Compression Ratio Calculator
Calculate an engine compression ratio from swept volume and clearance volume.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Compression ratio is (swept volume + clearance volume) ÷ clearance volume. Bore and stroke are entered in inches; clearance volume is per cylinder in cc. Compression ratio alone cannot determine required fuel octane; use engine-specific manufacturer or tuner guidance.
Worked example
calculated with the default values below
Bore 3.5 in · Stroke 3.5 in · Clearance volume per cylinder 60 cc · Number of cylinders 4 · Target compression ratio 10 :1 Compression ratio 10.20 :1 · Swept volume per cylinder 551.8 cc · Total displacement 2,207 cc · Clearance volume for your target 61.3 cc
General background references
Disclaimer
These calculators give estimates for general information only. Real fuel consumption, performance and range vary with driving style, load, terrain, temperature and vehicle condition. Do not rely on these results for towing limits, payload or braking decisions — use the rating plate of your vehicle.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
CR = (V_s + V_c) ÷ V_c; V_s = π × (bore ÷ 2)² × stroke × 16.387
- bore
- bore diameter in inches
- stroke
- stroke length in inches
- V_s
- swept volume per cylinder in cc
- V_c
- clearance volume in cc
- CR
- compression ratio
Uses swept cylinder volume plus clearance volume. Cylinder count must be a whole number from 1 to 16; dimensions and clearance must be positive.
Frequently asked questions
Can compression ratio alone determine the fuel octane I need?
It calculates static compression ratio from bore, stroke and clearance volume: (swept volume + clearance volume) ÷ clearance volume. The default inputs give about 10.20:1. The calculator does not recommend fuel octane; engine design, calibration, boost and the manufacturer’s specification matter, so use engine-specific guidance.