Sprinkler K-factor selection starts with one relationship: Q = K√P, with Q in gpm and P in psi. A larger K-factor delivers the required flow at a lower pressure but cannot be calculated below the 7 psi minimum, so the right choice keeps the most remote sprinkler at or above 7 psi, suits the water supply and respects the limits NFPA 13 sets for storage, high ceilings and thread size. Under NFPA 13, K5.6 remains the common choice for light and ordinary hazard spray sprinklers, K8.0 lowers the starting pressure for Ordinary Hazard Group 2 and higher densities, and K11.2 or larger is mandatory in specific storage and high-ceiling cases.
What the K-Factor Controls
Because Q = K√P, the pressure needed for a target flow is P = (Q/K)². At equal pressure a K8.0 discharges about 1.43 times the water of a K5.6, and a K11.2 twice as much. NFPA 13 also gives each K-factor a metric designation (1 gpm/psi½ is about 14.4 L/min/bar½), and older drawings often name sprinklers by orifice size, which matters when surveying an existing system.
| Nominal K (gpm/psi½) | Metric designation | Flow vs K5.6 at equal pressure | Legacy orifice name | Thread on new systems |
|---|---|---|---|---|
| K5.6 | K80 | 1.00 × | 1/2 in. | 1/2 in. NPT |
| K8.0 | K115 | 1.43 × | 17/32 in. | 3/4 in. NPT |
| K11.2 | K160 | 2.00 × | 5/8 in. | 3/4 in. NPT |
NFPA 13 does not allow sprinklers above K5.6 with a 1/2 in. NPT thread in new systems; 1/2 in. NPT K8.0 heads exist for retrofits of existing systems only. A new K8.0 or K11.2 layout needs 3/4 in. outlets, which is easy to miss when a takeoff is copied from a K5.6 job.
The 7 psi Floor: Why a Bigger K Is Not Always Better
NFPA 13 sets 7 psi (0.5 bar) as the minimum operating pressure for any sprinkler, and a sprinkler’s listing can set a higher one. If a larger K-factor could meet the required flow below that minimum, the calculation still starts at the minimum, and the sprinkler discharges more than the design needs at every flowing head.
Worked Example: The Most Remote Sprinkler
Required flow is the design density multiplied by the area each sprinkler protects. The example uses baseline NFPA 13 densities and common coverage areas: 225 ft² in light hazard (the maximum for hydraulically calculated systems under noncombustible, unobstructed construction), 130 ft² in ordinary hazard and 100 ft² in Extra Hazard Group 1.
| Hazard | Density × coverage | Required flow | K5.6 | K8.0 | K11.2 |
|---|---|---|---|---|---|
| Light hazard | 0.10 × 225 ft² | 22.5 gpm | 16.1 psi | 7.9 psi | 4.0 psi, so 7 psi applies: 29.6 gpm (+32%) |
| Ordinary Hazard Group 2 | 0.20 × 130 ft² | 26.0 gpm | 21.6 psi | 10.6 psi | 5.4 psi, so 7 psi applies: 29.6 gpm (+14%) |
| Extra Hazard Group 1 | 0.30 × 100 ft² | 30.0 gpm | 28.7 psi | 14.1 psi | 7.2 psi, no surplus |
Step by step for the ordinary hazard row: Q = 0.20 × 130 = 26.0 gpm. For K5.6, P = (26.0 ÷ 5.6)² = 21.6 psi. For K8.0, P = (26.0 ÷ 8.0)² = 10.6 psi. For K11.2, P = (26.0 ÷ 11.2)² = 5.4 psi, which is under the floor, so the sprinkler is calculated at 7 psi and flows 11.2 × √7 = 29.6 gpm.
Scaled to the design area: at 0.20 gpm/ft² over 1,500 ft² with 130 ft² per sprinkler, 12 sprinklers flow (1,500 ÷ 130 = 11.5, rounded up). K8.0 needs at least 12 × 26.0 = 312 gpm starting at 10.6 psi; K11.2 needs about 12 × 29.63 = 356 gpm starting at 7 psi. Friction raises both totals in the full calculation, but the trade holds: about 3.6 psi less starting pressure for about 44 gpm more flow.
The water supply decides. A supply short on pressure, such as a weak municipal main without a fire pump, points to the larger K-factor; a supply short on flow, or a tank sized by volume, points to the smaller one. Extra Hazard Group 1 is the easy case: K11.2 starts at 7.2 psi against 28.7 psi for K5.6, with no surplus.
Where NFPA 13 Restricts the Choice
- Small orifices. NFPA 13 limits heads below K5.6 to hydraulically calculated light hazard systems and, with narrow exceptions, to wet pipe, so most commercial layouts start at K5.6.
- High-piled storage. Standard-response CMDA sprinklers must be K8.0 or larger for design densities above 0.20 and up to 0.34 gpm/ft², and K11.2 or larger, listed for storage, above 0.34 gpm/ft². The text added to the 2019 edition by TIA 19-2 also permits K5.6 at 0.20 gpm/ft² or less and exempts modifications to existing systems using K8.0 or smaller heads.
