What is an area × density calculation?
At its simplest, the water demand of a sprinkler system is the design density times an area. The density is the amount of water that must fall on each square metre (or square foot) every minute; the area is the zone in which the sprinklers are assumed to open together in a fire. EN 12845 calls it the area of operation, NFPA 13 the design area. The calculator follows the same order for both standards: density and area from the hazard class; flow, pressure and number of the design sprinklers from the sprinkler choice; extra flows on top (the Turkish code hose reel and hydrant flows in EN 12845 mode, the hose allowance in NFPA 13 mode); and finally duration and water volume.
Qtotal = d × A + Qextra · V = Qtotal × tFor a permit in Turkey use EN 12845 mode: Article 96/5 of the Regulation on Fire Protection of Buildings (BYKHY) ties sprinkler design to EN 12845, and the regulation makes no reference to NFPA 13. NFPA 13 mode is for projects that call for NFPA 13.
EN 12845 mode: EN 12845 and the Turkish code
Density and area of operation (Table 3)
Dry and alternate systems increase the area by 25%; Light Hazard does not allow dry systems (OH1 is used instead) and OH4 uses HHP1.
| Class | Density (mm/min) | Area, wet / pre-action (m²) | Area, dry / alternate (m²) | d × A, wet (L/min) |
|---|---|---|---|---|
| LH | 2.25 | 84 | not allowed → OH1 | 189 |
| OH1 | 5.0 | 72 | 90 | 360 |
| OH2 | 5.0 | 144 | 180 | 720 |
| OH3 | 5.0 | 216 | 270 | 1,080 |
| OH4 | 5.0 | 360 | not allowed → HHP1 | 1,800 |
| HHP1 | 7.5 | 260 | 325 | 1,950 |
| HHP2 | 10.0 | 260 | 325 | 2,600 |
| HHP3 | 12.5 | 260 | 325 | 3,250 |
For HHP4 the standard gives no numeric design point: such hazards are usually protected by deluge systems, which are outside its scope. For high hazard storage (HHS) the density and area depend on the commodity category, storage configuration and height (Tables 4 and 5). In both cases the calculator asks for the sprinkler flow from a hydraulic calculation. Annex 8/B of the Turkish regulation matches this table except High Hazard 1 (7.7 instead of 7.5 mm/min); the calculator uses 7.7 for the Article 92/5 total (2,002 L/min) and 7.5 for the design sprinkler pressure, as Article 96/5 requires.
Sprinkler: K-factor, area and minimum pressure
| Class | Nominal K (Table 37a) | Maximum area per sprinkler (Table 19) | Maximum S and D | Minimum pressure (13.4.4) |
|---|---|---|---|---|
| LH | K57 | 21 m² | 4.6 m | 0.70 bar |
| OH | K80, K115 | 12 m² | 4.0 m (S 4.6 m in a staggered layout) | 0.35 bar |
| HHP and HHS ceiling, ≤ 10 mm/min | K80, K115, K160 | 9 m² | 3.7 m | 0.50 bar |
| HHP and HHS ceiling, > 10 mm/min | K115, K160 | 9 m² | 3.7 m | 0.50 bar |
The design sprinkler’s flow is q = d × As; from Q = K√P (clause 14.3) the required pressure is P = (q / K)². If that is below the class minimum, the minimum pressure governs. The minimum number of sprinklers in the area of operation is N = ⌈A / As⌉; by default the total uses N × q and never falls below the Article 92/5 flow. Example: OH3, K80, 12 m² → q = 60 L/min, P = (60 / 80)² ≈ 0.56 bar; N = ⌈216 / 12⌉ = 18. Table 19 covers sprinklers other than sidewall; sidewall sprinklers follow Table 20 (LH 17.0 m², OH 9.0 m²). Walls may be at most 2.0 m away in a standard layout, 2.3 m in a staggered layout and half the Table 19 or 20 spacing, and 1.5 m for open-joisted ceilings, the open faces of open-faced buildings and combustible or metal external walls; the smallest applicable value governs (clause 12.4.1).
Hose reels and hydrants (Turkish code Annex 8/C), duration and volume
| Building hazard class | Hose reel allowance | Hydrant allowance | Duration |
|---|---|---|---|
| Low hazard | 100 L/min | 400 L/min | 30 min |
| Ordinary Hazard 1–2 | 100 L/min | 400 L/min | 60 min |
| Ordinary Hazard 3–4 | 100 L/min | 1,000 L/min | 60 min |
| High hazard | 200 L/min | 1,500 L/min | 90 min |
Under Article 92/5 the Annex 8/C hose reel flow, and the hydrant flow where there is a hydrant system, are added to the sprinkler flow and multiplied by the Article 92/3 duration (low 30, ordinary 60, high 90 minutes). The reel design value is in Article 94/1-b-5 (100 L/min at 400 kPa) and the hydrant system design flow in Article 95/2 (at least 1,900 L/min with 700 kPa at the outlet). EN 12845 clause 8.1.1 requires 30 minutes for LH, 60 for OH and 90 for HHP and HHS; a combined supply must last as long as the most demanding system (clause 9.6.4 c). Article 92/4 also allows the Annex 8/A table, and the calculator shows that comparison. Example OH3, wet: 1,080 + 100 + 1,000 = 2,180 L/min, 130.8 m³ over 60 minutes; Annex 8/A (h ≤ 15 m) 135 m³, or 201 m³ with the reels and hydrants on the same tank.
