Sprinkler hydraulics has two fundamental inputs: the design density (mm/min) and the area of operation (m²). Together they determine the flow the system has to meet. TS EN 12845+A2:2026 gives these two values by hazard class in tables under Clause 7, and sets out in Clause 13.4 how they are to be verified in fully calculated systems. Source: TS EN 12845+A2:2026, Clauses 7.1 and 13.4, EN p.35-36 and p.98-102 (TSE p.38-39 and p.101-105).
1. Definitions
| Term | Definition | Clause |
|---|---|---|
| Design density | The minimum discharge density in millimetres per minute for which a sprinkler installation is designed, determined from the discharge in litres per minute of a given group of sprinklers divided by the area in square metres covered by them | Clause 3.19, EN p.17 |
| Area of operation | The maximum area over which it is assumed, for design purposes, that sprinklers will operate in a fire | Clause 3.9, EN p.16 |
| Area of operation, hydraulically most favourable | The location in an array of sprinklers of an area of operation of specified shape at which the water flow, for a given pressure measured at the control valve set, is a maximum | Clause 3.10, EN p.16 |
| Area of operation, hydraulically most unfavourable | The location in an array of sprinklers of an area of operation of specified shape at which the water supply pressure measured at the control valve set, to give the specified design density, is a maximum | Clause 3.11, EN p.16 |
2. Clause 7.1: LH, OH and HHP
Clause 7.1 (EN p.35) sets the basic rule:
The design density shall not be less than the relevant value given in this clause, with all the ceiling or roof sprinklers in the room concerned or those in the area of operation, whichever is the lesser, plus any in-rack and supplementary sprinklers, in operation.
The minimum requirements for design density and area of operation are given in Table 3; for HHS systems Clause 7.2 applies. The NOTE to the clause states that in pre-calculated systems the design criteria are met by applying the water supply and pipework requirements specified elsewhere in the standard (see Clauses 7.3, 9.3.2.2 and 10.7).
Table 3: LH, OH and HHP design criteria
| Hazard class | Design density (mm/min) | Area of operation, wet or pre-action (m²) | Area of operation, dry or alternate (m²) |
|---|---|---|---|
| LH | 2.25 | 84 | Not permitted, use OH1 |
| OH1 | 5.0 | 72 | 90 |
| OH2 | 5.0 | 144 | 180 |
| OH3 | 5.0 | 216 | 270 |
| OH4 | 5.0 | 360 | Not permitted, use HHP1 |
| HHP1 | 7.5 | 260 | 325 |
| HHP2 | 10.0 | 260 | 325 |
| HHP3 | 12.5 | 260 | 325 |
| HHP4 | deluge (see NOTE) | - | - |
Source: Table 3, Clause 7.1, EN p.36 (TSE p.39). NOTE: HHP4 requires special consideration; deluge systems are outside the scope of this standard.
The dry and alternate columns are 25 % greater than the areas in the wet column (72 → 90, 144 → 180, 216 → 270, 260 → 325). This reflects the assumption that more heads will open because water arrives late.
3. Clause 7.2: HHS
In HHS the density and the area do not come from a single hazard group but from the combination of storage configuration, category and height.
Clause 7.2.2.2 (EN p.36): Table 4 specifies the appropriate design density and area of operation, by category and maximum permitted storage height, for various types of storage with roof or ceiling protection only. The storage heights in the table are taken as the maximum values for effective sprinkler protection where sprinklers are provided in the roof or ceiling only. Where the storage height exceeds the limit, intermediate level in-rack sprinklers are provided in accordance with Clause 7.2.3.
As an example from Table 4, the ST4 (palletized racking) Category II rows (EN p.38):
| Maximum storage height (m) | Design density (mm/min) | Area of operation, wet or pre-action (m²) |
|---|---|---|
| 3.4 | 7.5 | 260 |
| 4.2 | 10.0 | 260 |
| 5.0 | 12.5 | 260 |
| 5.6 | 15.0 | 260 |
| 6.0 | 17.5 | 260 |
| - | 20.0 and above | 300 |
The NOTE to Table 4: dry and alternate systems should be avoided in High Hazard storage, particularly with the more combustible products (the higher categories) and with higher storage. Where a dry or alternate system nevertheless has to be installed, the area of operation shall be increased by 25 %.
