Duct Size Calculator
An online ductulator for HVAC ducts. Enter the airflow and either a friction rate (equal-friction method) or a target air velocity, and get the exact round duct diameter, the next standard size with its real velocity and friction, and equivalent rectangular duct sizes.
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How to use the Duct Size Calculator
- Enter the airflow the duct must carry, for example 100 CFM for a bedroom branch or 1,200 CFM for a 3-ton trunk.
- Choose the sizing method: friction rate (0.06–0.10 in. w.c. per 100 ft is typical for homes) or a target velocity.
- Pick the duct type so the velocity can be checked against typical limits.
- Read the round diameter, then the standard size and its velocity and friction.
- Use the rectangular table if the duct has to fit a joist or wall cavity.
Duct sizing formulas
Equal-friction method. For smooth galvanized round duct with standard air, friction loss in inches of water column per 100 ft is closely approximated by:
f = 0.109136 × Q1.9 ÷ D5.02 so D = (0.109136 × Q1.9 ÷ f)1/5.02
with Q in CFM and D in inches. Velocity method: V (fpm) = Q ÷ area (ft²), so D = √(4Q ÷ πV) × 12.
Rectangular equivalent (Huebscher): De = 1.30 × (a × b)0.625 ÷ (a + b)0.25. A rectangular duct with this equivalent diameter has the same friction loss as the round duct at the same airflow.
Unit conversions: 1 in. w.c. per 100 ft = 8.17 Pa/m; 1 m/s = 196.85 fpm; 1 L/s = 2.119 CFM; 1 m³/h = 0.5886 CFM.
Round duct capacity at 0.08 in. w.c. per 100 ft
| Round duct | Airflow (CFM) | Velocity (fpm) |
|---|---|---|
| 5 in | ~60 | ~440 |
| 6 in | ~97 | ~490 |
| 7 in | ~145 | ~545 |
| 8 in | ~205 | ~590 |
| 10 in | ~370 | ~685 |
| 12 in | ~600 | ~770 |
| 14 in | ~905 | ~850 |
| 16 in | ~1,290 | ~925 |
Values from the friction equation above for smooth metal duct. Flexible duct carries noticeably less at the same friction rate.
Frequently asked questions
How do I size a duct from CFM?
With the equal-friction method you pick a friction rate and solve the friction equation for the diameter. This calculator uses the standard fit for galvanized round duct: friction (in. w.c. per 100 ft) = 0.109136 × CFM^1.9 ÷ D^5.02, so D = (0.109136 × CFM^1.9 ÷ friction)^(1 ÷ 5.02). At 0.08 in./100 ft, 100 CFM needs about a 6 inch duct, and 400 CFM needs a little over 10 inches (a 10 inch duct would run at about 0.09 in./100 ft).
What friction rate should I use?
Residential systems are commonly designed between about 0.05 and 0.10 in. w.c. per 100 ft, depending on the blower's available static pressure and the total effective length of the longest run (ACCA Manual D). 0.08 or 0.10 is a typical starting point for short runs and 0.05 to 0.06 for long runs or quiet systems.
What air velocity is acceptable in ducts?
For quiet homes, supply trunks are usually kept under about 900 feet per minute, supply branches under about 600 fpm, return trunks under about 700 fpm and return grilles much lower. Commercial ducts often run faster. Higher velocity means smaller ducts but more noise and pressure loss.
How do I convert a round duct to a rectangular duct?
Use the equivalent diameter, which gives the same airflow and friction: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25, where a and b are the rectangle's sides. For example a 10 × 8 inch duct is equivalent to about a 9.8 inch round duct. Note that the rectangular duct has more cross-sectional area, so its air velocity is lower.
How many CFM per ton of air conditioning?
Air conditioners are usually designed for about 350 to 400 CFM per ton of cooling, so a 3-ton system moves roughly 1,050 to 1,200 CFM. Each room's share is proportional to its heating or cooling load.
Does duct material matter?
Yes. The formulas assume smooth galvanized steel. Flexible duct has much more friction, especially when compressed or sagging, so it is often sized one size larger than metal duct for the same airflow.
Is my data stored?
No. Everything is calculated in your browser.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.