HVAC duct sizing calculator

Use the calculator

Enter the design airflow, choose equal-friction sizing or velocity-based sizing, pick the duct material and shape, and the calculator returns the round equivalent diameter, air velocity, friction loss, and Reynolds number. For rectangular ducts, set the aspect ratio and the calculator returns dimensions via the Huebscher equivalence.

The four duct-sizing formulas

The fundamental energy-balance equation for any duct or pipe. Friction grows with the square of velocity, so doubling airflow in the same duct quadruples the pressure drop. For air at standard conditions this evaluates to roughly 0.65 Pa/m for a typical 400 mm duct at 5 m/s.

An explicit (non-iterative) approximation to the implicit Colebrook-White equation, accurate within ~1 % for the entire turbulent range typical in HVAC. Avoids the inner Newton-Raphson iteration that Colebrook requires while preserving the same physics.

HVAC ducts always operate in the fully-turbulent regime (Re > 10 000), so the friction factor depends mainly on relative roughness ε/D, not on Reynolds number. Below Re ≈ 2 300 (laminar), f = 64 / Re — never reached in normal HVAC ducting.

The Huebscher equivalence relates a rectangular duct to a round duct that would carry the same airflow with the same friction loss. Valid for aspect ratio b/a ≤ 4. At aspect ratio 1:1 (square), D_eq is roughly 1.10 × the side; at 4:1, D_eq is closer to 1.5 × the short side.

How to size HVAC duct, step by step

1. Compute the design airflow. For HVAC: ventilation airflow per ASHRAE 62.1 (typically 7.5 L/s per occupant + 0.3 L/s per m²) or by cooling load (1 CFM ≈ 1 ton of cooling load divided by 400). For exhaust: capture velocity × hood opening area, or industry-standard CFM per appliance (kitchen hood 100–300 CFM/ft² of cooking surface).
2. Pick the design friction rate. Standard low-pressure HVAC: 0.08 in.wc / 100 ft (0.65 Pa/m). Medium-pressure: 0.10–0.15 in.wc / 100 ft. Higher rates → smaller duct, more fan energy, more noise. Lower rates → larger duct, more space, quieter. Equal-friction means every section uses the same rate, simplifying selection while keeping pressure losses balanced.
3. Pick the duct material. Galvanised steel (ε = 0.15 mm) is the standard for rigid HVAC duct. Spiral wound (ε = 0.075 mm) is smoother and used for exposed runs in retail / restaurants. PVC / fibreglass duct (ε = 0.03 mm) is cleanest for hospitals and clean rooms. Flexible duct (ε = 0.91–3 mm depending on extension) has 5–10× the friction of rigid duct — use only for short final connections.
4. Choose round vs rectangular. Round duct is the most efficient — least friction per CFM, least leakage, smallest material. Use it whenever ceiling space allows. Rectangular duct fits better in tight ceilings; pick aspect ratio close to 1:1 (square is most efficient) and never exceed 4:1 — at higher ratios, friction grows fast and the Huebscher equivalence becomes inaccurate.
5. Iterate the diameter to hit the target friction rate. For each duct section: enter the section CFM, the target friction rate, the material. The calculator iterates the diameter using Darcy-Weisbach + Swamee-Jain friction factor until the computed friction loss equals the target. Round up to the next standard size (4, 5, 6, 7, 8, 10, 12, 14, 16, 18, 20, 22, 24 inch round; or rectangular sizes from a duct manufacturer's catalogue).
6. Cross-check velocity against ASHRAE comfort limits. Residential: 2.5–4 m/s mains, 1.5–3 m/s branches. Commercial office: 4–6 m/s mains, 2.5–4 m/s branches. Industrial: up to 10 m/s. If the equal-friction calculation lands outside the recommended velocity range for your application, use velocity-based sizing instead and re-check the friction.

Reference values

ASHRAE recommended air velocities

Maximum velocities for noise control. Lower limits avoid excessive duct cost; upper limits avoid noise (NC ratings) and energy waste from fan power.

Common standard duct sizes (galvanised steel)

Worked example: 2 000 CFM office supply branch

An office building supply branch carries 2 000 CFM (944 L/s) at the design friction rate of 0.08 in.wc / 100 ft (0.65 Pa/m), galvanised steel rigid duct, round shape preferred but ceiling space limits depth to 12 inches.

The 7.5 m/s velocity is at the high end of office-mains range (4–6 m/s preferred). Two options: (1) accept the slightly higher noise for the smaller duct, (2) step up to 18 in round to get velocity down to ~6 m/s and reduce friction to ~0.05 in.wc/100ft. The latter adds material cost but cuts fan energy and noise — common decision in commercial design.

Variants and special cases

Static-regain method

An alternative sizing method where the duct diameter is increased at each branch take-off so the static-pressure regain (from velocity drop) compensates the friction loss in the next section. Self-balancing (no dampers needed). More accurate for high-velocity systems above 10 m/s, but iterative — usually done with software (Trane TRACE, Carmel ductSIZER) rather than by hand.

Round vs rectangular trade-offs

Round: lowest friction per CFM, lowest leakage, smallest material weight, easiest to seal. Rectangular: better fit in tight ceilings, easier to attach branch take-offs from any side. Round is preferred whenever ceiling space allows; rectangular is universal in commercial low-ceiling buildings. Cross-over: rectangular at 1:1 aspect is roughly 10 % more material than equivalent round; at 4:1 it\'s 30 % more.