Clause 7.2.2.1 (EN p.36): for buildings exceeding 12 m the authorities should be consulted.
Clause 7.2.2.3 Excessive clearance (EN p.36-37): the values of Table 4 are used in determining the clearance. Where the clearance, that is the distance between the storage height and the ceiling sprinkler deflector, exceeds 4 m, one of two options is applied:
- Option 1: the density is increased by 2.5 mm/min for the first excess metre. For each additional metre the density is increased by 1 mm/min. A minimum K factor of K115 shall be used.
- Option 2: intermediate level in-rack sprinklers are provided in accordance with Clause 7.2.3.
Clause 7.2.3 Intermediate level in-rack sprinklers (EN p.37):
- 7.2.3.1: where more than 50 intermediate level sprinklers are installed in racks, they are not supplied from the same control valve set as the roof or ceiling sprinklers.
- 7.2.3.2: the design density for the roof or ceiling sprinklers shall be a minimum of 7.5 mm/min over an area of operation of 260 m². Where goods are stored above the top of the intermediate protection level, the design criteria for the roof or ceiling sprinklers are taken from Table 5.
- 7.2.3.3: for hydraulic calculation purposes, 3 sprinklers are assumed to operate simultaneously at the hydraulically most remote location at each in-rack sprinkler level, up to a maximum of three levels. Where the rack aisles are 2.4 m or wider, only one rack is assumed to be affected. Where the aisles are narrower than 2.4 m but equal to or wider than 1.2 m, two racks are assumed, and where they are narrower than 1.2 m, three racks. NOTE: it is not necessary to assume that more than three rows of sprinklers operate simultaneously in the vertical plane, nor more than three in the horizontal plane.
- 7.2.3.4: in-rack sprinklers and the associated ceiling sprinklers shall always be fully calculated (see Clause 13.1). NOTE: the minimum pressure at any operating sprinkler is 2.0 bar (see Clause 13.4.4).
4. Clause 13.4: Verification in fully calculated systems
13.4.1 Design density: the four sprinkler rule
Clause 13.4.1 (EN p.98):
The discharge density shall be taken as the total flow in litres per minute from the four adjacent sprinklers divided by the area in square metres covered by those four sprinklers. Where there are fewer than four sprinklers in open connection, the discharge density shall be taken as the lowest value of the flow from any one sprinkler divided by the area covered by that sprinkler.
It continues: the discharge density obtained with each available water supply or combination of supplies, from any area of operation containing the group of four sprinklers concerned or from the whole of the protected area, whichever is the smaller, shall not be less than the design density specified in Clause 7.
The area covered by each sprinkler is defined by centre lines drawn perpendicular to and midway along the lines joining adjacent sprinklers, and by the boundary of the protected area or half the distance to the nearest sprinklers, whichever is the greater (Figure 22). Where in-rack sprinklers are present, the calculation is carried out taking into account the simultaneous flow and pressure requirement of the roof or ceiling sprinklers and the intermediate level sprinklers.
13.4.2 Location of the area of operation
13.4.2.1 Hydraulically most unfavourable location (EN p.99): in determining the hydraulically most unfavourable location of the area of operation, account shall be taken of the sprinkler spacing, the layout, the elevation, the range centres, the sprinkler orifice size and changes in pipe diameters, and of all possible locations, whether on distribution pipes or between distribution pipes interconnected by range pipes.
In gridded installations the correct location of the most unfavourable area of operation shall be proved by moving the area one sprinkler step in each direction along the range pipes until the area with the highest pressure demand is identified. In looped installations the area is moved one sprinkler step in each direction along the distribution pipe.
13.4.2.2 Hydraulically most favourable location (EN p.99): in determining the most favourable location too, all possible locations shall be taken into account.
13.4.3 Shape of the area of operation
This is one of the most frequently misreported provisions. The distinction is not between wet and dry systems but between the unfavourable and the favourable location.
