The K-factor reduces how much water a sprinkler passes at a given pressure to a single number. It is an indirect expression of the orifice size and sits at the centre of sprinkler selection.
The formula
Q = K √P
In the metric system Q is in litres per minute and P in bar. So a K80 sprinkler delivers 80 L/min at 1 bar. At 4 bar it is 80 × root 4 = 160 L/min. Hold those two points and you can verify most site calculations by hand.
Two notations, one sprinkler
The same sprinkler is sold as K80 in Europe and K5.6 in America. The difference is the unit: L/min and bar on the EN side, gpm and psi on the NFPA side. The conversion factor is approximately 14.4.
| Metric | US units | K range, metric (NFPA Table 7.2.2.1) | Main use |
|---|---|---|---|
| K57 | ≈ K4.0 (not an NFPA nominal value; NFPA K4.2 = K60, range 57–63) | — | EN 12845: light hazard (LH) |
| K80 | K5.6 | 76–84 | EN 12845: ordinary hazard (OH) — the most common |
| K115 | K8.0 | 107–118 | EN 12845: OH, HHP/HHS ceiling, in-rack |
| K160 | K11.2 | 159–166 | EN 12845: HHP/HHS ceiling; smallest K for NFPA CMSA |
| K200 | K14.0 | 195–209 | ESFR |
| K360 | K25.2 | 349–387 | ESFR and CMSA |
For every nominal value (K20 to K480), K-factor ranges and thread types, see the complete K-factor table.
Which pressure is P in the formula?
P is the net pressure at the sprinkler nozzle inlet — not the pump discharge pressure. Pipe friction losses, static head and hose flow are added on the pump side; the flow of a single sprinkler is always calculated from its own inlet pressure.
Which K in which hazard class?
EN 12845 Table 37a ties the permitted K-factors to the hazard class: K57 for LH; K80 or K115 for OH; for HHP and HHS ceiling sprinklers K80, K115 or K160 at densities up to 10 mm/min and only K115 or K160 above 10 mm/min; K80 or K115 for HHS in-rack sprinklers. As the density rises, reaching the target with a small K produces unreasonable pressures, which is why the standard excludes K80 above 10 mm/min.
For concealed, flush and recessed sprinklers the highest K under EN 12845 is K115: Table 37a lists these bodies only in the LH and OH rows, and clause 14.2.2 prohibits them in OH4, HHP and HHS areas. The architect's wish for invisible sprinklers can therefore be met only in light and ordinary hazard areas.
The most expensive error: entering a metric K value while the software unit set is gpm and psi. The results inflate by a factor of 14.4 and the error usually goes unnoticed until commissioning. Verify one sprinkler's flow by hand before issuing the report.
Tolerance
EN 12259-1 defines a plus or minus 5 percent tolerance on the nominal K value, and NFPA 13-2025 Table 7.2.2.1 gives a range of the same order for each nominal K (76–84 for K80, for example). The nominal value is used in the hydraulic calculation; for dry-type sprinklers NFPA refers to the adjusted K-factor in 28.2.4.10.3.
How it is read on site
The K-factor is identified from the sprinkler's marking (NFPA 7.2.1: manufacturer letters and a sprinkler identification number that identifies the K-factor, deflector characteristic, pressure rating and thermal sensitivity). The colour code indicates the temperature rating, not the K-factor; the two are frequently confused. The spare sprinkler stock must include all types, orientations and ratings installed (NFPA 13-2025 16.2.7.5; EN 12845 20.1.4 also requires adequate spares of high-temperature, sidewall and other patterns).
Frequently Asked Questions
How much does a K80 sprinkler deliver at 1 bar?
80 L/min. Because Q = K root P, at 1 bar root 1 is 1 and Q equals the K value directly. At 4 bar it delivers 160 L/min.
Are K80 and K5.6 the same?
Yes, the same sprinkler. K80 is metric (L/min per root bar) and K5.6 imperial (gpm per root psi); the conversion factor is about 14.4.
Is P in the formula the pump pressure?
No. P is the net pressure at the sprinkler nozzle inlet. Pipe losses and static head are added on the pump side.
What is the highest K in a concealed sprinkler?
K115 under EN 12845. Table 37a allows concealed bodies only in the LH (K57) and OH (K80 or K115) rows, and clause 14.2.2 prohibits them in OH4, HHP and HHS areas.
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Download MEP Calc on the App StoreEN 12845:2015+A2:2026 · NFPA 13 (2025) · EN 12259-1 · FM Global Data Sheets. Definitions are for information; the full text of the standard governs in design.