Related Expert Tools
More precision tools in the everyday-life niche.
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Boat Speed Calculator
The Boat Speed Calculator works out actual boat speed in knots, mph, and km/h from a distance and time you enter, and separately estimates the theoretical hull speed of a displacement-hull boat from its waterline length. It supports nautical miles, statute miles, and kilometres. Use it to check your average speed on a passage or to see the practical top speed a displacement hull can reach efficiently.
Boost Horsepower Calculator
The Boost Horsepower Calculator estimates how much horsepower a turbocharger or supercharger adds at a given boost pressure, starting from your engine's naturally aspirated baseline. It applies an adjustable efficiency factor to account for intercooling, heat, and tuning quality rather than assuming a perfect, lossless gain. Use it to sanity-check a boost target before committing to a turbo, injector, or fuel system upgrade.
Compression Ratio to PSI Calculator Logic
What Is the Compression Ratio to PSI Calculator?
This calculator estimates the cylinder pressure a compression test gauge should read during cranking, based on the engine's static compression ratio. It uses the polytropic compression formula, pressure equals atmospheric pressure multiplied by compression ratio raised to a polytropic exponent, the same approximation engine builders and technicians use to sanity-check a compression test result against what an engine's known specifications predict. According to Calculate.co.nz's technical explainer on this formula, an exponent of 1.3 is the most widely used approximation for real engines under cranking conditions.
Mechanics and DIY enthusiasts use it two ways: forward, to predict what a healthy engine's compression test should read before performing the test, and in reverse, to sanity-check whether an actual gauge reading is in the right ballpark for the engine's known compression ratio, which can help distinguish a genuine mechanical problem from a normal reading that simply looked lower than expected.
The Polytropic Compression Formula
The formula used is pressure equals atmospheric pressure multiplied by compression ratio to the power of n, where n is the polytropic exponent. An exponent of 1.0 gives the simplest linear estimate and represents a lower bound; 1.4 represents the ideal adiabatic (no heat loss) case and is an upper bound; 1.3 sits between the two and is the figure most commonly cited as matching real cranking-speed compression tests, since some heat is lost during compression but not the theoretical zero-loss amount adiabatic compression assumes.
At sea level with a compression ratio of 9:1 and an exponent of 1.3, the formula gives approximately 132 psi, a useful reference point: most healthy engines with a 9:1 to 9.5:1 static compression ratio produce cranking compression test readings in the 120 to 160 psi range, once real-world losses below the theoretical figure are accounted for.
Why Real Cranking Tests Read Below the Theoretical Figure
An actual compression test gauge reading typically comes in 10 to 20 percent below the theoretical polytropic estimate. According to discussion among engine builders on BobIsTheOilGuy's technical forum, this gap comes from cranking speed being far slower than running speed, valve timing that was optimised for running conditions rather than cranking, and minor leakage past rings and valve seats that matters more at the lower pressures and speeds of a cranking test than it would at operating RPM.
A well-known empirical rule of thumb among engine builders multiplies compression ratio by 17 to 20 to estimate real-world cranking PSI directly, without going through the polytropic formula at all. This calculator shows both figures side by side, the polytropic theoretical estimate and its realistic 80 to 90 percent range, alongside the independent 17 to 20 times rule of thumb, so you can cross-check one against the other.
What a Low or Uneven Compression Reading Actually Means
A cylinder reading meaningfully below this calculator's expected range, or significantly lower than its neighbouring cylinders, points toward a real mechanical issue: worn piston rings, a burnt or leaking valve, or a head gasket problem between adjacent cylinders. A cylinder reading within the expected range but on the lower end is not automatically a problem; cylinder-to-cylinder variation of 10 to 15 percent is considered normal on many engines, and the absolute number matters less than consistency across all cylinders.
Accuracy and Limitations
This is an engineering estimate based purely on static compression ratio, not a diagnostic tool that accounts for your specific engine's condition, camshaft timing, or cranking speed. Camshaft timing in particular changes the effective (dynamic) compression ratio the engine actually experiences at cranking speed, which can shift real-world readings further from this calculator's static-ratio-based estimate than the general 10 to 20 percent real-world discount alone would suggest, especially on engines with aggressive cam profiles.
Use this calculator to establish a reasonable expected range before testing, not as a pass/fail threshold on its own. A genuine diagnosis should always compare actual readings across all cylinders for consistency, not just against this calculator's theoretical output. If you need to work out the compression ratio itself first, our compression ratio calculator computes it from bore, stroke, and chamber dimensions.