Flame Spread Index Classifier
Enter the FSI and SDI from an ASTM E84 report to get Class A (0–25), B (26–75) or C (76–200), then check it against the IBC interior-finish requirement for your occupancy and location and against 25/50 and insulation limits.
The flame spread index (FSI) is the ASTM E84 / UL 723 tunnel-test number comparing flame travel with red oak (100) and cement board (0).
Use the Flame Spread Index Classifier
IBC Table 803.13 requirement
Other ASTM E84 limits
Not a compliance decision: verify the exact tested product/assembly, the adopted code edition, footnotes and local amendments with the authority having jurisdiction.
Flame spread index classes (IBC 803.1.2, NFPA 101 10.2.3.4)
| Class | FSI | SDI | Older name |
|---|---|---|---|
| A | 0–25 | 0–450 | Class I |
| B | 26–75 | 0–450 | Class II |
| C | 76–200 | 0–450 | Class III |
A higher class satisfies a lower requirement: Class A may be used wherever B or C is required. Materials meeting NFPA 286 room-corner criteria may be used where Class A is required.
IBC Table 803.13: interior wall and ceiling finish by occupancy
Minimum class, exit stairways / corridors / rooms. IBC 2018, 2021 and 2024 (Table 803.11 in 2015).
| Group | Sprinklered | Not sprinklered |
|---|---|---|
| A-1 & A-2 | B / B / C | A / A / B |
| A-3, A-4, A-5 | B / B / C | A / A / C |
| B, E, M, R-1 | B / C / C | A / B / C |
| R-4 | B / C / C | A / B / B |
| F | C / C / C | B / C / C |
| H | B / B / C | A / A / B |
| I-1 | B / C / C | A / B / B |
| I-2 | B / B / B | A / A / B |
| I-3 | A / A / C | A / A / B |
| I-4 | B / B / B | A / A / B |
| R-2 | C / C / C | B / B / C |
| R-3 | C / C / C | C / C / C |
| S | C / C / C | B / B / C |
| U | No restriction | No restriction |
Key footnotes: in buildings under three stories (other than I-2), exit stairways may use Class B without sprinklers or Class C with them; Class C wainscoting or paneling up to 1,000 sq ft is allowed in a grade lobby. Sprinklered columns apply only with an NFPA 13 or 13R system (IBC 903.3.1.1 / 903.3.1.2).
Other ASTM E84 limits and floor finishes
| Use | Limit | Code |
|---|---|---|
| Materials exposed in plenums | FSI ≤ 25, SDI ≤ 50 | IMC 602.2.1 |
| Duct coverings, linings, insulation | FSI ≤ 25, SDI ≤ 50 | IMC 604.3 |
| Building insulation, concealed or exposed | FSI ≤ 25, SDI ≤ 450 | IBC 720.2, 720.3 |
| Foam plastic insulation | FSI ≤ 75, SDI ≤ 450 | IBC 2603.3 |
| Fire-retardant-treated wood | FSI ≤ 25 in a 30-min extended test, flame front ≤ 10.5 ft past burner | IBC 2303.2 |
| Floor finish Class I / Class II | ≥ 0.45 / ≥ 0.22 W/cm² critical radiant flux | ASTM E648 / NFPA 253, IBC 804 |
What the indices represent
ASTM E84 burns a 24 in × 25 ft specimen on the ceiling of a tunnel for 10 minutes. FSI is derived from the area under the flame-front distance–time curve, scaled so inorganic reinforced cement board is 0 and select-grade red oak is 100; SDI is scaled the same way from smoke obscuration. ASTM notes the two measurements are not necessarily related.
- It does not measure heat transmission through the surface.
- FSI alone does not establish that a material is noncombustible.
- Mounting, support, thickness, facing and substrate can change the reported result.
- It is not an hourly fire-resistance rating (ASTM E119).
Primary references
Quick answer
The flame spread index (FSI) is the ASTM E84 / UL 723 tunnel-test number comparing flame travel with red oak (100) and cement board (0). IBC and NFPA 101 classes are Class A FSI 0–25, Class B 26–75 and Class C 76–200, each with a smoke-developed index of 450 or less. A "25/50" rating means FSI ≤ 25 and SDI ≤ 50.
Examples
| Class / rating | Flame spread index | Smoke-developed index |
|---|---|---|
| Class A (Class I) | 0–25 | 0–450 |
| Class B (Class II) | 26–75 | 0–450 |
| Class C (Class III) | 76–200 | 0–450 |
| 25/50 (plenums, duct insulation) | 0–25 | 0–50 |
Enter the FSI and SDI from your report, then pick an occupancy and location to see the required class.
Summary
Interpret flame-spread index (FSI) and smoke-developed index (SDI) values reported by an ASTM E84 or UL 723 test. IBC Section 803.1.2 and NFPA 101 Section 10.2.3.4 class interior wall and ceiling finishes as Class A (FSI 0–25), B (26–75) or C (76–200), each with SDI 0–450. The tool then looks up the minimum class required by IBC Table 803.13 for the occupancy group, sprinkler status and location, and checks other common E84 limits: 25/50 for plenum materials and duct coverings, 25/450 for building insulation and 75/450 for foam plastic insulation. It interprets reported numbers; it is not a fire test or a code approval.
How it works
- Find the reported FSI and SDI on the test report or listing for the exact product and assembly.
- Enter both whole-number indices; the tool returns Class A, B or C, or "outside" if FSI > 200 or SDI > 450.
- Choose the occupancy group, whether the building is fully sprinklered (NFPA 13 or 13R) and where the finish is installed to see the IBC Table 803.13 minimum class.
- Read the pass/fail list for 25/50, insulation and foam plastic limits.
- Confirm the adopted code edition, local amendments and table footnotes with the authority having jurisdiction.
Use cases
- Check whether a wall panel with FSI 30 / SDI 300 can be used in a sprinklered office corridor.
- Explain what a flame spread index of 25 means and why SDI 450 still matters.
- Verify a "25/50 rated" claim for duct wrap or materials in a return-air plenum.
- Compare insulation and foam plastic E84 limits with a product data sheet.
- Prepare questions for a manufacturer, testing laboratory or code official.