13.4.3.1 Hydraulically most unfavourable location (EN p.99-100): the area of operation shall be as nearly rectangular as possible and symmetrical about the sprinkler layout (see Figure 23), and:
- a) in terminal and looped configurations the remote edge of the area is defined by a range, or by a pair of ranges where there is an end-centre layout. Sprinklers not forming a complete range or pair of ranges are grouped in the next range upstream of the rectangular area, as close as possible to the distribution pipe (see Figures 23 and 25);
- b) in gridded configurations in which the range pipes run parallel to the ridge of a roof with a pitch steeper than 6° or run along bays formed by beams deeper than 1.0 m, the remote edge of the area shall have a length L parallel to the ranges, and L shall be greater than or equal to 2 times the square root of the area of operation;
- c) in all other gridded configurations the remote edge of the area shall have a length L parallel to the ranges, and L shall be greater than or equal to 1.2 times the square root of the area of operation.
13.4.3.2 Hydraulically most favourable location (EN p.100): the area of operation shall be as nearly square as possible, and:
- a) in terminal and looped configurations the area shall, wherever possible, contain only sprinklers on one distribution pipe. The number of sprinklers calculated to operate on the ranges, or on pairs of ranges in end-centre installations, is placed at the hydraulically most favourable location on each range or pair of ranges. Sprinklers not forming a complete range or pair of ranges are placed at the hydraulically nearest locations in the next range (see Figures 24 and 26);
- b) in gridded configurations the area is placed at the hydraulically most favourable location on the ranges. Sprinklers not forming a complete range length are placed at the hydraulically nearest locations in the next range (see Figure 23).
Example: OH3 wet system, gridded, flat ceiling. The area of operation is 216 m². Under 13.4.3.1 c), L ≥ 1.2 x √216 = 1.2 x 14.7 = 17.6 m. If the same area lies under a roof with a pitch steeper than 6° and the ranges run parallel to the ridge, then under 13.4.3.1 b), L ≥ 2 x 14.7 = 29.4 m.
13.4.4 Minimum sprinkler discharge pressure
With all the sprinklers in the area of operation running, the pressure at the sprinkler at the hydraulically most unfavourable location shall not be less than that required to give the density specified in Clause 13.4.1, or than the following, whichever is the greater (EN p.102):
| Case | Minimum pressure |
|---|---|
| LH | 0.70 bar |
| OH | 0.35 bar |
| HHP and HHS (other than in-rack sprinklers) | 0.50 bar |
| K115 in-rack sprinklers | 1.00 bar |
| K80 in-rack sprinklers | 2.00 bar |
13.4.5 Minimum pipe diameters
Table 36 (EN p.103): 20 mm for LH; 20 mm for OH and HH in horizontal and upright pipes feeding a single sprinkler with a K factor not greater than 80; 25 mm in all other cases. Pipe diameters on the installation side of the control valve set may reduce only in the direction of water flow; grid and loop configurations are excepted. Upright sprinklers are not connected to any pipe larger than 65 mm in diameter (50 mm if insulated); pendent sprinklers are not connected directly to any pipe larger than 80 mm in diameter. For larger diameters an arm pipe is fitted such that the distance from the sprinkler deflector to the side of the main pipe is not less than 1.5 times the diameter of that pipe.
5. Pre-calculated systems: Tables 6 and 7
In systems that are not fully calculated the design density is not verified directly; instead the pressure and flow pairs to be provided at the control valve set are taken from a table.
Clause 7.3.1 (EN p.39): the water supply shall be capable of providing not less than the relevant flows and pressures specified in Table 6 at each control valve set. The pressure loss due to friction and static head between the water supply and each control valve set is calculated separately.
| Hazard class | Flow (l/min) | Pressure at the control valve set (bar) | Maximum demand flow (l/min) | Pressure at the control valve set (bar) |
|---|---|---|---|---|
| LH (wet and pre-action) | 225 | 2.2 + ps | - | - |
| OH1 wet and pre-action | 375 | 1.0 + ps | 540 | 0.7 + ps |
| OH1 dry and alternate; OH2 wet and pre-action | 725 | 1.4 + ps | 1 000 | 1.0 + ps |
| OH2 dry and alternate; OH3 wet and pre-action | 1 100 | 1.7 + ps | 1 350 | 1.4 + ps |
| OH3 dry and alternate; OH4 wet and pre-action | 1 800 | 2.0 + ps | 2 100 | 1.5 + ps |
Source: Table 6, Clause 7.3.1, EN p.39 (TSE p.42). NOTE: ps is the loss of static head, in bar, due to the height of the highest sprinkler in the array concerned above the 'C' gauge of the control valve set.
