Free Engineering Tool · NFPA 13
Sprinkler K-Factor Calculator
Solve the sprinkler discharge equation three ways: the flow at a given pressure, the pressure a sprinkler needs to deliver a flow, or the K-factor from a test point. Pick a nominal K or enter the listed value, switch between US and SI units, and subtract velocity pressure when the calculation uses normal pressure, following NFPA 13.
- Based on NFPA 13
- Free, runs in your browser; your inputs stay on your device
- Screening tool, not reviewed or sealed by a licensed engineer
- Last updated October 2026
Screening tool, not engineering advice. This calculator estimates results with simplified published methods. It has not been reviewed or sealed by a licensed engineer for your project, and DGA does not warrant that its results are accurate, complete or suitable for any use. Check every result independently and have the licensed professional responsible for your project review it before you rely on it. Use is subject to the Terms of Use.
Sprinkler
Solve For
Normal pressure (subtract velocity pressure)
Result
Flow and Pressure Curve
About This Tool
A sprinkler’s K-factor describes how much water its orifice passes for a given pressure. Flow follows the discharge equation Q = K · √P, so doubling the flow takes four times the pressure. In US units K is in gpm/psi½; in SI units it is in L/min/bar½, and the SI value is about 14.4 times the US value. NFPA 13 lists nominal K-factors from K-1.4 to K-33.6 (K-20 to K-480 in SI). The calculator uses the exact conversion and shows the result in both systems.
In a density and area design, the most remote sprinkler must deliver at least the design density times its area of coverage. The tool can take that product directly: enter the density and the area of coverage, and it returns the flow and the pressure P = (Q / K)2. NFPA 13 sets 7 psi (0.5 bar) as the minimum operating pressure for most standard spray sprinklers, so when the calculated pressure falls below it, the tool reports 7 psi and the larger flow that results.
Pressure in a flowing pipe has two parts. Normal pressure acts on the pipe wall and pushes water out through the orifice; velocity pressure is carried along with the flow. NFPA 13 lets the designer use normal pressure for sprinklers other than the end sprinkler on a line, with velocity pressure Pv = 0.001123 · Q2 / d4 (psi, with Q the upstream flow in gpm and d the inside diameter in inches) and Pn = Pt − Pv. Ignoring velocity pressure gives a slightly higher, more conservative demand.
Worked example
A light hazard room is protected by K-5.6 sprinklers on a 15 ft by 13 ft spacing (195 ft2) at 0.10 gpm/ft2. Each sprinkler needs 0.10 × 195 = 19.5 gpm, which takes (19.5 / 5.6)2 = 12.1 psi. With K-8.0 sprinklers the same flow needs only 5.9 psi, so the 7 psi minimum governs and each sprinkler actually discharges 8.0 × √7 = 21.2 gpm, which raises the system demand.
Assumptions and limits
The calculator assumes water at ordinary temperatures and uses the K-factor as listed for the sprinkler. ESFR, extended coverage, residential and other special sprinklers have listed minimum pressures and flows that override the density method, and those listings govern. Velocity pressure applies only to flowing pipe upstream of the sprinkler.
When to bring in an engineer
One sprinkler is the start of a hydraulic calculation, not the end. A remote area calculation carries every sprinkler, pipe, fitting and elevation change back to the water supply. DGA prepares sealed hydraulic calculations for new systems and modifications; the friction loss and supply analysis tools cover the next steps.
References
- NFPA 13, Standard for the Installation of Sprinkler Systems, National Fire Protection Association, 2025 edition: sprinkler discharge characteristics and K-factors, minimum operating pressure, hydraulic calculation procedures and the velocity pressure formula.
- U.S. Department of the Interior, Bureau of Reclamation, Water Measurement Manual, 3rd edition, 2001: orifice discharge and velocity head.
Frequently Asked Questions
How do you calculate sprinkler flow from a K-factor?
Multiply the K-factor by the square root of the pressure at the sprinkler: Q = K x sqrt(P). A K-5.6 sprinkler at 20 psi flows 5.6 x 4.47 = 25.0 gpm. In SI units, a K-80 sprinkler at 1.4 bar flows 80 x 1.18 = 94.7 L/min.
How do you convert a K-factor from US to SI units?
Multiply the US K-factor (gpm per psi to the one-half power) by about 14.4 to get the SI K-factor (L/min per bar to the one-half power). K-5.6 becomes K-80 and K-25.2 becomes K-360 in the nominal SI designations.
What is the minimum pressure at a sprinkler?
NFPA 13 sets 7 psi (0.5 bar) as the minimum operating pressure for most standard spray sprinklers. ESFR, extended coverage, residential and other special sprinklers have minimum pressures or flows in their listings, and those govern when they are higher.
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Engineering aid only, provided as is and without warranty. Results depend on the inputs and on the simplifying assumptions of the method summarized on each tool page, and DGA has not verified them for your application. They do not replace code-required design, testing or review by the licensed engineer responsible for the project. Use of the tools is subject to the Terms of Use.
Related Reading
Articles from DGA Insights on the code and the method behind this calculator.