Compression Ratio Calculator
Calculate engine compression ratio from bore, stroke, deck clearance, and chamber volume.
Helps determine octane requirements and engine performance potential.
The compression ratio is one of the most fundamental specifications of an internal combustion engine. It describes how much the air-fuel mixture is compressed before ignition — and it directly determines power output, efficiency, and the octane rating of fuel required.
Formula: Compression Ratio = (Swept Volume + Clearance Volume) ÷ Clearance Volume
Or equivalently: CR = (V_swept + V_clearance) ÷ V_clearance
What each variable means:
- Swept Volume (V_swept): also called displacement per cylinder; the volume the piston sweeps from Bottom Dead Center (BDC) to Top Dead Center (TDC).
- Clearance Volume (V_clearance): the volume remaining above the piston at TDC (combustion chamber volume).
- Bore: the diameter of the cylinder.
- Stroke: the distance the piston travels.
Swept Volume Formula: V_swept = π ÷ 4 × Bore² × Stroke (all in the same units)
Worked example: Bore = 86 mm, Stroke = 86 mm, Clearance Volume = 56.4 cc
V_swept = π ÷ 4 × 86² × 86 = 499.6 cc CR = (499.6 + 56.4) ÷ 56.4 = 556 ÷ 56.4 = 9.86:1
This is a typical naturally-aspirated gasoline engine — it runs comfortably on 91–93 octane fuel.
Typical compression ratios:
- Naturally-aspirated gasoline: 9:1–12:1
- High-performance naturally-aspirated: 12:1–14:1
- Turbocharged gasoline: 8:1–10:1
- Diesel: 14:1–25:1 (diesels ignite by compression, not spark)
Higher compression = more power and efficiency, but requires higher-octane fuel to prevent knock.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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