What does a flame spread index of 25 mean?

An FSI of 25 is the upper FSI boundary for Class A under the displayed ICC interior wall and ceiling finish classification. The product also needs a reported SDI from 0 through 450, and its tested product, mounting and application must match the intended use.

Can I determine the flame spread index from material dimensions?

No. FSI is a comparative index produced by a standardized fire test. It cannot be calculated reliably from thickness, density, ignition temperature or a generic material name.

Is Class A the same as fireproof or noncombustible?

No. ASTM states that FSI by itself does not classify or define a material as noncombustible. The test also does not provide a fire-resistance duration or measure heat transmission through the tested surface.

Why must I enter the smoke-developed index?

The displayed A/B/C classification uses both reported indices. Class A, B and C each require an SDI between 0 and 450 in addition to the applicable FSI band.

Does this result prove building-code compliance?

No. Required classification and acceptable test method depend on the adopted code edition, jurisdiction, occupancy, location, sprinkler status, product type and installed assembly. Verify the exact report and intended installation with the authority having jurisdiction.

Can I use a generic material value instead of a product report?

No for specification or compliance decisions. Results can change with product formulation, thickness, facing, substrate, joints, mounting and support. Use the report or listing for the exact product and assembly.

Flame Spread Index Classifier

Enter the FSI and SDI from a product test report to identify whether the values fall within the displayed Class A, B or C thresholds.

Interior finish classification thresholds

Class	FSI	SDI
A	0–25	0–450
B	26–75	0–450
C	76–200	0–450

These are the displayed ICC thresholds for interior wall and ceiling finish classification. They do not state which class your project requires.

What the indices represent

ASTM E84 is a comparative, controlled surface-burning test. Select-grade red oak and fiber-cement board surfaces provide reference behavior. The method reports FSI and SDI, but ASTM notes that 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 assembly details can affect the reported result.
It is not the same as an hourly fire-resistance rating.

Primary references

Summary

Enter the FSI and SDI from a product test report to identify whether the values fall within the displayed Class A, B or C thresholds.

How it works

Find the reported FSI and SDI on the exact product or assembly test report.
Enter both whole-number indices; neither value is optional for this classification check.
The tool compares them with the displayed Class A, B and C interior-finish thresholds.
Review the result explanation and the ASTM E84 scope limitations.
Confirm the required class, test method, assembly and adopted code with the authority having jurisdiction.

Use cases

Understand what a reported flame spread index of 25 means when SDI is also known.
Check the boundary between Class A, B and C interior wall or ceiling finish classifications.
Identify when SDI above 450 prevents an A/B/C result even if FSI is low.
Explain why an ASTM E84 index is not a fire-resistance rating or proof of noncombustibility.
Prepare questions for a manufacturer, testing laboratory or code official.

Frequently Asked Questions

Last updated: 2026-09-20 · Reviewed by Nham Vu