CR = (swept volume + clearance volume) / clearance volume. With a swept volume of 499.56 cc and a clearance volume of 63.99 cc, CR = 563.54 / 63.99 = 8.81 to 1.
What goes into the clearance volume?
The combustion chamber in the head, the head gasket volume (pi/4 x gasket bore squared x thickness), the gap between the piston top and the block deck, and any piston dish. A dome subtracts volume. Piston ring land crevice volume is ignored here, which makes the answer slightly high.
How do I enter a domed piston?
Enter the dome volume as a negative number. Changing the 5 cc dish in the example to a 5 cc dome cuts the clearance volume from 63.99 cc to 53.99 cc and lifts the ratio from 8.81 to 10.25 to 1.
What is dynamic compression ratio and does this calculator give it?
Dynamic compression ratio uses only the part of the stroke after the intake valve closes, so a late-closing cam lowers it. This page gives the static ratio only. As a rough rule, a later intake closing point reduces the effective ratio, which is why cammed engines can run a higher static ratio on the same fuel. Measure the real value with a cylinder pressure test.
What compression ratio is safe for my fuel?
That depends on the fuel octane, boost, cam timing, combustion chamber design and tuning, so no single number applies. Treat the result as one input to an engine builder's decision, not as a guarantee against knock.
What is a good compression ratio for a petrol engine?
As typical figures, naturally aspirated road engines often run roughly 9:1 to 11:1, and turbocharged ones are usually lower. For example an 86 mm x 86 mm cylinder with a 45 cc chamber, 0.8 mm gasket, 0.3 mm deck and 8 cc dish gives 9.38 to 1. The right value depends on fuel, boost and tuning.
How do I calculate compression ratio from cc?
Add the swept volume and the clearance volume, then divide by the clearance volume. An 84 mm bore by 90 mm stroke has 498.76 cc swept, and with a 40 cc chamber and nothing else that is (498.76 + 40) / 40 = 13.47 to 1.
How much does head gasket thickness change compression?
A thinner gasket reduces clearance volume and raises the ratio. In the 86 mm example, changing the gasket from 1.0 mm to 0.5 mm lifts the ratio from 8.81 to 9.2 to 1, and a 1.5 mm gasket lowers it to 8.45 to 1.