What is the minimum pressure at a sprinkler?

The full app enforces a floor of 7 psi at any operating sprinkler by default. Confirm the value against the edition of NFPA 13 your authority having jurisdiction has adopted, and against the listing for the specific sprinkler, which may require more.

How do I go from flow back to pressure?

Square the ratio: p = (q / K)². The calculator shows the pressure the selected head would need to deliver 25 gpm, which is the check most designers do when sizing against a density.

Does K change with pressure?

No. K is a property of the orifice and is constant across the sprinkler's listed pressure range. It is taken from the manufacturer's listing, not calculated.

How does density relate to K?

Density is gallons per minute per square foot over the design area. Multiply the density by the coverage area of one head to get the minimum flow that head must deliver, then use q = K√p in reverse to find the pressure that flow requires.

Sprinkler K-factor calculator

Discharge from a sprinkler at a given pressure, and the pressure needed for a given flow.

Preliminary. The reference tables behind this calculation are pending a second check against their printed source. Verify against your adopted edition before using a result in a submitted calculation.

14.8gpm

discharge at 7.0 psi

Pressure for 25.0 gpm: 19.9psi
K-factor: 5.6gpm/psi^0.5

The formula

Q = K sqrt(P) (reversed: P = (Q / K)^2)

Q: discharge from the sprinklergpm
K: the sprinkler's K-factor, from its listinggpm/psi^0.5
P: pressure at the sprinklerpsi

The K-factor is not dimensionless

A K of 5.6 means 5.6 gpm per square root of a psi. The same sprinkler quoted in metric units carries a different number for the same orifice, so a K-factor copied from a European datasheet into a US customary calculation will be wrong by a factor of about 14.4. Read the units on the datasheet before you use the number.

Because flow goes as the square root of pressure, doubling the pressure at a head raises its flow by about 41 percent, not 100. That is why the most remote head governs a calculation and why upsizing a main further upstream buys less than it looks like it should.

Run the whole system, not one pipe. Same solver, pass or fail against your flow test, NFPA 13 report as a PDF.

Try the full calc

Questions about this calculation

What is the minimum pressure at a sprinkler?: The full app enforces a floor of 7 psi at any operating sprinkler by default. Confirm the value against the edition of NFPA 13 your authority having jurisdiction has adopted, and against the listing for the specific sprinkler, which may require more.
How do I go from flow back to pressure?: Square the ratio: p = (q / K)². The calculator shows the pressure the selected head would need to deliver 25 gpm, which is the check most designers do when sizing against a density.
Does K change with pressure?: No. K is a property of the orifice and is constant across the sprinkler's listed pressure range. It is taken from the manufacturer's listing, not calculated.
How does density relate to K?: Density is gallons per minute per square foot over the design area. Multiply the density by the coverage area of one head to get the minimum flow that head must deliver, then use q = K√p in reverse to find the pressure that flow requires.

The other calculators

How these results are checked against worked examples · Pricing for the full app

Updated 2026-09-07