What an engine compression test measures
During cranking, a compression gauge indicates how much pressure a cylinder develops. The results are influenced by the mechanical sealing of valves, piston rings and the head gasket, but also by the test procedure. A pressure number is not a direct measurement of a particular component's condition.
Two questions matter: How do the cylinders compare to one another? And do the readings satisfy the minimum and variation limits published for this exact engine? An engine can have evenly low readings. In that case, the cylinders may appear balanced while all failing the minimum specification.
A repeatable test matters more than a pretty number
Consult the service manual before testing; follow its instructions for engine temperature, fuel and ignition disabling, throttle position, cranking revolutions, gauge adapters and other steps. Engine parts can be hot and moving; do not improvise the procedure if you are unfamiliar with working around rotating components, fuel systems or ignition systems.
- Record the vehicle year, model, engine and test conditions before collecting data.
- Use a compression gauge suitable for the engine and take the readings in the same way for each cylinder, following the service manual.
- Record dry compression PSI for every cylinder, including unusually low or zero measurements.
- Use the published engine minimum and allowable variation if available. Never substitute a generic online number as a factory specification.
- Repeat suspicious readings using the approved method before considering major repairs.
What cylinder spread means
Our calculator uses the following formula for dry compression variation:
This expresses how far the lowest measured cylinder is below the highest. It does not tell you whether either measurement is normal for your engine.
For a simple illustration, a highest reading of 180 PSI and a lowest reading of 162 PSI produces a spread of 10%. Whether that is acceptable depends on the factory specifications, the test method, and measurement uncertainty.
Dry vs. wet compression results
A wet test adds a small amount of oil in the manner and quantity described by the service procedure before retesting. Increased readings can support further investigation into sealing at the piston rings or cylinder wall. But the difference alone cannot prove worn rings: test technique, oil quantity, carbon deposits and other variables can change a measurement.
| Observation | What it may justify checking next |
|---|---|
| One cylinder substantially below peers | Repeat testing; assess sealing, valve operation, head gasket and testing consistency. |
| All cylinders similarly low | Check factory minimum, cranking speed, gauge accuracy and the entire test method. |
| One wet reading rises | Investigate ring/cylinder sealing as one possibility; confirm with other tests. |
| All cylinders meet entered limits | No abnormality flagged by this comparison, but not proof the engine is defect-free. |
Compression and leak-down are complementary
Compression testing measures cranking pressure. A leak-down test applies compressed air to a positioned cylinder to measure leakage and may offer clues about the escape path. Read the leak-down interpretation guide for the distinctions. Neither test, by itself, justifies an expensive repair without confirming evidence.
What the free calculator does
Enter readings for 3, 4, 5, 6, 8, 10 or 12 cylinders. The calculator identifies the highest and lowest dry readings, computes spread, and compares against optional limits you supply. Optional wet results and leak-down percentages remain recorded alongside the dry measurements. Print or save a worksheet after testing; values stay in your browser.