Clause 7.3.2 (EN p.40): in HHP and HHS systems without in-rack sprinklers the water supply shall be capable of providing the flow and pressure specified in Table 7 at the highest design point (or as modified in accordance with Clauses 7.3.2.2 to 7.3.2.5). A few rows from Table 7:
| Design density (mm/min) | Maximum demand flow, wet or pre-action (l/min) | Maximum demand flow, dry or alternate (l/min) |
|---|---|---|
| 7.5 | 2 300 | 2 900 |
| 10.0 | 3 050 | 3 800 |
| 12.5 | 3 800 | 4 800 |
| 15.0 | 4 550 | 5 700 |
| 20.0 | 6 400 | 8 000 |
| 30.0 | 9 650 | 12 000 |
Source: Table 7, Clause 7.3.2.1, EN p.40 (TSE p.43).
The flow scaling rule is in Clause 7.3.2.7 (EN p.41): Q2 = Q1 x (a2 / a1), where Q1 is the flow in Table 7, a1 is the area of operation for the design density (see Table 4) and a2 is the required area of operation. Under Clause 7.3.2.6, where the area is increased or decreased the flow changes proportionally, but the pressure at the design point does not change.
6. From design density to water volume
In fully calculated systems the minimum effective water volume is found, in accordance with Clause 9.3.2.3 (EN p.48), by multiplying the maximum demand flow (Qmax) by the duration in Clause 8.1.1. The durations are LH 30 min, OH 60 min, HHP 90 min and HHS 90 min (Clause 8.1.1, EN p.42). No hose or fire brigade allowance is added to this formula by the standard.
In pre-calculated systems the volume is read directly from Table 9 (LH and OH) or Table 10 (HHP and HHS).
7. Determining the hazard class
Since the design density comes from the hazard class, the starting point of the whole calculation is the classification. Clause 6.1 (EN p.30): the hazard class for which the sprinkler system is to be designed shall be determined before the design work is started. The buildings and areas to be protected are classified as Light Hazard (LH), Ordinary Hazard (OH) or High Hazard (HH). The classification depends on the occupancy and the fire load; examples of occupancies are given in Annex A.
At the end of Clause 6.1 (EN p.31) there is a transition rule: where there are areas of different hazard classification in open connection, the higher design criteria shall be extended into the area of lower classification by at least two rows of sprinklers.
Frequently Asked Questions
How is design density defined in EN 12845?
TS EN 12845+A2:2026 Clause 3.19 (EN p.17): design density is the minimum discharge density in millimetres per minute for which a sprinkler installation is designed, determined from the discharge in litres per minute of a given group of sprinklers divided by the area in square metres covered by them. The definition of the area of operation is in Clause 3.9: the maximum area over which it is assumed, for design purposes, that sprinklers will operate in a fire.
What are the LH, OH and HHP values in Table 3?
Table 3 (Clause 7.1, EN p.36), for wet or pre-action: LH 2.25 mm/min and 84 m²; OH1 5.0 and 72; OH2 5.0 and 144; OH3 5.0 and 216; OH4 5.0 and 360; HHP1 7.5 and 260; HHP2 10.0 and 260; HHP3 12.5 and 260; HHP4 deluge. In the dry or alternate column, LH reads 'not permitted, use OH1', OH1 90, OH2 180, OH3 270, OH4 reads 'not permitted, use HHP1' and HHP1 to HHP3 are 325 m².
Which sprinklers is the density measured from in the hydraulic calculation?
