Free Engineering Tool · NFPA 92
Smoke Layer Descent Calculator
Estimate how fast the smoke layer comes down in an atrium, mall or other large space with no smoke exhaust running. Enter the design fire, the ceiling height and the floor area, and the calculator gives the layer height at any time and the time to reach a target height, using the empirical correlations in NFPA 92.
- Based on NFPA 92
- Free, runs in your browser; your inputs stay on your device
- Screening tool, not reviewed or sealed by a licensed engineer
- Last updated October 2026
Screening tool, not engineering advice. This calculator estimates results with simplified published methods. It has not been reviewed or sealed by a licensed engineer for your project, and DGA does not warrant that its results are accurate, complete or suitable for any use. Check every result independently and have the licensed professional responsible for your project review it before you rely on it. Use is subject to the Terms of Use.
Advanced method. The correlations behind this tool come from research and test data with narrow ranges of validity. Outside those ranges results can be wrong by a wide margin, and they are no substitute for a fire model or an analysis by a qualified fire protection engineer.
Design Fire
Space and Target
Heights are measured from the base of the fire. The space is taken as a uniform box or cylinder with the fire near its center and no smoke exhaust running.
Result
Smoke Layer Height Over Time
About This Tool
When a fire starts in a tall space, the plume carries smoke to the ceiling, where a layer forms and thickens. NFPA 92 gives two empirical correlations for the height of the bottom of that layer, z, above the base of the fire, for a space of uniform cross-section with no smoke exhaust. For a steady fire of heat release rate Q: z/H = 1.11 − 0.28 ln[t Q1/3 H−4/3 / (A/H2)], with Q in kW and H in m, or z/H = 0.67 − 0.28 ln[…] with Q in Btu/s and H in ft. For a fire that grows as a t-squared fire with growth time tg: z/H = 0.91 [t tg−2/5 H−4/5 (A/H2)−3/5]−1.45 in SI, or 0.23 […]−1.45 with H in ft. H is the ceiling height above the fire, A the floor area and t the time after ignition in seconds.
Both correlations come from fire tests in large spaces with shape factors A/H2 from 0.9 to 14 and are considered valid down to z/H = 0.2. They track the lower edge of the smoke, where the first wisps appear, so they are conservative compared with zone models that compress the layer into a sharp interface. They also include the time the plume needs to reach the ceiling, which is why a growing fire shows the layer still at the ceiling for the first minute or two. Rearranging each equation gives the time for the layer to reach a target height. The US and SI forms published in the standard round their constants separately, so switching units shifts the answer by up to about one percent of H. For a growing fire the heat release rate shown is 1,000 Btu/s (1,055 kW) times (t/tg)2; the t-squared fire growth calculator builds that curve in detail.
Worked example
An atrium 120 by 100 ft (12,000 ft2) has a ceiling 80 ft above the floor, so A/H2 = 1.88. A fast t-squared fire (tg = 150 s) starts on the floor. At 120 s the correlation still puts the layer at the ceiling; it begins to descend at about 131 s. At 180 s the layer is 50.3 ft above the floor (z/H = 0.63) while the fire is about 1,440 Btu/s (1,520 kW). The layer reaches a target height of 40 ft at about 211 s, three and a half minutes after ignition. A steady 3,000 Btu/s fire in the same space brings the layer to 40 ft in about 82 s.
Assumptions and limits
The space is a box or upright cylinder with vertical walls and a flat ceiling, the fire is near the middle of the floor, and the layer is not disturbed by HVAC, stack effect, wind, sprinkler spray or exhaust. The t-squared correlation assumes the fire keeps growing; once a real fire levels off, the steady form or a zone model is needed. Spaces with sloped ceilings, balconies or varying cross-sections, layers below 0.2 H and shape factors outside 0.9 to 14 call for a zone or CFD model. Stratification under a hot ceiling, which can keep smoke from ever reaching it, is not modeled.
When to bring in an engineer
The filling time is the start of an atrium smoke control design: it sets when the exhaust must run and how high the layer must stay. DGA designs smoke control systems and runs the fire and smoke modeling that NFPA 92 expects for complex spaces.
References
- NFPA 92, Standard for Smoke Control Systems, National Fire Protection Association, 2024 edition: smoke layer interface position for steady and t-squared fires.
- Heskestad, G., and Delichatsios, M.A., Environments of Fire Detectors, Phase I: Effect of Fire Size, Ceiling Height and Material, Vol. I and II, NBS-GCR-77-86 and NBS-GCR-77-95, National Bureau of Standards, 1977.
- Cooper, L.Y., Harkleroad, M., Quintiere, J., and Rinkinen, W., “An Experimental Study of Upper Hot Layer Stratification in Full-Scale Multiroom Fire Scenarios,” Journal of Heat Transfer, Vol. 104, 1982, pp. 741 to 749.
- Klote, J.H., AtriumCalc: Atrium Smoke Control Calculator, Technical Information and User Guide, Version 1.1, ASHRAE, 2015.
Frequently Asked Questions
How long does it take smoke to fill an atrium?
It depends on the fire, the ceiling height and the floor area. With no exhaust, a fast-growing fire in an 80 ft high atrium of 12,000 square feet brings the smoke layer halfway down in about three and a half minutes. Larger floor areas and taller ceilings take longer; faster-growing or larger steady fires fill faster.
What does z/H mean in the smoke filling equations?
It is the height of the bottom of the smoke layer above the base of the fire, z, divided by the ceiling height above the fire, H. A value of 1 means the layer has not started to come down; NFPA 92 considers the correlations valid down to 0.2.
What is the shape factor A/H2?
It is the floor area of the space divided by the square of its height. Tall narrow spaces have small values and wide low spaces have large values. The NFPA 92 correlations were developed from tests with values between 0.9 and 14.
Why does the layer stay at the ceiling at first?
Smoke needs time to rise to the ceiling and spread before a layer forms, and a growing fire is small at first. The empirical correlations include this lag, so early times can give z/H greater than 1, which the calculator shows as the layer still at the ceiling.
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Engineering aid only, provided as is and without warranty. Results depend on the inputs and on the simplifying assumptions of the method summarized on each tool page, and DGA has not verified them for your application. They do not replace code-required design, testing or review by the licensed engineer responsible for the project. Use of the tools is subject to the Terms of Use.
Related Reading
Articles from DGA Insights on the code and the method behind this calculator.