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 | Imperial | Typical use |
|---|---|---|
| K57 | K4.2 | Light hazard (LH) |
| K80 | K5.6 | Ordinary hazard (OH) — the most common |
| K115 | K8.0 | Heavy OH, HHP, in-rack |
| K160 | K11.2 | High hazard, CMSA |
| K200 | K14.0 | CMSA, ESFR under lower ceilings |
| K360 | K25.2 | ESFR, the high-bay standard |
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 ties the permitted K-factors to the hazard class: K57 for LH, K80 or K115 for OH, and K80 through K240 for HHP and HHS. As the density rises, reaching the target with a small K produces unreasonable pressures, so K115 is preferred at 12.5 mm/min and above.
Concealed, flush and recessed bodies cannot in practice go above K115. The architect's wish for invisible sprinklers has a direct hydraulic consequence wherever a high density is required: achieving the same density with a smaller K multiplies the pump pressure.
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. The nominal value is used in the calculation; even if the real nozzle deviates by 5 percent the system stays safe, because the design density is already provided at the most unfavourable sprinkler.
How it is read on site
The K-factor is stamped on the sprinkler body or yoke arm. The colour code indicates the temperature rating, not the K-factor; the two are frequently confused. The spare sprinkler cabinet must hold spares of every K-factor and temperature type.
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 in practice. Where a high density is required, that limit pushes the pump pressure directly upwards.
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 · NFPA 13 (2025) · EN 12259-1 · FM Global Data Sheets. Definitions are for information; the full text of the standard governs in design.