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CFM calculator

Airflow from a heat load or from air changes per hour — standard-air factors, corrected for altitude.

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Btu/h
°F
ft

Standard-air factors assume sea level; altitude scales them by air density.

Result

Airflow
1,091cfm
Factor used
1.1
  1. 1.Standard (sea-level) air: factor = 1.1
  2. 2.cfm = q / (factor × Δt) = 24,000 / (1.1 × 20) = 1,091 cfm

How it's tested on the exam

Shows up on: PE Mechanical (HVAC & Refrigeration)

  • 1.10 is the Handbook's sea-level factor: 60 min/h × 0.075 lb/ft³ × (0.24 + 0.45W) Btu/lb·°F. Many textbooks use 1.08 (dry air only). On the PE, use the Handbook's 1.10, and its 0.92 at 5,000 ft — at altitude the density and the factor drop, so the same load needs more cfm.
  • Latent load: q_l = 4,840 × cfm × ΔW with ΔW in lb of water per lb of dry air — divide grains by 7,000 first. 4,840 is the Handbook's number (60 × 0.075 × 1,076); the 0.68-per-grain form some texts use rests on a different latent heat (about 1,060 Btu/lb rather than 1,076), and it is not 4,840 / 7,000 (0.691).
  • Δt is supply to room, not outdoor to room, when you size supply air for a space load.
  • cfm is volume, not mass. Two airflows with the same cfm at different temperatures carry different mass.

Worked example

A zone has a 24,000 Btu/h sensible load, held at 75 °F with 55 °F supply air. How much supply air at sea level, and at 5,000 ft?

  1. 1.Sea level: cfm = 24,000 / (1.10 × 20) = 1,091 cfm
  2. 2.At 5,000 ft the density ratio is (1 − 5,000 × 6.875 × 10⁻⁶)^5.256 = 0.832, so the factor is 1.10 × 0.832 = 0.915
  3. 3.cfm = 24,000 / (0.915 × 20) = 1,311 cfm (the Handbook's rounded 5,000-ft factor, 0.92, gives 1,304)

About 1,090 cfm at sea level; about 1,310 cfm at 5,000 ft.

Where it is in the Handbook

  • Heating, Ventilation, and Air Conditioning — Heat Gain Calculations Using Standard Air and Water Values — PE Mechanical Reference Handbook

Section names as printed in the current NCEES PE Reference Handbooks (Mechanical 2.1, Civil 2.2); numbering can shift between versions. On exam day you'll use your own approved calculator — here's which models NCEES allows.