Spiral duct

A round duct fabricated from a continuous helical seam — smoother interior than longitudinal-seam round duct (ε ≈ 0.075 mm vs 0.15 mm), 5–10 % less friction at same diameter. Common in exposed installations (retail, restaurants, design-forward offices) for both performance and aesthetics. Costs 20–30 % more than rolled-and-seamed round.

Flexible duct limits

Flex duct has 5–10× the friction of rigid duct because of its corrugated inner surface. Use only for the last 1–1.5 m connecting rigid duct to a diffuser or register. Long runs of flex (over ~3 m) are a common cause of low airflow and high fan energy in residential systems. Never use flex on the supply trunk.

The ASHRAE equal-friction method

The equal-friction method sizes ducts so that the friction loss per unit length is constant throughout the system. This results in a balanced design when section lengths are similar, and provides reasonable balance even when section lengths differ significantly. The friction rate is typically selected in the range of 0.5 to 1.5 Pa/m (0.05 to 0.15 in. of water/100 ft) for low-pressure systems.

Related calculators and references

Frequently asked questions

What is the equal-friction method?

A duct sizing method where every section of the duct system is sized so it has the same friction loss per unit length (typically 0.08 in.wc / 100 ft for low-pressure HVAC). This naturally balances the pressure losses across branches without complex iterative analysis. ASHRAE Fundamentals Chapter 21 codifies it as the preferred method for systems where balancing dampers will fine-tune the final flow distribution.

What size duct do I need for 1 000 CFM?

At 0.08 in.wc/100ft target friction in galvanised steel: ~14 in (350 mm) round duct, giving velocity ~5 m/s (980 fpm). For lower noise: 16 in round at ~3.8 m/s. Rectangular equivalent at 2:1 aspect: 12 × 24 in (300 × 600 mm). Use the calculator to fine-tune for your specific friction target and material.

How do I convert round to rectangular duct?

The Huebscher equivalent diameter formula: D_eq = 1.30 · (a · b)^0.625 / (a + b)^0.25, where a and b are the rectangular sides. For the rectangular section to have the same friction loss as a round duct of equal D_eq at the same airflow, both must be in the same units (mm or inches). Aspect ratio above 4:1 reduces accuracy; the calculator flags this.

What is a typical duct velocity?

Per ASHRAE: 2.5–6 m/s in commercial supply air (lower for quieter spaces, higher for industrial). 6–10 m/s for industrial low-pressure HVAC. 10–25 m/s for dust collection (must keep particles airborne). Below 2 m/s, ducts oversize and may not transport contaminants. Above 12 m/s, noise (NC ratings) and energy use both jump sharply.

What is friction factor in HVAC?

A dimensionless number that relates pressure loss to velocity-pressure: ΔP = f · (L/D) · ρ·v²/2 (Darcy-Weisbach). For air in galvanised duct at typical velocities, f ranges 0.018–0.030. Computed from the Colebrook-White equation or its explicit Swamee-Jain approximation, both of which depend on Reynolds number and relative roughness ε/D.

Equal-friction vs static-regain — which method should I use?

Equal-friction — simpler, faster, balanced; standard for most low-pressure HVAC. Requires balancing dampers at branches. Static-regain — sizes each section so the static-pressure regain at downstream junctions cancels the upstream friction loss. Self-balancing (no dampers needed) but iterative and best for high-velocity systems (>10 m/s). ASHRAE Fundamentals Ch 21 covers both; equal-friction wins for everyday HVAC.

How does flexible duct change the calculation?

Flex duct is 5–10× the friction of smooth rigid duct because of its corrugated wall. Roughness ε ≈ 0.91 mm when smooth-pulled, 3 mm when partially compressed. Use flex only for short (≤ 1.5 m) final connections to diffusers / registers. Always verify it's fully extended and supported every 1.5 m to prevent additional sag friction.

Should I include fitting losses?

Yes — for the full system pressure drop. The equal-friction calculator only sizes the straight duct. Add dynamic losses (elbows, tees, transitions) using ASHRAE Ch 21 loss coefficients × velocity pressure. Rule of thumb: fittings add 50–100 % to the straight-duct friction loss in a typical HVAC system. Rule-of-thumb total: 0.10 in.wc per 100 ft + 0.05 in.wc per fitting.

Sources and methodology

1. ASHRAE. ASHRAE Handbook — Fundamentals, latest edition. Chapter 21 — Duct Design (equal-friction, static-regain, T-method).
2. SMACNA. HVAC Systems Duct Design, 4th Edition. Sheet Metal and Air Conditioning Contractors\' National Association.
3. Idelchik, I.E. Handbook of Hydraulic Resistance, 4th Edition. Begell House, 2007. Comprehensive loss-coefficient reference.
4. Swamee, P.K. & Jain, A.K. Explicit equations for pipe-flow problems. Journal of the Hydraulics Division, ASCE, Vol. 102, 1976.
5. Colebrook, C.F. Turbulent flow in pipes, with particular reference to the transition region between the smooth and rough pipe laws. J. Inst. Civil Engineers, 1938–39.
6. ASHRAE Standard 62.1 — Ventilation for Acceptable Indoor Air Quality, 2022.