Strobe Candela Calculator
Pick the right candela rating for a fire alarm strobe the way NFPA 72 Chapter 18 requires: the wall-mount table by room size, the ceiling-mount table by room size and lens height, the corridor spacing rule, and the sleeping-area 110/177 cd thresholds — all four code paths in one flow. Compare one appliance against four (and square subdivisions) priced in real UL-max current, map the table minimum to the setting you can actually buy, and print a coverage justification sheet. Free, no login.
Space
Justification sheet header (project, space, preparer)
Required candela
More than two appliances in a field of view must be synchronized (18.5.5.7.2)
NFPA 72 (2022) Tables 18.5.5.7.1(a)/(b) — values unchanged since 2013; 18.5.5.8 corridors; 18.5.5.10 sleeping areas; 18.5.5.7.2 synchronization. Currents: UL-max RMS (16–33 V), System Sensor datasheets. The AHJ has the final say on layout.
Strobe Coverage Justification
Visible notification appliance selection per NFPA 72 Chapter 18 — prepared with the NORDIX Strobe Candela Calculator (nordixhq.com/tools/strobe-candela-calculator)
| Project: — | Space: — |
| Prepared by: — | Date: — |
| Space type | Room — wall-mounted (lens 80–96 in AFF, 18.5.5.1) |
| Dimensions | 35 × 22 ft — governing square 35 × 35 ft |
| Code basis | NFPA 72 (2022) Table 18.5.5.7.1(a) |
| Required | 1 × 60 cd minimum (selected setting: 75 cd) |
| Alternatives considered | 1 × 60 cd (158 mA); 4 × 15 cd (264 mA); 4 × 15 cd (264 mA); 2 × 30 cd (188 mA); 2 × 34 cd (316 mA) |
Basis: NFPA 72 (2022) Chapter 18 — table values unchanged since the 2013 edition (2022 renumbering: 18.5.5.7 rooms / 18.5.5.8 corridors / 18.5.5.9 performance-based / 18.5.5.10 sleeping). Non-square rooms governed by the longest dimension or the off-center doubling rule; more than two appliances in any field of view must flash in synchronization (18.5.5.7.2). Appliance currents where shown are manufacturer UL-max RMS across the 16–33 V window. This is a design aid — the adopted code edition and the AHJ govern.
How this calculator works
The candela tables are square-room tables, so the first step is the governing square: for a rectangular room, the longest dimension; for an appliance not centered on its wall, the greater of the distance to the farthest wall and twice the distance to the farthest adjacent wall. Round up to the next listed row — there is no 36×36 row, so a 35 ft room reads the 40×40 row. Wall strobes (lens at 80–96 in per 18.5.5.1) read Table 18.5.5.7.1(a), which offers one appliance or four (one per wall, allowed at 40×40 ft and up). Ceiling strobes read Table 18.5.5.7.1(b), banded by lens height at 10/20/30 ft — and the code’s own footnote lets a taller band’s row be used at any lower height, which is why a 50×50 ft room takes 95 cd at any lens height up to 30 ft. Rooms beyond the tables are subdivided into squares, each covered by its own appliance — NFPA’s annex example covers a 40×80 ft room with two 60 cd appliances. Wherever more than two appliances (or synchronized groups) share a field of view, they must flash in synchronization (18.5.5.7.2).
Worked example: one strobe or four?
A 50×50 ft room, wall-mounted, has two table options — and the current draw decides:
- One appliance: table minimum 94 cd → buy the 95 cd setting → 181 mA (strobe-only, UL max).
- Four appliances (one per wall): 30 cd each → 4 × 94 mA = 376 mA — plus four boxes, four drops, and the synchronization requirement.
- The single 95 cd strobe uses less than half the current and a quarter of the labor — which is why fewer, higher-candela appliances are the default recommendation.
That current difference flows straight into the circuit loading, voltage drop, and battery calculations — the same device library powers all four tools.