- Ceilings over 30 ft. The 2025 edition, as revised by TIA 25-5 (effective December 22, 2025), bars sprinklers below K11.2 in Ordinary Hazard Group 2 and higher occupancies under ceilings more than 30 ft (9.1 m) high. Standard-coverage heads in Ordinary Hazard Group 2 must then be quick-response, extra hazard needs at least K16.8 (K25.2 for extended coverage), and densities of 0.37 to 0.45 gpm/ft² apply. The annex cites large-scale tests in which K8.0 heads did not control the fire and K11.2 heads did, a result worth raising for high-ceiling retail and production space even on older editions.
Density/Area Versus Room Design
Both methods use the same hazard density. Density/area flows every sprinkler in a design area set by the hazard: 1,500 ft² for light and ordinary hazard and 2,500 ft² for extra hazard at the baseline points. Room design flows every sprinkler in the room that creates the greatest demand, provided its walls carry a fire-resistance rating equal to the required water supply duration and its openings are protected. In light hazard, unprotected openings are allowed if some sprinklers in the adjoining space are added to the calculation.
| Criterion | Density/area method | Room design method |
|---|---|---|
| Sprinklers calculated | All in the hazard’s design area | All in the room creating the greatest demand |
| Construction needed | Nothing extra | Walls rated to the water supply duration used (30 min light hazard, 60 to 90 min ordinary hazard), protected openings |
| Pays off in | Open floors, large rooms | Rated mechanical rooms, stockrooms, classrooms |
| Typical mistake | Wrong hazard group | Unrated partitions used as room boundaries |
Worked Comparison: An Ordinary Hazard Stockroom
Take a 40 ft × 26 ft (1,040 ft²) Ordinary Hazard Group 2 stockroom with walls rated to the water supply duration and rated self-closing doors, protected by K8.0 sprinklers at 10 ft × 13 ft (130 ft²). Density/area flows 12 sprinklers: at least 312 gpm from 10.6 psi. Room design flows the 8 sprinklers in the room at the same 0.20 gpm/ft²: at least 208 gpm, a third less, before friction and hose allowance.
Fewer flowing heads also shrink the K11.2 penalty: the surplus of about 3.6 gpm per sprinkler totals 29 gpm across 8 heads instead of 44 gpm across 12. Design-area adjustments, such as the 30% increase for dry-pipe systems or the quick-response reduction, change the density/area count and can shift the comparison.
Canada, the United States and EMEA: Which Rules Apply
According to the Canadian Automatic Sprinkler Association, the 2020 and 2025 National Building Code of Canada reference NFPA 13-2019, where any point on the density/area curve is still an acceptable design basis. NFPA 13-2022 requires fixed design points for new systems, and the 2025 edition drops the curves except through the original edition when an existing system is evaluated. Provinces adopt the national code on their own schedules and can designate different editions, so check the referenced-documents table of the code governing your permit. In the US the edition follows the adopted model code; the 2021 International Building Code, for example, references NFPA 13-2019.
Listing is the second check. Data sheets list each approval separately (UL, UL for use in Canada, FM and others), and approvals vary by model, finish, coating and temperature rating. Confirm the Canadian listing for the exact configuration; our guide to UL, FM and ULC approvals in Canada explains how AHJs read those marks.
EN 12845 projects in the UK and EMEA write K-factors in metric (K80 is the counterpart of K5.6) and use their own hazard classes, densities and areas of operation, so convert the design basis, not just the heads. Where FM Global is the insurer, its data sheets set their own sprinkler and pressure criteria, which can differ from NFPA 13.
Submittal and Field Checks
- Read the SIN. The K-factor is often not marked on the sprinkler; the Sprinkler Identification Number, usually on the deflector, leads to the data sheet, which states it in US and metric units.
- Match thread to fitting. A 1/2 in. NPT K8.0 head on a new system is a rejection item; check outlet sizes on the shop drawings before ordering.
- Recalculate any K change. Swapping K5.6 heads for K8.0 or K11.2 in a renovation changes the flow at every head and needs a new hydraulic calculation.
- Stock the spare cabinet by type. NFPA 13 requires at least 6 spares below 300 sprinklers, 12 for 300 to 1,000 and 24 above 1,000, covering every type and rating installed, with a wrench for each sprinkler type. Mixing K5.6 office heads with K8.0 stockroom heads means stocking both.
- Check room boundaries. A Canadian fire protection engineer has described room design calculations built on unrated partitions inside apartment suites, where the real room was the whole suite inside the rated construction.
Selection Checklist
- Confirm which NFPA 13 edition your code references.
- Classify the hazard and pick the design method before the K-factor.
- Calculate remote-sprinkler flow (density × coverage) and P = (Q/K)² for each candidate K.
- Flag any K below 7 psi and count the surplus flow.
- Check the water supply: short on pressure points to a larger K, short on flow to a smaller one.
- Apply the storage floors and, under the 2025 edition, the high-ceiling minimums.
- For room design, verify wall rating, door protection and room boundaries.
- On new systems, specify 3/4 in. NPT outlets for K8.0 and K11.2.
- Check the data sheet: SIN, K-factor in both units, response, temperature rating, Canadian listing.
- Stock spares for every K-factor and type installed.
Next steps: our worked example of NFPA 13 equivalent lengths for valves and our comparison of NFPA 13, 13R and 13D. The fire sprinklers page lists the sprinkler types we supply, and the fire protection products page covers the valves and trim around them.
Need Help With a Sprinkler Submittal?
Send the hazard classification, design method, K-factors and listing requirements through our contact page or to info@valve-atlas.com, and we will quote sprinklers and the valves around them with the data sheets your AHJ will ask for.
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