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Cylinder head flow to horsepower calculator

How much power a set of heads can support, based on their intake port flow. Useful when comparing heads, as long as you remember it's a ceiling, not a promise.

Enter your numbers
Flow bench numbers
Most published figures are at 28 in.

Horsepower potential

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The most the heads can support in a well-matched combination.
Flow corrected to 28 in
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Per cylinder
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A ceiling, not a promise

Heads only reach this potential with a cam, intake, exhaust and compression that let them. A mismatched combination will fall short of it, sometimes well short.

The rule of thumb

Horsepower potential ≈ peak intake flow (cfm at 28 in H₂O) × 0.257 × cylinders

This widely used rule, popularized by engine builder David Vizard, reflects how much air a two-valve naturally aspirated engine needs per horsepower. It gives a realistic upper limit for a well-developed combination.

Different test pressures

Flow at 28 in = measured flow × √(28 ÷ test pressure)

Flow benches are sometimes run at 10 or 25 inches of water instead of 28. Airflow rises with the square root of pressure, so the calculator corrects to 28 in first. Compare heads using figures from the same bench whenever you can.

Frequently asked questions

Why is this described as a ceiling and not a promise?

Cylinder head airflow sets the upper limit on how much power the engine could theoretically make, but the actual output also depends on cam timing, intake and exhaust design, compression, and tuning — a great set of heads on a poorly matched engine won't reach that ceiling.

What test pressure should my flow numbers be measured at?

This formula is calibrated around the common 28 inches of water test standard most flow benches and published head-flow data use — flow numbers taken at a different test pressure won't plug in directly without converting first.

Why does peak flow matter more than flow at any single lift point?

Peak intake flow reflects the port's best-case ability to feed the cylinder, which correlates well with the engine's overall power potential — flow at just one low-lift point can look fine on a head that actually chokes badly higher up, where power is actually made.

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