Critical Velocity Calculator

This calculator is open-source and covered under the MIT License. The source code is on GitHub. Contributions and feedback are welcome through GitHub or here.

The 2026 NFPA 502 quotes new equations for critical velocity that give credible numbers. Here is a calculator to generate a critical velocity estimate and plot for a given tunnel.

The equations were developed by Beyer, Stacey and Brenn, and are explained in their paper published in the Fire Technology journal. You can download a paper from either source below.

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Note that the NFPA is currently fixing the 2026 version, as Annex D final editing contained several editorial errors, see the note on Stacey Agnew’s website. Until fixed, it is suggested that you use the equations as printed in the original paper above and reproduced here.

Thanks to Stacey Agnew and Technische Universität Graz for supporting the tunnelling industry by developing and sharing this model, which is adopted in NFPA 502. Special recognition goes to Michael, Conrad, and Günter.