Engine compression ratio is one of the most important numbers in automotive engineering. It governs power output, fuel efficiency, combustion stability, and the choice of fuel octane rating. This calculator solves the classic compression-ratio formula in five directions: find the CR from known dimensions, or reverse-engineer the bore, stroke, clearance volume, or swept volume needed to hit a specific target.
How it works
The swept volume (Vₛ) of a single cylinder is:
Vₛ = (π / 4) × bore² × stroke
where bore and stroke are in centimetres (divide millimetres by 10). The result is in cubic centimetres (cc). For example, a bore of 86 mm and a stroke of 86 mm gives Vₛ = (π/4) × 8.6² × 8.6 ≈ 499.9 cc per cylinder.
Compression ratio then follows from:
CR = (Vₛ + Vᴄ) / Vᴄ
where Vᴄ is the clearance volume — the space above the piston at TDC comprising the combustion chamber, deck-height gap, and compressed head gasket. This is the industry standard static compression ratio (as opposed to the dynamic CR, which accounts for inlet valve closing angle but is only relevant for high-revving race engines).
Reverse solve paths use simple algebra:
- Clearance needed: Vᴄ = Vₛ / (CR - 1)
- Bore needed: bore = √(4 × Vₛ / (π × stroke_cm)) × 10
- Stroke needed: stroke = Vₛ / ((π/4) × bore_cm²) × 10
Worked example
A performance four-cylinder engine with 86 mm bore, 86 mm stroke, and a measured clearance volume of 46.5 cc per cylinder:
- Vₛ = (π/4) × 8.6² × 8.6 = 499.9 cc
- CR = (499.9 + 46.5) / 46.5 = 546.4 / 46.5 = 11.75 : 1
- Total displacement = 499.9 × 4 = 1999.6 cc (2.0 L)
That puts the engine firmly in the “high-performance naturally-aspirated” band — suitable for 98 RON or 99 RON premium petrol.
To check a target instead: if you wanted 10.5 : 1 with the same bore and stroke, the required clearance volume would be Vᴄ = 499.9 / (10.5 - 1) = 52.6 cc per cylinder.
| Bore (mm) | Stroke (mm) | Clearance (cc) | CR | Total (4 cyl) |
|---|---|---|---|---|
| 86 | 86 | 52.6 | 10.5 : 1 | 2.0 L |
| 86 | 86 | 46.5 | 11.75 : 1 | 2.0 L |
| 86 | 86 | 41.7 | 13.0 : 1 | 2.0 L |
| 96 | 86 | 52.0 | 12.0 : 1 | 2.5 L |
Formula note
The formula used here is the static geometric compression ratio — the standard definition used by manufacturers, engine builders, and tuning software. A separate concept called dynamic compression ratio adjusts for the intake valve closing angle (relevant only for high-lift cam profiles at high RPM) but is not modelled here, since static CR is the figure stamped on engine data plates and used to select octane rating.
All calculations run entirely in your browser. No engine data is sent anywhere.