Enter the design fire heat release rate and the clear height from the top of the fuel to the bottom of the smoke layer you want to maintain, and the page estimates the smoke a steady axisymmetric fire plume carries into the layer, then converts it to an exhaust volume in cfm with the SI value in parentheses. It suits early sizing checks for atriums, malls, arenas and other large open spaces.
Basis: the axisymmetric plume equations of NFPA 92, Standard for Smoke Control Systems, which IBC Section 909 (Smoke Control Systems) points to for smoke exhaust in large spaces. The convective heat release is Qc = 0.7 Q in kW and the limiting flame height is zl = 0.166 Qc^(2/5) in m. Above the flame (z > zl) the mass flow is m = 0.071 Qc^(1/3) z^(5/3) + 0.0018 Qc in kg/s; at or below it, m = 0.032 Qc^(3/5) z. The volume is m divided by an ambient density of 1.2 kg/m³ (about 0.075 lb/ft³), and 1 m³/s equals 2,118.88 cfm. For example, a 5,000 Btu/s (about 5,275 kW) fire with a 20 ft clear height gives roughly 51,000 cfm. Reference as of October 2026; check the edition of NFPA 92 and the IBC adopted in your jurisdiction.
This is a simplified reference estimate, not a design. It ignores smoke temperature above ambient, wall and corner plumes, balcony spill and window plumes, plugholing limits on each exhaust inlet, makeup air velocity and sprinkler effects. The engineer of record, the local code (IBC 909, NFPA 92) and the authority having jurisdiction govern the final exhaust rate. Pressurization airflow for stairs and vestibules is not covered.
Frequently Asked Questions
NFPA 92 does not set exhaust by floor area. For a steady fire it sizes exhaust to remove the smoke the fire plume entrains at the design smoke layer height, so the inputs are the design fire size and the clear height, not the room area.
A design fire of 5,000 Btu/s (about 5,275 kW) is a common figure for atrium and large space smoke control, but the fire size must come from the fuel load actually expected in the space and be accepted by the engineer of record and the authority having jurisdiction.
No. It is a simplified reference estimate of plume mass flow converted at ambient air density. A real design under IBC 909 and NFPA 92 also accounts for smoke temperature, makeup air, plugholing, wall or corner plumes, sprinkler effects and system testing, and the engineer of record and local code govern.