Reference: wall-mount candela by room size
NFPA 72 (2022) Table 18.5.5.7.1(a) — values unchanged since the 2013 edition. The four-appliance column is genuinely non-monotonic (19 cd at 45×45, back to 30 cd through 60×60) — that is what the code prints. Note for older references: the 1996 edition had 30/95/185/305 cd at the 30/50/70/90 ft rows; the 2002 edition revised them to 34/94/184/304, and some old manufacturer guides still reprint the 1996 figures.
| Max room size | One appliance | Four (one per wall) |
|---|---|---|
| 20 × 20 ft | 15 cd | Not allowable |
| 28 × 28 ft | 30 cd | Not allowable |
| 30 × 30 ft | 34 cd | Not allowable |
| 40 × 40 ft | 60 cd | 15 cd |
| 45 × 45 ft | 75 cd | 19 cd |
| 50 × 50 ft | 94 cd | 30 cd |
| 54 × 54 ft | 110 cd | 30 cd |
| 55 × 55 ft | 115 cd | 30 cd |
| 60 × 60 ft | 135 cd | 30 cd |
| 63 × 63 ft | 150 cd | 37 cd |
| 68 × 68 ft | 177 cd | 43 cd |
| 70 × 70 ft | 184 cd | 60 cd |
| 80 × 80 ft | 240 cd | 60 cd |
| 90 × 90 ft | 304 cd | 95 cd |
| 100 × 100 ft | 375 cd | 95 cd |
| 110 × 110 ft | 455 cd | 135 cd |
| 120 × 120 ft | 540 cd | 135 cd |
| 130 × 130 ft | 635 cd | 185 cd |
Section numbers by edition
The table values have been stable since 2013, but the section numbers move between editions — the 2022 edition inserted two new sections (ceiling-appliance minimum height and obstructions), pushing everything after them down. Cite the numbers for the edition your jurisdiction adopted:
| Rule | 2013 / 2016 | 2019 | 2022 |
|---|---|---|---|
| Room spacing tables | 18.5.5.4.1(a)/(b) | 18.5.5.5.1(a)/(b) | 18.5.5.7.1(a)/(b) |
| Corridors | 18.5.5.5 | 18.5.5.6 | 18.5.5.8 |
| Performance-based alternative | 18.5.5.6 | 18.5.5.7 | 18.5.5.9 |
| Sleeping areas | 18.5.5.7 | 18.5.5.8 | 18.5.5.10 |
Frequently asked questions
What candela strobe do I need for my room size?
For a wall-mounted strobe, NFPA 72 Table 18.5.5.7.1(a) keys the minimum candela to the room's governing square: 15 cd covers 20×20 ft, 34 cd covers 30×30, 60 cd covers 40×40, 94 cd covers 50×50, 135 cd covers 60×60, and it climbs to 635 cd at 130×130. Use the longest dimension of a rectangular room and round up to the next listed row — a 35 ft room reads the 40×40 row because there is no 36×36 row. Ceiling-mounted strobes use a separate table keyed to both room size and lens height. This calculator does all the lookups and cites the row.
Is the table minimum the candela I actually buy?
Usually not — the tables print minimums like 34, 60, 94, or 184 cd, but strobes sell in fixed selectable settings (typically 15/30/75/95/110/115/135/150/177/185). You buy the next setting at or above the table minimum: a 60 cd requirement means a 75 cd setting, 94 cd means 95, 184 cd means 185. Mixing those two number sets up is a classic NICET-exam and field mistake. This calculator shows both — the table minimum with its citation, and the purchasable setting with its UL-max current.
Is it better to use one big strobe or several small ones?
Usually one bigger strobe. In a 50×50 ft room the table allows either one 94 cd appliance or four 30 cd appliances (one per wall) — but the single 95 cd setting draws about 181 mA while four 30 cd strobes draw about 376 mA, plus four back-boxes, four wire drops, and a synchronization requirement. Fewer, higher-candela appliances usually win on current, labor, and battery size. The four-light option and square subdivisions earn their keep in rooms too big for any single setting — above 184 cd required, no standard setting reaches it.
