How this works. This tool does not read the charts for you. Use the figures and Table 3 further down this page to determine each factor, then enter the values below. The calculator multiplies them and applies the minimum-distance rules of §2.3.1.1.

Calculator Per FM DS 1-42 §2.3.1.2

1 Base separation distance — S_B

Read S_B from the chart that matches the exposing fire and the exposed-wall category. Choose the exposed-wall / exposure type to jump to the right source.

Exposed wall / exposure type

S_B value

2 Unprotected opening adjustment factor — U

For an SFR (stable fire-resistive) exposing wall, read U from Figure 40 using the percent unprotected openings and number of stories. For a Non-SFR wall, U = 1.0. For an SFR wall and SFR roof, U = the actual unprotected-opening fraction (do not use Figure 40). See §2.3.3.

U value (0–1)

3 Exposure angle adjustment factor — M

When the buildings are parallel, M = 1.0. When they are at an angle, determine the exposure angle θ (see Figures 47–48) and read M from Figure 49. See §2.3.6.

M value (0–1)

Minimum-distance rules §2.3.1.1

Exposure presents an explosion hazard, BLEVE, PV rupture, or run-away reaction — enforce 125 ft (38 m) minimum (§2.3.1.1.B). Exposed wall has a fire rating ≥ 3 hours (F180/EI180) — the general 10 ft (3.0 m) minimum does not apply (§2.3.1.1.A).

Enter S_B, U, and M above to compute the MFL space separation distance.

S_B — Base separation distance Figures 37–39 & Table 3

The information needed to use Figures 37 through 39 is the exposed-wall category (§2.3.2), the classification of exposing-wall construction (§2.3.3), the exposing fire hazard category (§2.3.4), and the exposing-wall length L (§2.3.5, maximum 500 ft / 150 m). Use Table 3 instead for fire-rated exposed walls.

Figure 37 — Base separation distance for exposed noncombustible walls — **Fig. 37.** Base separation distance for exposed noncombustible walls.

Figure 38 — Base separation distance for exposed combustible walls — **Fig. 38.** Base separation distance for exposed combustible walls.

Figure 39 — Base separation distance for ignitable liquids — **Fig. 39.** Base separation distance for ignitable liquids. The required separation is from the exposed wall to the edge of the pool fire (§2.3.4.5).

Table 3 — Base separation distance for fire-rated exposed walls

Fire rating (hours)	HC-1 / HC-2 / HC-3	Storage occupancy			Ignitable liquids
Fire rating (hours)	HC-1 / HC-2 / HC-3	<30 ft (<9 m)	30–45 ft (9–14 m)	≥45 ft (≥14 m)	Ignitable liquids
<1	Categorize the exposed wall as either combustible or noncombustible (use Figure 37 or 38).
1	15 ft(4.5 m)	40 ft(12 m)	50 ft(15 m)	60 ft(18 m)	40 ft(12 m)
2	10 ft(3 m)	30 ft(9 m)	40 ft(12 m)	50 ft(15 m)	25 ft(8 m)
3	5 ft (1.5 m)
≥4	None — only structural separation and minimum clearance for thermal expansion are required.

U — Unprotected opening adjustment factor Figure 40

For an SFR exposing wall, determine the total unprotected openings as a percentage of total wall area, then read U from Figure 40 for the corresponding number of stories (1–3 or 4+). For Non-SFR walls, U = 1.0. See §2.3.3.1–2.3.3.2.

Figure 40 — Exposing wall adjustment factor for unprotected openings in an SFR wall — **Fig. 40.** Exposing-wall adjustment factor for unprotected openings in an SFR wall.

M — Exposure angle adjustment factor Figures 47–49

When the exposed buildings are at an angle to one another, determine the exposure angle θ using Figures 47–48, then read the adjustment factor M from Figure 49. See §2.3.6.

Figure 47 — Exposed buildings at angles — **Fig. 47.** Exposed buildings at angles — defining the exposing angle θ.

Figure 48 — Angle to circular buildings — **Fig. 48.** Angle to circular buildings.

Figure 49 — Angular exposure adjustment factor (M) — **Fig. 49.** Angular exposure adjustment factor (M) vs. θ for non-parallel exposures.

How to use & methodology

Pick units (U.S. or metric) at the top of the calculator.
Step 1 — S_B: Select the exposed-wall / exposure type, read the base separation distance from the matching figure (37/38/39) or Table 3, and enter it.
Step 2 — U: Read the unprotected-opening factor from Figure 40 (SFR walls), or use 1.0 for Non-SFR walls.
Step 3 — M: Read the exposure-angle factor from Figure 49, or use 1.0 for parallel exposures.
The tool returns S_M = S_B × U × M, then applies the §2.3.1.1 minimum-distance rules.

Minimum distances (§2.3.1.1)

The minimum space separation is 10 ft (3.0 m) for any situation except exposed walls with a fire rating ≥ 3 hours.
Maintain at least 125 ft (38 m) from any occupancy presenting an explosion hazard, BLEVE, PV rupture, or run-away reaction.

Scope & references

Background on the maximum foreseeable loss (MFL) concept is in FM DS 1-22, Maximum Foreseeable Loss. The space-separation methodology, figures, and Table 3 reproduced here are from FM DS 1-42, MFL Limiting Factors, Section 2.3. This tool covers the base calculation; it does not handle exposure-length adjustment for offset/overlapping buildings (Figures 42–46, §2.3.5.2), side-wall separation (§2.3.8), or the loading-station, rail, and vehicle-parking scenarios (§2.3.10–2.3.12) — consult the data sheet directly for those.

Engineering aid only. This calculator is a convenience interface to the FM DS 1-42 (Section 2.3) space-separation method. It does not replace the source document and is not a substitute for independent review by a qualified fire protection engineer. The user is responsible for correctly determining each factor (S_B, U, M) from the governing figures and tables, for confirming the exposed-wall and exposing-fire classifications, and for evaluating all applicable scenarios and exceptions.