An online ductulator for round, rectangular and flat oval duct
The duct sizer above does the job of the slide-rule ductulator. Give it an airflow and a friction rate or a velocity and it returns the round duct diameter; add a width or a height and it returns the matching rectangular or flat oval size. Give it a duct size instead and it returns the pressure drop and velocity. It takes IP or SI units in any mix, corrects for material roughness, elevation and air temperature, and uses the equivalent diameter and friction equations of the ASHRAE Handbook, Fundamentals.
Using the duct sizer
- Enter the airflow the duct must carry. For a trunk, that is the sum of the outlets downstream of it.
- Enter either a pressure drop (Pd) in inches of water column per 100 ft or a velocity in fpm. Table 2 gives the usual starting values; the project specification or ACCA Manual D sets the real target.
- Read the round diameter, and whichever of velocity or pressure drop you did not enter.
- For a rectangular or flat oval duct, enter the height you have room for and the width comes back at the same pressure drop. Advanced Settings holds the aspect ratio, the shape (Rnd/Rect or Rnd/Oval) and the air conditions, and the + beside the result lists alternate sizes.
- To check an existing duct, enter the airflow and its size (diameter, or width and height) and read the pressure drop and velocity.
Example
1,000 CFM at 0.10 in. wc per 100 ft, galvanized steel, sea level. The calculator returns a 13.7 in round duct at 983 fpm. With a 10 in height the rectangular equivalent is 16 x 10 in (aspect ratio 1.6:1, 900 fpm); with an 8 in height it is 22 x 8 in. Checking a 20 x 10 in duct at the same airflow gives 720 fpm and 0.06 in. wc per 100 ft. Open this example in the calculator.
Round duct sizes by airflow
Calculated by this page for galvanized steel duct at sea level and 70 °F. Round up to the next available diameter. For a rectangular duct, enter the airflow and one side in the calculator to get the other.
| Airflow CFM |
0.08 in. wc / 100 ft | 0.10 in. wc / 100 ft | ||
|---|---|---|---|---|
| Diameter in | Velocity fpm |
Diameter in | Velocity fpm |
|
| 100 | 6.0 | 500 | 5.8 | 550 |
| 150 | 7.0 | 560 | 6.7 | 610 |
| 200 | 7.8 | 600 | 7.5 | 660 |
| 300 | 9.1 | 660 | 8.7 | 730 |
| 400 | 10.1 | 710 | 9.7 | 780 |
| 500 | 11.0 | 760 | 10.5 | 830 |
| 600 | 11.8 | 790 | 11.3 | 870 |
| 800 | 13.1 | 850 | 12.6 | 930 |
| 1,000 | 14.3 | 900 | 13.7 | 980 |
| 1,200 | 15.3 | 940 | 14.6 | 1,030 |
| 1,500 | 16.7 | 990 | 15.9 | 1,090 |
| 2,000 | 18.6 | 1,070 | 17.7 | 1,170 |
| 2,500 | 20.2 | 1,130 | 19.3 | 1,230 |
| 3,000 | 21.6 | 1,180 | 20.7 | 1,290 |
| 4,000 | 24.1 | 1,260 | 23.0 | 1,380 |
| 5,000 | 26.2 | 1,330 | 25.1 | 1,460 |
Velocities are rounded to 10 fpm. A velocity above the ranges in Table 2 means the duct is small for the airflow and will be noisy; size it at a lower friction rate.
Friction rate and velocity
The usual starting values for straight duct. Noise, the fan's available static pressure and the project specification decide the final numbers.
| Application | Friction rate in. wc / 100 ft | Velocity fpm |
|---|---|---|
| Residential supply trunk | 0.08 to 0.10 | 700 to 900 |
| Residential supply branch | 0.08 to 0.10 | 600 to 900 |
| Residential return | 0.05 to 0.08 | 500 to 700 |
| Commercial low pressure main (offices, schools) | 0.08 to 0.15 | 1,000 to 1,500 |
| Commercial branch | 0.08 to 0.10 | 600 to 1,000 |
| Medium and high pressure main | 0.15 and up | 2,000 to 4,000 |
Method
Pressure drop comes from the Darcy-Weisbach equation, with the friction factor solved iteratively from the Colebrook equation for the duct's Reynolds number and relative roughness. This is the calculation behind the ASHRAE friction chart; the calculator does the iteration instead of the chart lookup.
A rectangular duct is sized through its equivalent diameter, the round duct with the same pressure drop at the same airflow, from the Huebscher relation
De = 1.30 (a b)0.625 / (a + b)0.25
where a and b are the duct sides in inches. A flat oval duct uses the Heyt and Diaz form of the same relation, from its cross-sectional area A and perimeter P:
De = 1.55 A0.625 / P0.25
Velocity is always the airflow divided by the duct's actual free area, not the equivalent round area, which is why a 20 x 10 in duct at 1,000 CFM reads 720 fpm.
Air density comes from the ISO 2533 standard atmosphere at the entered elevation, corrected to the entered temperature with the ideal gas law, and viscosity from the 1985 NIST correlation for dry air. The defaults are sea level and 70 °F (0.075 lb/ft3). Material roughness defaults to 0.0003 ft, the ASHRAE value for galvanized steel duct, and Table 3 lists the other classes. The difference is not small: 1,000 CFM through a 14 in duct drops 0.09 in. wc per 100 ft in galvanized steel and 0.17 in fully extended flex duct.
| Duct material | Roughness ft |
|---|---|
| Uncoated carbon steel, aluminum, PVC (smooth) | 0.0001 |
| Galvanized steel, longitudinal or spiral seams (medium smooth, the default) | 0.0003 |
| Galvanized steel, short joints (average) | 0.0005 |
| Fibrous glass duct, or lined duct (medium rough) | 0.003 |
| Flexible duct, fully extended (rough) | 0.01 |
Questions
What size duct do I need for 400 CFM?
At 0.1 in. wc per 100 ft in galvanized steel, 400 CFM needs a round duct of about 9.7 in, so a 10 in duct, running at about 780 fpm. Enter 400 and 0.1 in the calculator and add a height to get the matching rectangular size.
How many CFM can a 12 inch duct carry?
At 0.1 in. wc per 100 ft a 12 in round galvanized duct carries about 710 CFM at about 900 fpm. At the same friction rate a 6 in duct carries about 115 CFM, 8 in about 240 CFM, 10 in about 440 CFM and 14 in about 1,070 CFM.
Does elevation or air temperature change the size?
Yes, through air density. At 5,000 ft the air is thinner, so 1,000 CFM at 0.1 in. wc per 100 ft needs a 13.2 in duct instead of 13.7 in at sea level. Set elevation and temperature in Advanced Settings; the defaults are sea level and 70 °F.
Does the pressure drop include fittings?
No. The pressure drop is for straight duct only. Add elbows, takeoffs and transitions with the loss coefficients of the ASHRAE Duct Fitting Database, or by equivalent length as in ACCA Manual D.
See also
- Pipe Sizer, for hydronic water and glycol piping.
- Steam Pipe Sizer, for saturated steam.
- Psychrometric Calculator and Psychrometric Chart, for moist air properties and processes.
- Thermist for Revit, which routes the duct you just sized with clash avoidance.
References
- ASHRAE Handbook, Fundamentals, chapter 21, Duct Design: equivalent diameter, friction chart and duct roughness classes.
- ISO 2533:1975, Standard Atmosphere.
- Kadoya, Matsunaga and Nagashima, Viscosity and Thermal Conductivity of Dry Air in the Gaseous Phase, Journal of Physical and Chemical Reference Data, 1985.