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Volume Ratio to Ideal Isothermal Pressure

Calculate the absolute-pressure change for a fixed amount of ideal gas held at constant temperature.

Formula & worked example
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How to use this calculator

Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

Compressing an ideal gas from 10 volume units to 1 at constant temperature multiplies absolute pressure by 10. Starting at 14.7 psi absolute gives 147 psi absolute.

Final absolute pressure = initial absolute pressure × initial volume / final volume

Worked example

Enter these known values and leave the other values blank.

Initial volume ratio term: 10
Final volume ratio term: 1
Initial absolute pressure: 14.7 psi

Final ideal absolute pressure: 147 psi

Assumptions and limitations

The amount of ideal gas and its temperature remain constant. This is Boyle’s-law scaling, not a general thermodynamic compression model.
Both ratio terms and pressures must be positive. Use the same volume unit for the two ratio terms; their common scale cancels.
Both pressures are absolute. A gauge pressure must be converted to absolute pressure using the applicable ambient reference before entry.
A combustion engine compression ratio alone cannot determine a compression-test gauge reading. Valve timing, temperature change, cranking speed, leakage and other details are outside this model.
A ratio below one represents expansion at constant temperature. The retained equations support either positive ratio direction.

Common questions

Is this a prediction for an engine compression test?

No. It calculates an ideal constant-temperature pressure ratio. A real compression test needs a different physical model and the relevant test conditions.

Why must the pressures be absolute?

Boyle’s law relates gas pressure measured from vacuum. Gauge pressure uses a shifted zero and cannot be substituted directly into the ratio.

References

Pounds-force per square inch use the exact international pound and inch with standard gravity, replacing the rounded captured factor.

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