BTU to CFM → Static Pressure → Air Velocity →
Calculator
Imperial (CFM / inches)
Metric (L/s / mm)
Duct Shape
Round
Rectangular
Airflow (CFM)
Friction Rate (in w.g. per 100 ft)
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Recommended Round Size
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Enter values above and click calculate.
How It Works
This calculator sizes ductwork with the equal-friction method — the same
approach behind a ductulator and ACCA Manual D. You choose a friction
rate, and every duct in the system is sized so it loses that much
pressure per 100 feet of run.
Round Duct
D = (0.109136 × CFM1.9 ÷ Friction Rate)1 / 5.02
This is the ASHRAE Fundamentals friction equation for galvanized sheet
metal duct with standard air, solved for diameter. The result is rounded
up to the next standard stock size.
Rectangular Duct
De = 1.30 × (W × H)0.625 ÷ (W + H)0.25
A rectangular duct does not carry the same air as a round duct of equal
area. The Huebscher equivalent-diameter equation finds the rectangular
size that produces the same friction as the round duct above. Enter the
height you are limited to — a joist space, a bulkhead, a ceiling cavity —
and the calculator solves for the width, rounded up to the next even inch.
Friction Rate
0.08–0.10 in w.g./100 ft is typical for residential
supply and return trunks. Commercial systems often run 0.10–0.20. A lower
friction rate gives larger, quieter ducts; a higher rate gives smaller
ducts with more pressure drop. In metric, 0.1 in w.g./100 ft is about
0.8 Pa/m.
Velocity Check
The calculator reports air velocity at the recommended size and
flags anything above 900 FPM (about 4.6 m/s) , where duct
noise becomes noticeable in occupied spaces. Residential branch runs are
usually kept to 600–700 FPM; trunks up to 900.
Common results at 0.1 in w.g./100 ft:
Start with the
BTU to CFM calculator to get your
airflow, size the duct here, then check the run with the
Static Pressure calculator .