Duct Sizer Calculator
Size round and rectangular ductwork from any two of flow rate, friction rate, velocity or diameter, using Colebrook-White friction factors and the Huebscher equivalent-diameter relation.
- Density
- 0.075 lb/ft³
- Dynamic viscosity
- 0.0432 lb/(ft·h)
- Specific heat
- 0.24 Btu/(lb·°F)
- Energy factor
- 1.08 Btu/(h·°F·CFM)
Results
Enter both known values to size the duct.
Sized the duct? Design the system.
Use MEPFlow to lay out complete duct systems visually, with fitting losses and critical-path pressure carried through automatically.
How the Duct Sizer Works
Duct sizing connects four quantities: the airflow the duct must carry, the friction rate it loses along its length, the air velocity inside it, and its diameter. Fix any two and the other two follow. This calculator is a generalized solver, so every one of the six possible input pairs is handled by the same engine rather than by separate modes.
Calculations are performed on the equivalent round duct, matching the classic McQuay DesignTools DuctSizer calculation basis. Four relationships do the work:
- Continuity relates flow, area and velocity.
- Reynolds number characterises the flow regime from the air density and viscosity.
- Colebrook-White gives the Darcy friction factor.
- Darcy-Weisbach converts that friction factor into a friction rate.
The Equations
Q = A × V
Continuity — flow rate equals cross-sectional area times velocity.
Re = ρVD / μ
Reynolds number — dimensionless, so it is the same in Imperial and Metric.
1/√f = -2 log₁₀[ ε/(3.7D) + 2.51/(Re√f) ]
Colebrook-White — implicit in f, solved by iteration to a tolerance of 1e-10.
R = (1200 f / D) × Pv
Darcy-Weisbach in Imperial working form — friction rate per 100 ft, with D in inches.
De = 1.30 (a b)^0.625 / (a + b)^0.25
Huebscher — equivalent round diameter of a rectangular duct, on an equal-friction basis.
Round, Rectangular and Which Velocity Is Which
When you enter a rectangular duct, two different areas exist and they are not interchangeable. The primary results use the equivalent round duct, because that is the basis on which the friction calculation is performed. The actual rectangular geometry section reports the true width × height area and the velocity that airflow actually travels at inside that section.
The actual velocity is the number to use for acoustic assessment and for sizing grilles, diffusers and coils. The equivalent-round velocity belongs to the pressure-loss calculation. The legacy tool only ever showed the equivalent-round basis; both are reported here so the distinction is explicit.
Why Are the Warnings Advisory?
The calculator flags high velocities, high friction rates and elongated aspect ratios, but it never blocks a result. Acceptable duct velocity is not a fixed number: it depends on whether the duct is a main or a branch, how close it runs to occupied space, the acoustic criteria for that space, and the trade-off between duct size and fan energy. A shaft riser and a bedroom branch have legitimately different limits, so the tool reports the context and leaves the judgement to the engineer.
Is This a Complete Duct Design?
No. This calculator sizes an individual duct section from straight-duct friction. A complete design may also require:
- fitting, transition and takeoff losses
- total pressure along the critical path
- system effect at the fan inlet and outlet
- acoustic analysis and attenuation
- duct liner or external insulation
- air density corrections for elevation and temperature
- balancing dampers and commissioning tolerance
- local codes and the authority having jurisdiction
Frequently Asked Questions
How do I use a two-variable duct sizing solver?
Check exactly two of the four variables — flow rate, friction rate, velocity and equivalent round diameter — to mark them as known inputs. The calculator solves the remaining two. All six combinations are supported, so you can size on a friction rate, hold a velocity limit, or check an existing duct size.
What friction rate should I design to?
Many low-pressure supply systems are sized in the region of 0.08 to 0.10 in. w.c. per 100 ft, but the appropriate value depends on the available fan pressure, the duct run length, acoustic criteria and the energy cost of the system. There is no single correct figure.
Why is the equivalent round diameter not based on equal area?
A rectangular duct has more wetted perimeter than a round duct of the same area, so it produces more friction. The Huebscher relation gives the diameter of a round duct with the same pressure loss for the same airflow and length, which is the equivalence that matters for pressure-loss calculations. Sizing on equal area would under-size the rectangular duct.
What is the friction factor and why is it solved iteratively?
The Darcy friction factor describes how much pressure a duct loses to wall friction. The Colebrook-White equation defines it implicitly — the friction factor appears on both sides — so it has to be solved numerically rather than evaluated directly.
What duct roughness does this calculator use?
An absolute roughness of 0.0007 ft (0.21336 mm) is used, matching the classic McQuay DesignTools DuctSizer so results agree with that reference tool. Real installed roughness varies with material, joint construction, liner and age.
Does switching between Imperial and Metric change the result?
No. Units are only a representation. Every value you have entered is converted, the underlying calculation is unchanged, and switching back returns the original numbers. Reynolds number and the friction factor are dimensionless, so they read identically in both systems.
Does this calculator size a complete duct system?
No. It sizes an individual duct section. A complete design also requires fitting and transition losses, the critical path total pressure, system effect at the fan, balancing, acoustics and the applicable codes.