Duct Sizer – Free HVAC Duct Size Calculator
Free online calculator · No signup required
Size round or rectangular ductwork from any two of airflow, friction rate, velocity or diameter.
- 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.
Your next design step
Move from one duct section to a connected layout.
Draw supply and return ductwork over your PDF plan, connect air terminals and review sizes in the context of the system.
Keep this check: Carry your reviewed airflow and sizing criteria into the project; check connectivity and pressure losses.
30 days free. Then $50 USD/month. No credit card required. Create a project and choose your workflow. Calculator inputs do not transfer automatically.
How to use the duct sizer without guessing
Choose two known values from airflow, friction rate, velocity and round diameter. The calculator solves the others. For equal-friction sizing, airflow and friction rate are usually the useful starting pair; velocity and diameter are useful for checking an existing duct.
- 1
Establish the airflow
Use the sum of connected terminal design airflows for this section. Use ventilation or load calculations to establish airflow before choosing a duct size.
- 2
Set the friction criterion
Use the available fan-pressure budget and effective duct length. A familiar friction rate such as 0.1 in. w.g./100 ft is an assumption, not a universal requirement.
- 3
Review round or rectangular geometry
A rectangular equivalent diameter compares resistance; it is not the same as equal area. Check the actual rectangular cross-section and velocity as well as the selected equivalent diameter.
- 4
Review the full system
Check noise, aspect ratio, fitting losses, terminal pressure and the critical fan-to-terminal route. A straight-duct result alone does not size the fan.
Worked example: check velocity in a 12-inch round duct
Start with 600 CFM through a 12-inch round duct. Convert diameter to feet before calculating area. This is a geometry and velocity check; the friction result depends on the calculator's air and roughness assumptions.
A = π × 1² / 4 = 0.7854 ft²; V = 600 / 0.7854 = 764 FPM
The same duct is 304.8 mm diameter and carries approximately 283 L/s at 3.88 m/s. For a rectangular duct, use its actual width × height to calculate area.
| Duct | Actual area | Actual velocity |
|---|---|---|
| 12-inch round | 0.7854 ft² | 764 FPM |
| 12 × 8-inch rectangular | 0.6667 ft² | 900 FPM |
| 12 × 10-inch rectangular | 0.8333 ft² | 720 FPM |
Free HVAC Duct Sizer and Duct Size Calculator
Use this duct sizing calculator for round or rectangular ductwork. Enter any two of airflow, friction rate, velocity or equivalent round diameter and solve the remaining two. Switch between Imperial and Metric units without changing the underlying calculation.
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.
R = (1200 f / D) × Pv
Darcy-Weisbach in Imperial working form — friction rate per 100 ft.
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 vs Rectangular Duct Sizing
Rectangular ducts are converted to an equivalent round diameter for friction calculations. The actual rectangular area and actual velocity are still reported separately, which is important for acoustic review, grille and diffuser selection, and checking real section velocity.
What Friction Rate Should I Use?
There is no universal friction rate. Many low-pressure systems are initially checked around 0.08 to 0.10 in. w.c. per 100 ft, but the correct design value depends on fan pressure, run length, energy use, acoustics, available shaft space and project-specific constraints. Use the calculator as a sizing aid, not as a substitute for engineering judgement.
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 and transition losses, total pressure along the critical path, system effect, acoustic review, balancing, insulation, density corrections and applicable codes.
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.