NFPA 13 mode: Table 19.2.3.1.1 and the hose allowance
NFPA 13-2025 no longer has the density/area curves of earlier editions: Table 19.2.3.1.1 gives each hazard class a single density and design area, and a smaller area of operation still uses that density (19.2.3.1.4). The standard’s primary units are US customary, and the first stated value is the requirement (1.6.1.3); the metric conversions stated in the standard may also be used, even though they are not always exact (1.6.3). The calculator works in gpm/ft², ft², gpm and psi in US mode and with the tables’ own metric values in SI mode. The hose allowance and duration come from Table 19.2.3.1.2.
| Class | Density | Design area | d × A | With concealed space (3,000 ft², 280 m²) | Hose allowance (inside + outside) | Duration (min) |
|---|---|---|---|---|---|---|
| LH | 0.10 gpm/ft² (4.1 mm/min) | 1,500 ft² (140 m²) | 150 gpm (574 L/min) | 0.07 gpm/ft² (2.9 mm/min) | 100 gpm (380 L/min) | 30 |
| OH1 | 0.15 gpm/ft² (6.1 mm/min) | 1,500 ft² (140 m²) | 225 gpm (854 L/min) | 0.12 gpm/ft² (4.9 mm/min) | 250 gpm (950 L/min) | 60 or 90 |
| OH2 | 0.20 gpm/ft² (8.1 mm/min) | 1,500 ft² (140 m²) | 300 gpm (1,134 L/min) | 0.17 gpm/ft² (6.9 mm/min) | 250 gpm (950 L/min) | 60 or 90 |
| EH1 | 0.30 gpm/ft² (12.2 mm/min) | 2,500 ft² (230 m²) | 750 gpm (2,806 L/min) | 0.28 gpm/ft² (11.4 mm/min) | 500 gpm (1,900 L/min) | 90 or 120 |
| EH2 | 0.40 gpm/ft² (16.3 mm/min) | 2,500 ft² (230 m²) | 1,000 gpm (3,749 L/min) | 0.38 gpm/ft² (15.5 mm/min) | 500 gpm (1,900 L/min) | 90 or 120 |
Design area adjustments
- Dry pipe and double-interlock preaction systems: area +30%, density unchanged (19.2.3.2.6).
- Quick-response sprinklers: in wet systems, light and ordinary hazard, with ceilings up to 20 ft (6.1 m), the area is reduced by Figure 19.2.3.2.3.1: 40% below 10 ft (3.0 m), 25% at 20 ft. The design area never holds fewer than five sprinklers (19.2.3.2.3).
- Extra hazard: with ceilings up to 30 ft (9.1 m) and high-temperature or K11.2 (K160) and larger sprinklers, the area may be reduced by 25%, but not below 2,000 ft² (185 m²) (19.2.3.2.7).
- Unsprinklered combustible concealed space: next to it the area is at least 3,000 ft² (280 m²) with the density from the table’s second column; this rule is applied last (19.2.3.1.5, 19.2.3.2.8.2).
Multiple adjustments are compounded on the table area (19.2.3.2.8.1); for example, OH2 on a dry system is 1,500 × 1.30 = 1,950 ft². The further rules for ceilings over 30 ft (9.1 m) in Ordinary Hazard Group 1 and above (19.2.3.2.5) and for slopes over 2 in 12 (19.2.3.2.4) are not in the calculator, and rack and pile storage is designed to Chapters 20–25.
Sprinkler, pressure, hose allowance and duration
For standard spray sprinklers in a hydraulically calculated system, Table 10.2.4.2.1 allows at most 225 ft² (20 m²) per sprinkler for LH, 130 ft² (12 m²) for OH and 100 ft² (9 m²) for EH. Nominal K-factors come from Table 7.2.2.1: K5.6 (K80), K8.0 (K115), K11.2 (K160). The design sprinkler’s flow is q = d × As and its pressure P = (q / K)², and no sprinkler may run below 7 psi (0.5 bar) (28.2.4.11.1). The Table 19.2.3.1.2 allowance combines inside and outside hose; its inside hose part is 0, 50 or 100 gpm (50 gpm for one hose connection, 100 gpm for several; 19.1.6.3). Total demand is sprinklers plus hose (19.1.4.2). The lower duration in the table applies only where the alarm and supervisory devices are electrically supervised and monitored at an approved, constantly attended location (19.2.3.1.3). A tank serving sprinklers only can leave the hose water out of its volume (Annex A.19.1.5.2).
