Duct size calculator
Duct sizing trades size against noise. Too small and it whistles; too large and it does not fit.
m3/h
m/s
Main ducts 4 to 7 m/s; branches 2 to 4; supply grilles under 2.5.
Round duct diameter
210mm
- Equivalent rectangular width
- 264
- Equivalent rectangular height
- 132
- Equivalent round diameter of that rectangle
- 201
- Duct area
- 0.034 72 m2
- Flow in cubic feet per minute
- 294.3
- Approximate friction loss
- 13.70 Pa
- Velocity pressure
- 9.60 Pa
Show the working
- Air flow = 500
- Target velocity = 4
- Rectangular aspect ratio = 2
- Duct length = 15
- flow_m3s = 500 / 3600 = 0.138 889
- Duct area = 0.138 889 / 4 = 0.034 722 2
- Round duct diameter = sqrt(4 * 0.034 722 2 / pi) * 1000 = 210.261
- Flow in cubic feet per minute = 500 * 0.5885777777777777 = 294.289
- Equivalent rectangular height = sqrt(0.034 722 2 / 2) * 1000 = 131.762
- Equivalent rectangular width = 131.762 * 2 = 263.523
- Equivalent round diameter of that rectangle = 1.30 * (263.523*131.762)^0.625 / (263.523 + 131.762)^0.25 = 200.722
- Approximate friction loss = 210.261 > 0 ? 0.02 * 15 / (210.261/1000) * 1.2 * 4^2 / 2 : 0 = 13.6973
- Velocity pressure = 0.5 * 1.2 * 4^2 = 9.6
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- area = flow / velocity
- diameter = sqrt(4 x area / pi)
- equivalent round diameter = 1.30 (ab)^0.625 / (a+b)^0.25
A rectangular duct of the same cross-sectional area carries less air than a round one, which is what the equivalent diameter corrects for.