The EN notation from K57 to K360, the SI formula Q = K × √P, and how to choose the right K-factor.
On a logistics warehouse project I was called in to review a calculation. The design team had used K57 sprinklers for an OH3 area, because "it was the most common one in the catalogue". The hydraulic calculation passed on paper, but at site acceptance the most remote sprinkler delivered 0.4 bar — not even half the 5.0 mm/min design density. Enlarging the pump was not the answer either, because the losses were already at their limit. The only fix was moving to K80 and increasing pipe diameters at several points. The cause was a single number: the K-factor.
What the K-factor is, and where the EN notation comes from
The K-factor is the constant describing a sprinkler nozzle's flow-pressure characteristic. EN 12845 and EN 12259-1 define it in SI units. The basic formula:
Q = K × √P
Q is sprinkler flow in L/min and P is the pressure at the nozzle inlet in bar. This underpins every hydraulic calculation in EN 12845. It belongs to a different K notation family from the NFPA 13 version, Q (gpm) = K × √P (psi) — confuse them and the calculation comes out wrong by an order of magnitude.
The EN K values form a standardised series: K57, K80, K115, K160, K200, K240, K280, K320, K360. These numbers are the approximate value of Q divided by √P. A K80 sprinkler discharges 80 L/min at 1.0 bar, and 80 × √2.25 = 120 L/min at 2.25 bar.
Which K-factor for which hazard class?
The pre-calculated tables fix the K selection indirectly. The HHP and HHS pressure and flow requirements are given explicitly for K80 and K115 — so the standard itself defines K limits for particular densities. Our working selection on site:
| Hazard class | Recommended K | Typical use |
|---|---|---|
| LH | K57 or K80 | Hotel rooms, offices, schools |
| OH1, OH2 | K80 | Restaurants, car parks, office blocks |
| OH3, OH4 | K80 | Manufacturing, shopping centres, workshops |
| HHP1 (7.5 mm/min) | K80 | Small spray booths, plastic injection |
| HHP2/3 (10–12.5 mm/min) | K115 | Rubber, foam, plastics processing |
| HHS category III/IV | K115 or K160 | High racking, ST4 pallet racks |
| CMSA / high control mode | K200, K240 | High-bay storage, special projects |
| ESFR (EN 12845-2) | K200, K280, K320, K360 | Fast suppression, high racking |
Using Q = K√P in practice
Design density (mm/min), sprinkler coverage area (m²) and K-factor are locked together. The minimum flow a sprinkler must deliver:
Qmin = density × area
For OH3: 5.0 mm/min × 12 m² = 60 L/min. Which K delivers that, and at what pressure?
| K-factor | Pressure needed for 60 L/min | Comment |
|---|---|---|
| K57 | (60/57)² = 1.11 bar | Marginal; will not hold at the remote point |
| K80 | (60/80)² = 0.56 bar | Comfortable; the standard choice |
| K115 | (60/115)² = 0.27 bar | Below the EN 12845 minimum — not valid |
This is where the field rule bites: EN 12845 requires at least 0.5 bar at the sprinkler inlet. Using K115 for 60 L/min solves the formula but the standard does not accept it. At its 0.5 bar minimum a K115 head delivers 81 L/min — excessive for OH, appropriate for HHP.
Converting to NFPA notation
Ask a European supplier for K80 and the equivalent on an American list is K5.6. The conversion table:
| EN K (SI) | NFPA K (US) | Note |
|---|---|---|
| K57 | K4.2 | L/min·bar-1/2 versus gpm·psi-1/2 |
| K80 | K5.6 | The most common wet system sprinkler |
| K115 | K8.0 | The HHP/HHS standard |
| K160 | K11.2 | High flow, storage |
| K200 | K14.0 | Storage CMSA, entry-level ESFR |
| K240 | K16.8 | ESFR high racking |
| K280 | K19.6 | ESFR / large drop |
| K320 | K22.4 | ESFR-25 |
| K360 | K25.2 | ESFR high ceiling |
The rough rule: EN K ≈ NFPA K × 14.4. NFPA K8.0 × 14.4 = 115.2, i.e. K115. It derives from the combined unit conversions of litres to gallons and bar to psi; because the standards bury different square-root units in the constant, the numbers look different while the physical nozzle is identical.
Common field errors
1. Using K57 for ordinary hazard
Chosen because it is the cheapest in the catalogue. As shown above, achieving 60 L/min with K57 needs 1.1 bar at the sprinkler. Add static head, pipe losses and ring losses and the pump pressure climbs to 6–7 bar. Moving to K80 eases the pump, the pipe sizing and the certification all at once.
2. Mixing K-factors
Do not mix K80 and K115 heads on the same distribution pipe within one area of operation. The EN 12845 hydraulic calculation assumes a single K type; a mixture does not guarantee the flow at the most remote head. At transitions between HHP and OH, a separate control valve set is required.
3. Feeding an NFPA K value into the EN formula
Spreadsheets arriving from abroad frequently take K5.6 and write Q = 5.6 × √P in bar — producing a flow fourteen times too low. Check the K notation first; supplier catalogues usually print both values side by side.
4. Forgetting the 0.5 bar minimum
EN 12845 never operates a sprinkler below 0.5 bar. Even if the hydraulic software says "mathematically sufficient", a calculation with the most remote head below 0.5 bar will not be accepted.
The technical difference from NFPA 13
NFPA 13 describes the same physics with different notation: Q in gpm, K as a single decimal figure such as 5.6, 8.0 or 11.2, and P in psi. EN works entirely in SI: L/min and bar. If a design team is using NFPA calculations on an EN project, every K value and every pressure and flow unit must be converted — convert only some of them and the error hides itself somewhere in the model.
Application in Turkey
BYKHY requires sprinkler installations to comply with TS EN 12845, so the K notation is the SI series K57 to K360. A common site situation: a European supplier's sprinkler is labelled K80 while an American import reads K5.6. If both carry UL/FM or EN approval, they are physically the same nozzle. Do not reject a product because the label notation differs — read the listing.
Frequently asked questions
Is the EN K-factor the same as the NFPA K-factor?
No. The numerical values come from different unit sets, so the same nozzle reads differently in each notation. EN K80 and NFPA K5.6 are the same product.
Is K57 sufficient for OH1?
In practice, no. At its 0.5 bar minimum a K57 head delivers only 40 L/min; achieving the OH1 requirement of 5.0 mm/min × 12 m² = 60 L/min needs over 1.1 bar, which hurts the pressure budget. K80 is the standard choice.
When is K115 used?
K115 becomes necessary at HHP and HHS design densities of 12.5 mm/min and above. Reaching the same density with K80 would push the sprinkler pressure to around 6 bar, making pump selection impossible.
Is P in the formula the pressure after losses?
No — P is the net pressure at the sprinkler nozzle inlet in bar. Pipe losses and static head are added to the pump pressure; individual sprinkler flow is always calculated from its own inlet pressure.
What is the K-factor tolerance?
EN 12259-1 permits ± 5 % on the nominal K value. Calculations use the nominal figure; since the design is based on the worst-case sprinkler, a 5 % deviation still leaves the system safe.
SprinkCalc — Fire Sprinkler Design Across Three Standards
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 Fixed firefighting systems — Automatic sprinkler systems. EN 12845-2:2024 (CMSA & ESFR sprinkler systems). NFPA 13 Standard for the Installation of Sprinkler Systems. Turkish Regulation on Fire Protection of Buildings (BYKHY). FM Global Property Loss Prevention Data Sheet 2-0.