How do I size a ceiling-mounted strobe?
NFPA 72 Table 18.5.5.7.1(b) keys ceiling strobes to room size AND lens height, in three bands: at 10 ft a 20×20 room needs 15 cd, at 20 ft it needs 30 cd, at 30 ft it needs 55 cd — light spreads farther before reaching eye level from a taller mount. Each band caps at a different room size (44×44 at 10 ft, 46×46 at 20 ft, 70×70 at 30 ft), and the code's footnote permits using a taller band's row at any lower height, which is why a 50×50 room takes 95 cd at any lens height up to 30 ft. Above 30 ft: suspend the appliance, use the performance-based alternative, or wall-mount.
How many strobes does a corridor need?
In a corridor 20 ft wide or less, strobes may be 15 cd regardless of length — the requirement is placement: within 15 ft of each end and no more than 100 ft between appliances (NFPA 72 18.5.5.8). That arithmetic gives one strobe up to 30 ft, two up to 130 ft, three up to 230 ft, and one more per 100 ft after that. Any fire door, elevation change, or obstruction interrupting the view splits the run into separate corridors, each counted on its own. Corridors wider than 20 ft use the room tables instead.
What candela is required in a sleeping room?
It depends on how close the lens is to the ceiling: 110 cd when the lens is 24 inches or more below the ceiling, 177 cd when it is within 24 inches — including ceiling-mounted units and strobes integral to the smoke alarm (NFPA 72 Table 18.5.5.10.3). The rationale is smoke stratification: a lens near the ceiling may be inside the smoke layer exactly when it needs to be seen. In rooms with any dimension over 16 ft, the appliance must also be within 16 ft of the pillow. Where sleeping-room strobes are required at all comes from the building code and ADA scoping, not NFPA 72.
My room is bigger than the largest strobe setting — what now?
The table keeps going past what you can buy — a 90×90 ft room reads 304 cd for one appliance, but commercial strobes top out around 185 cd. The code's answer is more appliances, not brighter ones: the four-per-wall column (95 cd each at 90×90), or subdividing the space into squares with one appliance per square — NFPA's own annex example covers a 40×80 ft room with two 60 cd appliances on 40×40 squares. Every appliance-or-group beyond two in a field of view must flash in synchronization. This calculator generates and prices those options automatically.
Why do different references show different section numbers and values for the same table?
Two kinds of drift. Values: the 1996 edition printed 30/95/185/305 cd at the 30/50/70/90 ft rows; the 2002 edition revised them to 34/94/184/304, and some manufacturer application guides still circulate the 1996 figures. Section numbers: the room tables were 18.5.5.4.1 in 2013/2016, 18.5.5.5.1 in 2019, and 18.5.5.7.1 in 2022, when two new sections pushed the numbering down — corridors moved to 18.5.5.8 and sleeping areas to 18.5.5.10. This calculator uses the current values (unchanged since 2013) with 2022 numbering and notes the older numbers, so cite whichever edition your jurisdiction adopted.
Method: NFPA 72 (2022) Tables 18.5.5.7.1(a)/(b) and sections 18.5.5.1 (80–96 in lens height), 18.5.5.7 (rooms, subdivision, off-center rule), 18.5.5.7.2 (synchronization), 18.5.5.7.6 (ceilings above 30 ft), 18.5.5.8 (corridors), 18.5.5.10 (sleeping areas) — values verified against NFPA committee documents and manufacturer application guides; unchanged since the 2013 edition. Where visible notification is required at all comes from the adopted building code and ADA scoping (IBC 907.5.2.3; ADA 2010 §215/702), not NFPA 72. The performance-based alternative (18.5.5.9, 0.0375 lumens/ft²) can substitute for the tables except in sleeping areas. Device currents are manufacturer UL-max RMS figures. This is a design aid — the adopted edition and the AHJ govern.
The tables behind this tool: wall strobe candela chart, ceiling strobe candela chart, and corridor strobe spacing chart.