Compression Ratio Calculator
Work out your engine’s static compression ratio in seconds. Enter the cylinder bore and stroke in millimetres, then the clearance volume in cc, that is the combustion chamber plus head gasket plus any deck clearance at top dead centre. The calculator returns the compression ratio expressed as X.X:1 along with the swept (displaced) volume of one cylinder in cubic centimetres. It is ideal for engine builders, tuners and hobbyists checking how a head skim, thicker gasket or different piston changes the CR.
How to use the compression ratio calculator
-
1
Enter bore and stroke
Type the cylinder bore and stroke in millimetres to set the swept volume.
-
2
Add the clearance volume
Enter the total volume above the piston at TDC in cc (chamber + gasket + deck).
-
3
Read the result
See the compression ratio as X.X:1 and the swept volume per cylinder in cc.
The compression ratio formula
Static compression ratio compares the total cylinder volume at bottom dead centre to the volume left at top dead centre:
swept volume = (π ÷ 4) × bore² × stroke
compression ratio = (swept volume + clearance volume) ÷ clearance volume
The swept volume is the cylinder the piston displaces between TDC and BDC. The clearance volume is everything that remains above the piston at TDC: the combustion chamber, the head-gasket bore, the deck clearance and any valve pockets or piston dish. Because bore and stroke are usually in millimetres, divide the swept volume by 1,000 to express it in cc.
Worked example
Take a single cylinder with an 86 mm bore, an 86 mm stroke and a 50 cc clearance volume.
- Swept volume = (π ÷ 4) × 86² × 86 ÷ 1000 ≈ 499.6 cc
- Compression ratio = (499.6 + 50) ÷ 50 ≈ 10.99:1
Drop the clearance volume (skim the head, thinner gasket) and the ratio climbs; raise it (dished pistons, thicker gasket) and the ratio falls.
How clearance volume moves the ratio
| Bore (mm) | Stroke (mm) | Clearance (cc) | Swept (cc) | Compression ratio |
|---|---|---|---|---|
| 86 | 86 | 60 | 499.6 | 9.33:1 |
| 86 | 86 | 50 | 499.6 | 10.99:1 |
| 86 | 86 | 45 | 499.6 | 12.10:1 |
| 86 | 86 | 40 | 499.6 | 13.49:1 |
Common pitfalls
- Mixing units. Keep bore and stroke in millimetres so the cc conversion (÷1000) holds; do not mix inches and millimetres.
- Forgetting gasket and deck. Clearance volume is not just the chamber, add the gasket bore volume and any deck height above the piston.
- Static vs dynamic CR. This is the static (geometric) ratio. Dynamic compression, which factors in valve timing, is always lower.
Frequently Asked Questions
It is the total volume left above the piston when it is at top dead centre: the combustion chamber, the head-gasket bore, the deck clearance, plus any piston dish or valve reliefs. Measure or sum all of these in cc and enter the total.
Naturally aspirated petrol engines typically run about 9:1 to 12:1, with high-performance or E85 builds going higher. Turbo and supercharged engines usually run lower, around 8:1 to 10:1, to control knock. Diesels are far higher, often 16:1 to 22:1.
It gives the static (geometric) compression ratio based purely on swept and clearance volumes. Dynamic compression, which depends on intake valve closing timing, is always lower and needs cam data to calculate.
The numbers you enter are sent to our server only to compute your results. They are not stored, saved or shared. In the step-by-step view, the values also travel in the page link between steps, so avoid sharing links that contain numbers you want to keep private.
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