Compression ratio compares the cylinder volume with the piston at the bottom of its stroke to the volume left when it is at the top. Higher compression extracts more energy from the fuel but needs higher-octane fuel to avoid knock; most modern petrol engines run 10:1 to 13:1, diesels 15:1 to 20:1.
How to use the Compression Ratio Calculator
- Bore
- Stroke
- Cylinders
- Combustion chamber volume
- Piston dish (+) or dome (−) volume
- Head gasket bore
- Head gasket thickness (compressed)
- Deck clearance (piston below deck)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the bore, stroke and number of cylinders, the combustion chamber volume, the piston dish (positive) or dome (negative) volume, the head gasket bore and compressed thickness, and the deck clearance (piston below deck positive).
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Bore | Units: mm, in |
| Stroke | Units: mm, in |
| Cylinders | A number |
| Combustion chamber volume | Units: cc, in³ |
| Piston dish (+) or dome (−) volume | Units: cc, in³ |
| Head gasket bore | Units: mm, in |
| Head gasket thickness (compressed) | Units: mm, in |
| Deck clearance (piston below deck) | Units: mm, in |
What each result means
- Swept volume per cylinder = pi÷4×[Bore]^2×[Stroke]
- Clearance volume = [Combustion chamber volume] + [Piston dish (+) or dome (−) volume] + pi÷4×[Head gasket bore]^2×[Head gasket thickness (compressed)] + pi÷4×[Bore]^2×[Deck clearance (piston below deck)]
- Compression ratio = ([Swept volume per cylinder] + [Clearance volume])÷[Clearance volume]
- Engine displacement = [Swept volume per cylinder]×[Cylinders]
Example values
How the main result changes when Bore changes and everything else stays at the example values above:
| Bore (mm) | Compression ratio (: 1) |
|---|---|
| 43 | 3.22 |
| 64.5 | 5.94 |
| 86 | 9.66 |
| 107.5 | 14.3 |
| 129 | 19.78 |
| 172 | 32.76 |
Formula
Swept volume Vd = π/4 × bore² × strokeClearance volume Vc = chamber + piston dish (− dome) + π/4 × gasket bore² × gasket thickness + π/4 × bore² × deck clearanceCR = (Vd + Vc) ÷ VcWorked example
Inputs — Bore: 86 mm; Stroke: 86 mm; Cylinders: 4; Combustion chamber volume: 45 cc; Piston dish (+) or dome (−) volume: 5 cc; Head gasket bore: 87 mm; Head gasket thickness (compressed): 1 mm; Deck clearance (piston below deck): 0.3 mm.
Results — Swept volume per cylinder = 499.6 cc; Clearance volume = 57.69 cc; Compression ratio = 9.66 : 1; Engine displacement = 1.998 L.
Frequently asked questions
What is a normal compression ratio?
9:1 to 13:1 for petrol engines (higher with direct injection), 15:1 to 20:1 for diesels.
How do I raise compression?
Use domed pistons, a thinner head gasket, a smaller chamber (milling the head) or reduce deck clearance.
What is dynamic compression?
The effective ratio after the intake valve closes, which is lower than static compression and decides knock tendency with a given camshaft.
What formula does the Compression Ratio Calculator use?
Swept volume Vd = π/4 × bore² × stroke.