Airflow / CFM & Static Pressure Calculator
Find airflow (CFM) from heat, velocity, or velocity pressure, verify delivered capacity with the 1.08 sensible formula, and check total external static pressure against field targets.
Use Calculator in AppWhat is Airflow & Static Pressure?
Airflow is the volume of air a system moves, measured in cubic feet per minute (CFM). Proper airflow is critical: too little starves the coil (frozen evaporator, poor dehumidification), while too much hurts latent removal and comfort. The industry rule of thumb is roughly 400 CFM per ton of cooling (typically 350–450 CFM/ton depending on climate and equipment).
Total external static pressure (TESP) is the resistance the blower works against, measured in inches of water column (iwc) with a manometer at the supply and return plenums. Most residential systems target 0.50 iwc or less; readings above 0.80 iwc signal restricted ducts, a dirty filter or coil, or undersized returns.
These calculations are estimates for reference. Always verify against the equipment nameplate, the blower performance table, and manufacturer specifications before making a diagnosis.
Target Airflow by Cooling Tonnage (~400 CFM/ton)
Typical required airflow for residential cooling systems at 350, 400, and 450 CFM per ton:
| Tons | Nominal BTU/hr | 350 CFM/ton | 400 CFM/ton | 450 CFM/ton |
|---|---|---|---|---|
| 1.5 | 18,000 | 525 CFM | 600 CFM | 675 CFM |
| 2 | 24,000 | 700 CFM | 800 CFM | 900 CFM |
| 2.5 | 30,000 | 875 CFM | 1,000 CFM | 1,125 CFM |
| 3 | 36,000 | 1,050 CFM | 1,200 CFM | 1,350 CFM |
| 3.5 | 42,000 | 1,225 CFM | 1,400 CFM | 1,575 CFM |
| 4 | 48,000 | 1,400 CFM | 1,600 CFM | 1,800 CFM |
| 5 | 60,000 | 1,750 CFM | 2,000 CFM | 2,250 CFM |
Total External Static Pressure (TESP) Guide
Interpreting a total external static pressure reading (measured in inches of water column):
| TESP (iwc) | Status | Typical Cause | Action |
|---|---|---|---|
| ≤ 0.50 | Good | Correctly sized ducts, clean filter | Airflow within spec |
| 0.51 – 0.80 | Elevated | Dirty filter, mild duct restriction | Inspect filter/coil, monitor |
| 0.81 – 1.00 | High | Undersized returns, dirty coil, closed dampers | Correct restriction — airflow reduced |
| > 1.00 | Critical | Severe blockage or crushed/collapsed duct | Find and clear before running |
Key Airflow Formulas
CFM from Heat
CFM = BTU / (1.08 × ΔT). Rearranged sensible-heat formula; ΔT is the return-minus-supply temperature split across the coil.
CFM from Velocity
CFM = Velocity (FPM) × Duct Area (ft²). Measure velocity with an anemometer; use ft², not square inches.
Velocity from Velocity Pressure
V = 4005 × √VP. Convert a pitot-tube velocity pressure (iwc) into air velocity in FPM.
Static Pressure Target
Total external static pressure should be ≤ 0.50 iwc; above 0.80 iwc indicates a restriction that is choking airflow.
Airflow Calculation Example
Problem:
A 3-ton system is delivering about 27,000 BTU/hr of sensible cooling. The tech measures a 20°F temperature split (ΔT) across the coil and a total external static pressure of 0.45 iwc. Find the airflow and check it against the 400 CFM/ton rule.
Solution:
CFM from heat: 27,000 / (1.08 × 20) = 1,250 CFM
Target airflow (400 CFM/ton): 3 × 400 = 1,200 CFM → 1,250 CFM is right on target
Velocity cross-check: a 20" × 10" plenum = 1.39 ft²; pitot VP of 0.05 iwc → V = 4005 × √0.05 ≈ 896 FPM; CFM = 896 × 1.39 ≈ 1,245 CFM
Static pressure: 0.45 iwc ≤ 0.50 iwc → healthy, no restriction
Result: ~1,250 CFM (≈417 CFM/ton) with 0.45 iwc TESP — airflow and static pressure both within spec
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