Compression Ratio Calculator
Calculate static compression ratio from bore, stroke, chamber volume, gasket, deck clearance and piston dome or dish — plus the effective compression ratio once you add boost. The number that decides how much power and octane your engine needs.
How to Use
- Enter your bore and stroke, switching between mm and inches as you like. These set the swept volume the piston moves through.
- Enter the combustion chamber volume in cc (from the head spec or a cc measurement).
- Add the rest of the clearance volume: the head gasket bore and thickness, the piston-to-deck clearance, and the piston dome (use a negative cc) or dish (positive cc).
- Read the static compression ratio — the squeeze before the spark fires.
- Optionally add boost in psi to see the effective compression ratio, which is what really decides whether you will knock.
Gasket bore, thickness and deck clearance use the same length unit as bore/stroke. A flat-top piston is about 0 cc; a dome is negative, a dish is positive.
Show Work
About the Compression Ratio Calculator
Need a hand with automotive and vehicle projects? The Compression Ratio Calculator does the work for you — free, and right here in your browser. Calculate static compression ratio from bore, stroke, chamber volume, gasket, deck clearance and piston dome or dish — plus the effective compression ratio once you add boost. The number that decides how much power and octane your engine needs.
How it works
Type your numbers into the boxes. The answer shows up right away — you do not have to press a button. If you change a number, the answer changes too. So you can try different numbers and watch what happens, or check an answer you worked out yourself. Just make sure each box has the right kind of number in it.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What is a good compression ratio?
It depends on the engine and fuel. Most modern naturally-aspirated gasoline engines run roughly 10:1 to 12:1 on pump premium. Boosted engines usually run lower static compression — often 8.5:1 to 10:1 — to leave headroom for the pressure the turbo or supercharger adds. Diesels run far higher, around 16:1 to 22:1, because they rely on compression heat to ignite the fuel.
How does boost change compression?
Boost stacks on top of your static ratio. A turbo or supercharger packs extra air into the cylinder before the piston even starts squeezing, so the real pressure at ignition is much higher than the static number. The effective ratio shown here approximates that: a 9:1 engine at 14.7 psi of boost behaves roughly like an 18:1 engine in terms of cylinder pressure — which is why boosted builds drop static compression.
What is clearance volume?
Clearance volume is the small space left above the piston when it is at the very top of its stroke. It is the sum of the combustion chamber, the head gasket’s bore volume, the piston-to-deck gap, and any piston dish (which adds volume) minus any dome (which subtracts it). Shrinking clearance volume raises compression; growing it lowers compression.
Why does a piston dome lower the number I enter?
A domed piston sticks up into the chamber and takes up space, so it reduces clearance volume and raises compression — enter it as a negative cc. A dished piston has a bowl that adds space, lowering compression — enter it as a positive cc. A flat-top piston is roughly zero. Getting this sign right matters; a dome and a dish of the same size move the ratio in opposite directions.
Does higher compression always make more power?
Higher compression generally makes more power and better efficiency from the same fuel, but only up to the knock limit set by your octane. Push compression too high for your fuel and the engine knocks, which costs power and can destroy pistons. The goal is the highest compression your fuel and tune can safely support — see the linked knowledge-base article on compression ratio and octane.
How do I use the Compression Ratio Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Planning an engine build
Try different head gaskets, chamber sizes and pistons to hit a target compression ratio before you buy a single part.
Choosing a head gasket
See exactly how much a thicker or thinner gasket moves your compression, and dial in the squeeze you want.
Setting up a boosted combo
Check the effective compression with boost so you can pick a static ratio that stays safely under the knock limit on your fuel.
Matching fuel and octane
Know your real compression so you can run the right octane and avoid both knock and wasted money on premium you do not need.
Verifying a measured engine
Plug in cc measurements from a burette to confirm the actual compression of an assembled or freshly machined engine.
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