Duct Velocity and Sizing Calculator
Air velocity in a rectangular duct, its equivalent round diameter, and the size a target velocity needs.
Results
What this tool does
The equivalent diameter is the round duct that would have the same pressure drop at the same flow, which is what lets rectangular ducts be sized from round-duct charts. It is not the duct that has the same area — a flat duct always performs worse than its area suggests, because more of the air is rubbing against a wall.
Formula
v = Q ÷ A ; Dₑ = 1.3 (ab)^0.625 ÷ (a+b)^0.25
Variables
| Symbol | Meaning | Unit |
|---|---|---|
qq | Flow rate | m³/h |
aa | Duct width | mm |
bb | Duct height | mm |
vm | Speed | m/s |
VV | Speed | m/s |
DE | Equivalent round diameter | mm |
AR | Area | m² |
AN | Area | m² |
DR | Duct width | mm |
Worked example
- Flow rate1500 m³/h
- Duct width400 mm
- Duct height250 mm
- Speed5 m/s
- Speed4.1667 m/s
- Equivalent round diameter343.333 mm
- Area0.10000 m²
- Area0.08333 m²
- Duct width325.74 mm
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
What velocity should I design for?
Low, if you want it quiet. Common guidance keeps main ducts in homes and offices at 4 to 6 m/s, branches at 3 to 4, and outlets below 2 — noise rises steeply with velocity, roughly as its fifth or sixth power, so doubling the speed is not twice as loud but many times worse. Industrial systems run faster because nobody is trying to hold a conversation next to them.