Round duct diameter, velocity, and rectangular equivalent for any airflow and friction rate. Uses the Wright (Loeffler) friction equation for galvanized steel.
How equal-friction duct sizing works
The equal-friction method picks one target pressure drop rate (typically 0.08 in WC per 100 ft for residential supply) and sizes every duct segment to hit that rate at its design CFM. The Wright (Loeffler) friction equation gives diameter directly:
ΔP/100ft = 0.109136 × Q^1.9 / D^5.02 (Q in CFM, D in inches, ΔP in in WC)
Solving for D: D = (0.109136 × Q^1.9 / ΔP)^(1/5.02)
Velocity falls out of continuity: V = 183.35 × Q / D² (FPM, with Q in CFM and D in inches).
Velocity guidance
- Residential supply (main): 700–900 FPM
- Residential branch: 500–700 FPM
- Residential return: 500–700 FPM
- Commercial low-velocity: 1,200–1,800 FPM
- Anything > 2,000 FPM: high-velocity, needs lined duct for noise
Rectangular equivalent
Two duct dimensions, one round equivalent — the equation accounts for the extra surface area drag of rectangular shape:
De = 1.30 × (a × b)^0.625 / (a + b)^0.25
The calculator runs this backward: pick aspect ratio, get width and height that match the round duct's air-handling capacity. Aspect ratios over 4:1 lose efficiency rapidly — try to keep ducts as square as the framing allows.
Standard round duct sizes
Manufactured in 1" increments through 12", then 2" increments thereafter: 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36 inches. Round up your calculated diameter to the next standard size.
Frequently asked questions
What friction rate should I use for residential duct sizing?
0.08 in WC per 100 ft is the standard for residential supply ducts. Use 0.05 for return air and quiet bedrooms — lower friction rate means larger, quieter ducts. 0.10–0.15 is acceptable for short commercial branch runs. Never exceed 0.20 except for short high-velocity commercial systems with proper acoustic lining.
What duct velocity is too loud for a home?
For residential applications, keep main supply ducts under 900 FPM and branch runs under 700 FPM to avoid noise complaints. Above 1,200 FPM you will typically hear air noise at registers. The calculator flags anything over 2,000 FPM as very high — that's commercial territory and requires lined ductwork.
Does supply and return duct need to be the same size?
No. Return air moves more slowly than supply, so return ducts are typically larger for the same CFM. A good rule of thumb: size return ducts at 0.05 in WC per 100 ft friction rate while sizing supply at 0.08. Also, the total return CFM must equal total supply CFM — balanced system pressure prevents door pressure issues and HVAC short-cycling.
What is rectangular duct equivalent diameter?
The equivalent diameter (De) is the round duct diameter that would carry the same airflow at the same friction rate as a given rectangular duct. The formula is De = 1.30 × (a × b)^0.625 / (a + b)^0.25, where a and b are the width and height in inches. This calculator uses that equation in reverse to give you rectangular dimensions from a calculated round size.
Why should I keep the aspect ratio under 4:1 for rectangular duct?
High aspect ratios increase the perimeter-to-area ratio of the duct cross-section, which increases surface friction and heat gain/loss. A 4:1 duct has roughly 25% more friction surface than a square duct of equal area. Above 4:1, pressure loss penalties grow quickly and duct fabrication cost increases. Use the 4:1 limit as a rule of thumb and go square whenever the framing allows.