Airflow (CFM) Estimator
Optional humidity inputs
Enter Return/Supply temperature + RH to estimate today’s SHR and enable the enthalpy method.
CFM Estimator – Input Notes
  • System Tonnage (tons) – Nameplate cooling size. 1 ton = 12,000 BTU/h.
  • Measured ΔT (Return DB − Supply DB) – Actual dry-bulb temp drop across the indoor coil (cooling mode). Take readings in the return/supply plenums, not a random grille.
  • Assumed SHR – Sensible Heat Ratio (share of cooling that lowers air temperature). Typical 0.70–0.80; 0.75 is a good default.
  • Use auto SHR from humidity – If you enter return/supply DB + RH, the tool estimates today’s SHR and copies it into “Custom SHR” so you can see/edit it.
  • Custom SHR – Manual override (0–1). Use this when you know the job’s sensible/latent split or want to tweak the auto value.
  • Altitude (ft) – Used to adjust for lower air density at elevation (roughly −3% per 1,000 ft). If unsure, enter city elevation.
  • Return Dry-Bulb (°F) – Temperature of air entering the coil (return plenum). Avoid radiant hits; shield the probe.
  • Return RH (%) – Relative humidity of return air. Needed for auto-SHR and enthalpy method.
  • Supply Dry-Bulb (°F) – Temperature of air leaving the coil (supply plenum, ~2–3 ft downstream if possible).
  • Supply RH (%) – RH of supply air. Often high (90–100%) right off the coil—this is normal.
Tip: Let the system stabilize ~10–15 minutes before readings. Verify clean filter, correct stage, and reasonable duct leakage to avoid misleading ΔT/RH.