Compression Ratio Calculator — Engine CR from Bore & Stroke
Enter bore, stroke, and clearance volume to find the compression ratio (CR) of a petrol or diesel engine. The calculator also shows swept volume per cylinder and total engine displacement for your chosen cylinder count.
mm
mm
cc
Number of cylinders
Ratio of maximum cylinder volume (at BDC) to clearance volume (at TDC)
- 1
Swept volume per cylinder
(π ÷ 4) × 86² × 86 ÷ 1000 = 499.56 ccVolume displaced by the piston as it travels from BDC to TDC. - 2
Compression ratio
(499.56 + 56) ÷ 56 = 9.92
How does this calculator work?
Compression ratio CR = (V_d + V_c) / V_c, where V_d = (π/4) × bore² × stroke is the per-cylinder swept volume and V_c is the clearance volume at top dead centre. Typical range: 9–12:1 for petrol, 14–23:1 for diesel. Higher CR gives better efficiency but needs higher-octane fuel to avoid knock.
Formula
How this is calculated
The compression ratio compares the maximum cylinder volume (piston at bottom dead centre, BDC) to the minimum cylinder volume (piston at top dead centre, TDC). At TDC only the clearance volume V_c (combustion chamber) remains; at BDC the swept volume V_d is added. Dividing (V_d + V_c) by V_c gives CR. The swept volume per cylinder is derived from the bore and stroke: V_d = (π/4) × bore² × stroke, converting from mm³ to cc by dividing by 1 000.
Typical CR values: naturally aspirated petrol engines 9–12:1; high-performance petrol 12–15:1; diesel engines 14–23:1 (diesel needs higher CR to reach the auto-ignition temperature without a spark plug). Turbocharged petrol engines often run a lower static CR (7–10:1) because the boost pressure raises effective cylinder pressure. Increasing CR improves thermal efficiency and power output but raises knock risk, requiring higher-octane fuel.
This calculator gives the static (geometric) compression ratio from cylinder geometry alone. The effective (dynamic) compression ratio, which is slightly lower due to late intake valve closing timing, cannot be computed from bore and stroke alone — it requires cam timing data. Use the static CR as an upper bound.
Frequently asked questions
Most modern naturally aspirated petrol engines run 10–12:1 on regular-grade fuel. High-performance engines may push 13–15:1 on premium fuel. Above ~13:1 most pump fuels will cause knock (pre-detonation) without additional knock-suppression strategies.
The clearance volume includes the combustion chamber recess in the cylinder head, any bowl in the piston crown, and the compressed head gasket volume. It is usually quoted in engine specifications. You can measure it directly by inverting the head and filling the chamber with a calibrated syringe until it is flush with the deck surface.
No. Compression ratio is a per-cylinder property — it depends only on the bore, stroke, and clearance volume of one cylinder. Adding more cylinders increases total displacement and power potential but does not change CR.
Also known as
TG we-Calculate Editorial Team. (2026). Compression Ratio Calculator — Engine CR from Bore & Stroke [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/compression-ratio-calculator
TG we-Calculate Editorial Team. "Compression Ratio Calculator — Engine CR from Bore & Stroke." TG we-Calculate. 2026. https://we-calculate.com/calculator/compression-ratio-calculator.
TG we-Calculate Editorial Team, "Compression Ratio Calculator — Engine CR from Bore & Stroke," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/compression-ratio-calculator
@misc{wecalculate_compression_ratio_calculator, title = {Compression Ratio Calculator — Engine CR from Bore & Stroke}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/compression-ratio-calculator}}, year = {2026}, note = {TG we-Calculate} }
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