Free bushfire attack level (BAL) calculator

Australia-only. Runs Simplified and Appendix B methods simultaneously to AS 3959-2018 - outputs BAL rating and AS 3959 construction section reference.

Free · No sign-up requiredInstant resultsAU onlyAS 3959-2018 Construction of buildings in bushfire-prone areas

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How to use the free bushfire attack level calculator

1
Enter site parameters
Input Fire Danger Index, BAL Low exclusion status, vegetation classification, distance from classified vegetation, and effective slope under vegetation.
2
Review both methods
The calculator runs the simplified method and the Appendix B detailed method simultaneously so you can compare results.
3
Read the BAL rating
The resulting BAL (LOW, 12.5, 19, 29, 40, or FZ) determines the construction requirements for the building under AS 3959.

About this calculator

Two-method BAL assessment to AS 3959-2018 - the Simplified method (FDI lookup tables, Cl 2) and the Appendix B detailed method (radiant heat flux calculation) - running simultaneously so you can compare and select the governing result. Covers Fire Danger Index values for all Australian states and territories.

Standard: AS 3959-2018 Construction of buildings in bushfire-prone areas

How to use the free bushfire attack level calculator

The BAL calculator allows you to find the BAL for any site in Australia with five simple inputs, using both the simplified method and the detailed (Appendix B) method simultaneously.

Signing up for a Calcs.com account unlocks further advanced features for design and analysis. The sheet has Key Properties (inputs), Summary (calculated BAL and design considerations), Advanced Parameter Overrides, and Detailed Method Results.

Only five key inputs are required for most BAL calculations: Fire Danger Index, BAL Low Threat Exclusion, Vegetation Classification, Distance Between Site and Classified Vegetation, and Effective Slope Under Classified Vegetation.

Effective slope under classified vegetation

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Frequently asked questions

What is Bushfire Attack Level (BAL)?

BAL is a measure of the potential severity of a building's exposure to ember attack, radiant heat, and direct flame contact from a bushfire. There are six BAL ratings: BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40, and BAL-FZ (Flame Zone). Higher ratings require more stringent construction to AS 3959.

What is the difference between the simplified method and the Appendix B method?

The simplified method uses AS 3959 Table 2.4 lookup tables with five inputs. It is conservative and suitable for most standard sites. The Appendix B method calculates radiant heat intensity from flame geometry - it can produce a lower (less conservative) BAL where justified, but requires more detailed vegetation assessment.

What inputs does this calculator require?

The five key inputs are: (1) Fire Danger Index (FDI) for your region, (2) whether BAL-LOW exclusion applies, (3) vegetation classification (e.g. grassland, shrubland, forest), (4) distance between the building and the classified vegetation boundary, and (5) effective slope of the land under the vegetation.

Which Australian standard does this calculator use?

This calculator implements AS 3959-2018 (Construction of buildings in bushfire-prone areas). It uses the 2018 edition, which includes updated FDI values and the Appendix B detailed method.

Is this calculator appropriate for a building permit application?

This free tool is for indicative assessment only. A formal BAL assessment for a building permit must be certified by a qualified bushfire consultant. A full Calcs.com trial adds the ability to save, export, and customise the calculation for formal documentation.

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