Clause 13.4.1 (EN p.98): the discharge density is taken as the total flow in litres per minute from the four adjacent sprinklers divided by the area in square metres covered by those four sprinklers. Where fewer than four sprinklers are in open connection, the lowest value of the flow from any one sprinkler is divided by the area covered by that sprinkler.
What shape should the area of operation have?
Clause 13.4.3 (EN p.99-100) distinguishes by location: at the hydraulically most unfavourable location the area shall be as nearly rectangular as possible and symmetrical about the sprinkler layout; at the hydraulically most favourable location it shall be as nearly square as possible. The distinction is not between wet and dry systems but between the unfavourable and the favourable location.
What is the 1.2 x square root rule in gridded systems?
Clause 13.4.3.1 c) (EN p.100): in all other gridded configurations the remote edge of the area shall have a length L parallel to the range pipes, and L shall be greater than or equal to 1.2 times the square root of the area of operation. Under Clause 13.4.3.1 b) this factor is 2 for gridded configurations in which the range pipes run parallel to the ridge of a roof with a pitch steeper than 6° or run along bays formed by beams deeper than 1.0 m.
What is the minimum pressure at the most unfavourable sprinkler?
Clause 13.4.4 (EN p.102): with all the sprinklers in the area of operation running, the pressure at the sprinkler at the hydraulically most unfavourable location shall not be less than that required to give the density specified in 13.4.1, or than the following values, whichever is the greater: 0.70 bar for LH; 0.35 bar for OH; 0.50 bar for HHP and HHS other than in-rack sprinklers; 1.00 bar for K115 in-rack sprinklers; 2.00 bar for K80 in-rack sprinklers.
References
- TS EN 12845+A2:2026 Clauses 3.9, 3.10, 3.11 Definitions of area of operation (EN p.16 / TSE p.19)
- TS EN 12845+A2:2026 Clause 3.19 Definition of design density (EN p.17 / TSE p.20)
- TS EN 12845+A2:2026 Clause 6.1 General and the two row rule (EN p.30-31 / TSE p.33-34)
- TS EN 12845+A2:2026 Clause 7.1, Table 3 (EN p.35-36 / TSE p.38-39)
- TS EN 12845+A2:2026 Clauses 7.2.2.1, 7.2.2.2, 7.2.2.3, Table 4 (EN p.36-38 / TSE p.39-41)
- TS EN 12845+A2:2026 Clauses 7.2.3.1-7.2.3.4 (EN p.37 / TSE p.40)
- TS EN 12845+A2:2026 Clause 7.3.1, Table 6 (EN p.39 / TSE p.42)
- TS EN 12845+A2:2026 Clause 7.3.2.1, Table 7 (EN p.40 / TSE p.43)
- TS EN 12845+A2:2026 Clauses 7.3.2.6, 7.3.2.7 (EN p.41 / TSE p.44)
- TS EN 12845+A2:2026 Clause 8.1.1 Duration (EN p.42 / TSE p.45)
- TS EN 12845+A2:2026 Clause 9.3.2.3 Calculated systems (EN p.48 / TSE p.51)
- TS EN 12845+A2:2026 Clause 13.4.1, Figure 22 (EN p.98-99 / TSE p.101-102)
- TS EN 12845+A2:2026 Clauses 13.4.2.1, 13.4.2.2 (EN p.99 / TSE p.102)
- TS EN 12845+A2:2026 Clauses 13.4.3.1, 13.4.3.2 (EN p.99-100 / TSE p.102-103)
- TS EN 12845+A2:2026 Clause 13.4.4 Minimum sprinkler discharge pressure (EN p.102 / TSE p.105)
- TS EN 12845+A2:2026 Clause 13.4.5, Table 36 Minimum pipe diameters (EN p.102-103 / TSE p.105-106)
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Download MEP Calc on the App StoreTS EN 12845+A2:2026 (EN 12845:2015+A2:2026) Fixed firefighting systems — Automatic sprinkler systems — Design, installation and maintenance; TS EN 12845-2:2025 (EN 12845-2:2024) ESFR and CMSA sprinkler systems. Every figure in this article is taken from the published standard text; clause, table and page references are listed under References. General information only, not a substitute for the standard or for a design review.