Design density is the water flow per unit area. In SI it is given in mm/min, numerically identical to L/min/m²: 5 mm/min means 5 litres per square metre per minute.
Why a rainfall unit?
mm/min is in fact a rainfall unit: it states how many millimetres of water are applied to the surface each minute. That notation gives the engineer an intuitive check; 5 mm/min is 300 mm of rainfall an hour, an intensity never seen in nature. That is what a sprinkler does.
Unit conversion
- 1 mm/min is about 0.0245 gpm/ft²
- 1 gpm/ft² is about 40.75 mm/min
For example, the 0.20 gpm/ft² NFPA 13 gives for ordinary hazard corresponds to about 8.2 mm/min. EN 12845 uses 5.0 mm/min in the same hazard band — but because the area of operation differs, the total flows converge.
Density alone is meaningless
If a page states only "5 mm/min", the information is incomplete. Density is always read together with the area of operation, because the total flow is the product of the two:
Q = density × area of operation
OH-3 example: 5.0 mm/min × 216 m² = 1,080 L/min
EN 12845 values by hazard class
| Class | Density | Area (wet) | Area (dry) |
|---|---|---|---|
| LH | 2.25 mm/min | 84 m² | Not permitted |
| OH1 | 5.0 mm/min | 72 m² | 90 m² |
| OH2 | 5.0 mm/min | 144 m² | 180 m² |
| OH3 | 5.0 mm/min | 216 m² | 270 m² |
| OH4 | 5.0 mm/min | 360 m² | Not permitted → HHP1 |
| HHP1 | 7.5 mm/min | 260 m² | 325 m² |
| HHP2 | 10.0 mm/min | 260 m² | 325 m² |
| HHP3 | 12.5 mm/min | 260 m² | 325 m² |
Note that across the OH groups the density stays fixed and the area changes. Moving from OH1 to OH4 does not mean more water per square metre; it means wetting a larger area at the same density.
Why does the area grow on a dry system?
On dry and alternate installations, other sprinkler heads can open while the water is filling the pipe. EN 12845 compensates by increasing the area of operation by at least 25 percent. The density does not change; because the area grows, the total flow and the tank volume rise.
A common error: treating the density as an average. The design density must be achieved at every sprinkler within the area of operation, including the most remote and most unfavourable point. An average calculation leaves the far corner dry.
The minimum pressure rule
Even where the density calculation gives a low pressure, EN 12845 sets floors: 0.7 bar for LH, 0.35 bar for OH, and 0.5 bar for HHP and HHS. The rule is explicit — the density or that pressure, whichever is higher.
Frequently Asked Questions
What does 5 mm/min mean?
Five litres of water per square metre per minute. The unit mm/min is numerically identical to L/min/m² and is a rainfall unit.
Is density alone sufficient?
No. Density is always read with the area of operation; the total flow is the product of the two. For OH-3: 5.0 mm/min × 216 m² = 1,080 L/min.
Does the density increase on a dry system?
No, the density stays fixed; the area of operation grows. EN 12845 increases it by at least 25 percent on dry and alternate systems.
What is the conversion between mm/min and gpm/ft²?
1 mm/min is about 0.0245 gpm/ft²; in reverse, 1 gpm/ft² is about 40.75 mm/min.
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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.