Example: OH2, wet system, K5.6 (K80)
- Table 19.2.3.1.1: 0.20 gpm/ft² × 1,500 ft² = 300 gpm (with the SI column, 8.1 mm/min × 140 m² = 1,134 L/min)
- Design sprinkler: 0.20 × 130 ft² = 26 gpm → P = (26 / 5.6)² ≈ 21.6 psi ≥ 7 psi; N = ⌈1,500 / 130⌉ = 12
- Sprinklers: 12 × 26 = 312 gpm; hose allowance 250 gpm → total 562 gpm
- 90 minutes (upper value): 50,580 gal ≈ 191.5 m³; 60 minutes where 19.2.3.1.3 is met: 33,720 gal ≈ 127.6 m³
Why preliminary? Density × area is a lower bound: sprinklers nearer the supply run at higher pressure than the most unfavourable one and discharge more, so the hydraulic flow is higher. The final value comes from a hydraulic calculation to EN 12845 clause 13.4 or NFPA 13 Chapter 28; under Article 92/4 of the Turkish code a hydraulic result governs where one has been made.
Frequently Asked Questions
How is sprinkler demand calculated by area × density?
The sprinkler flow is the design density times the area. In EN 12845 mode, under Article 92/5 of the Turkish fire code the hose reel flow from Annex 8/C, and the hydrant flow where there is a hydrant system, are added to the sprinkler flow (Annex 8/B: density × area, identical to EN 12845 Table 3 except 7.7 mm/min for High Hazard 1); the total is multiplied by the Article 92/3 duration (low 30, ordinary 60, high 90 minutes). In NFPA 13 mode the Table 19.2.3.1.1 density and design area are multiplied and the Table 19.2.3.1.2 hose allowance is added.
What is the total flow for an OH3 building?
In EN 12845 mode, for a wet system the sprinkler flow is 5.0 mm/min × 216 m² = 1,080 L/min; adding the Annex 8/C Ordinary Hazard 3–4 allowances (hose reels 100 L/min, hydrants 1,000 L/min) gives 2,180 L/min. By the Article 92/5 method the preliminary volume over 60 minutes is 130.8 m³. Under Article 92/4 the Annex 8/A table may be used instead (Ordinary Hazard 3 wet, h ≤ 15 m: 135 m³, or 201 m³ if the reels and hydrants on the same tank are added); compare the routes in the fire water tank tool.
What is the NFPA 13-2025 demand for Ordinary Hazard Group 2?
Table 19.2.3.1.1 gives 0.20 gpm/ft² over 1,500 ft² for OH2: 300 gpm (with the SI column, 8.1 mm/min × 140 m² = 1,134 L/min). On a wet system a K5.6 (K80) sprinkler covering 130 ft² needs 26 gpm at about 21.6 psi; 12 sprinklers × 26 gpm = 312 gpm. With the 250 gpm hose allowance of Table 19.2.3.1.2 the total is 562 gpm, for 60 or 90 minutes (the lower value under 19.2.3.1.3). A dry pipe system increases the area by 30% (1,950 ft²).
Which K-factors does EN 12845 allow for each hazard class?
Table 37a allows K57 for LH; K80 or K115 for OH; and for HHP and HHS ceiling sprinklers K80, K115 or K160 up to a design density of 10 mm/min and K115 or K160 above it. The pressure at the most unfavourable sprinkler may not be below 0.70 bar for LH, 0.35 bar for OH and 0.50 bar for HHP and HHS (clause 13.4.4). In NFPA 13 the minimum nominal K is K5.6 (K80) (9.4.4.1) and every sprinkler needs at least 7 psi (0.5 bar) (28.2.4.11.1).
Can NFPA 13 be used for a permit in Turkey?
Article 96/5 of the Turkish fire code ties sprinkler design to EN 12845, and the regulation makes no reference to NFPA 13. Permit-stage water and tank calculations are therefore made in EN 12845 mode, including the Annex 8/C hose reel and hydrant flows. NFPA 13 mode is for projects that call for NFPA 13, or for comparing the two standards.
Does this replace the hydraulic calculation?
No. Density × area is a lower bound: in a real pipe network sprinklers nearer the supply run at higher pressure and discharge more, so the hydraulic flow is higher. The calculator is for preliminary sizing and checking; the final flow and pressure come from a hydraulic calculation to EN 12845 clause 13.4 or NFPA 13 Chapter 28.
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