Duct Size Calculator

Duct dimensions are always inner dimensions

Every dimension this duct sizer reports is a clear inside dimension. Add liner, wrap or sheet thickness separately when you detail the duct.

Duct sizing chart

The most air each round duct carries at 0.08 and 0.10 in. wg per 100 ft, the low-velocity friction range in the design criteria below. Every figure comes from this calculator's own model, rounded down to the nearest 5 cfm, so in that model the airflow shown keeps the duct at or below the friction rate at the top of its column. Fittings, joints and installation are not included. Velocities are in parentheses. For any other airflow, friction rate, material or a rectangular duct, use the calculator above.

Flex is figured at the calculator's flex roughness. Flex that sags or is compressed carries less air at the same friction rate.

For a version to print, with round ducts to 36 in. and rectangular metal ducts, see the printable duct sizing chart.

Duct sizing questions

How many cfm can a 12 × 12 duct handle?

A 12 × 12 in. galvanized metal duct, measured inside, carries the same air at the same friction rate as a 13.1 in. round duct. In this calculator's model that is:

• 800 cfm at 0.08 in. wg per 100 ft (800 fpm)
• 905 cfm at 0.10 in. wg per 100 ft (905 fpm)

Duct board of the same inside size carries less, because it is rougher. Other rectangular sizes are in the printable duct sizing chart.

What size duct do I need for a given cfm?

It depends on the friction rate or velocity you design to and on the duct material. Round inside diameters from this calculator, before rounding up to a size you can buy:

Round up to the next available size, not down. For any other airflow, enter it in the calculator above.

What is the rule of thumb for duct sizing?

There is no single rule that replaces a friction or velocity check. For low-velocity comfort ductwork, a friction rate of 0.08 to 0.10 in. wg per 100 ft is common practice. Size on that, then check the velocity against the ranges under design criteria below. Noise or the fan's available static pressure often governs instead. Converting a round duct to a rectangle of the same area under-sizes it (see section 3 of the methodology).

How do I calculate the size of a duct?

From the airflow and either a design velocity or a friction rate. By velocity, use the formula in the next section. By friction rate, the equation has to be solved by iteration, which is what this calculator does.

Duct size formula

By velocity, the round duct diameter follows from the airflow and the velocity you design to:

D = 13.54 × √(Q / V)

Where D is the inside diameter in inches, Q the airflow in cfm and V the velocity in fpm. For example, 400 cfm at 700 fpm needs a 10.2 in. round duct.

By friction rate, which is how a ductulator works, there is no closed-form answer: the diameter and the friction factor depend on each other. The calculator solves the Darcy-Weisbach equation for D, as set out in the methodology below.

HVAC duct sizing calculator methodology

Overview

This duct size calculator sizes HVAC ductwork by the equal friction method, the same principle as a traditional Ductulator, and reports both the round duct and the rectangular duct that carries the same air at the same friction rate. It is based on ASHRAE Fundamentals ch. 21 (and Manual D's circular equivalents), for low- and medium-pressure comfort ductwork at standard air density.

Friction rate is always quoted per 100 ft (per 30 m).

1. Round duct from a friction rate

Pressure drop comes from the Darcy-Weisbach equation:

ΔP/L = f × (ρ/2) × V² / D

with the friction factor from the Swamee-Jain (1976) explicit approximation to Colebrook-White, which avoids iterating on f:

f = 0.25 / [log₁₀(ε/3.75D + 5.74/Re^0.9)]²

Where:

• ΔP/L = pressure drop per unit length (in. wg/100 ft)
• f = Darcy friction factor
• ρ = air density, 0.0735 lb/ft³
• V = air velocity (ft/min)
• D = duct diameter
• ε = absolute roughness of the material
• Re = Reynolds number

Sizing is the same relation solved for D rather than a separate approximation:

D = [8 f L Q² / (π² g hf)]^0.2

Because f depends on D, this converges by iteration, but it converges to the same model the forward direction uses. That matters in practice: size a duct on 0.08 in. wg/100 ft, then enter the diameter you were given, and the calculator returns 0.08. Through V1.25 it did not.

2. Air velocity

Once the diameter is known, velocity follows from continuity:

V = Q / A, with A = π(D/12)²/4

3. Rectangular duct: equal friction, not equal area

Rectangular ducts are reported as the circular equivalent of Huebscher (1948), adopted by ACCA Manual D Appendix 3 and ASHRAE Fundamentals Chapter 21:

De = 1.30 × (W × H)^0.625 / (W + H)^0.25

This holds friction rate and airflow constant, not cross-sectional area. The equal-friction rectangle always has more free area than its round equivalent, and therefore runs at a lower velocity. The calculator reports both velocities so the difference is visible rather than implied.

Sizing a rectangle on equal area instead is a common shortcut and it under-sizes: it gives a duct that carries the air faster and at a higher friction rate than the round duct it was meant to replace.

4. Material roughness

The material sets the absolute roughness ε used above:

Material	ε
Metal (galvanized steel)	0.0003 ft
Duct board	0.0019 ft
Flex	0.00775 ft
Fabric (DurkeeSox)	0.024 ft

Design criteria

These are the ranges most often used in practice. They are guidance, not code requirements. The governing criterion on a real project is usually noise or available static pressure.

Velocity

• Supply ducts: 800–1,500 fpm
• Return ducts: 600–1,000 fpm
• Main supply trunks: 1,000–2,000 fpm

Friction rate

• Low-velocity systems: 0.08–0.10 in. wg/100 ft
• Medium-velocity systems: 0.10–0.20 in. wg/100 ft
• High-velocity systems: 0.20–0.50 in. wg/100 ft

Return air ducts

Return ducts are sized the same way as supply ducts: enter the return airflow and a friction rate or velocity. The velocity ranges above are lower for returns than for supply. A return or transfer opening between rooms is sized with the Return Air Transfer Duct/Pass Thru Calculator.

Worked example

Given: 2,000 CFM, galvanized steel, 0.08 in. wg/100 ft.

Step 1: round duct. Solving Darcy-Weisbach with the Swamee-Jain friction factor at ε = 0.0003 ft gives D = 18.49 in.

Step 2: velocity. A = π(18.49/12)²/4 = 1.865 ft², so V = 2,000 / 1.865 = 1,072 fpm, within the range for a main supply trunk.

Step 3: rectangular equivalent. The square that satisfies De = 1.30 a/2^0.25 at De = 18.49 is 16.92 × 16.92 in., running at 1,006 fpm, slower than the round duct, because the equal-friction rectangle has more free area.

Step 4: constrain the depth. Fix the height at 12 in. and the same duct becomes 24.61 × 12.00 in. at 975 fpm, still at 0.080 in. wg/100 ft.

Note that the equal-area rectangle for an 18.49 in. round duct would be 16.39 in. square. That is the shortcut this calculator does not take.

What changed in V1.26

Two corrections, both of which move published numbers:

One friction model in both directions. Earlier versions used one formulation to size a duct from a friction rate and a different one to report the rate from a diameter. They disagreed, so the calculator was not self-consistent. Both directions now use Swamee-Jain. Sizing moved by about 0.8% on metal and duct board, −3.4% on flex and −7.8% on fabric duct; the gap grew with roughness.

Metric friction input. A unit conversion on the metric friction-loss input scaled it by roughly 1/3.33, so entering 20 Pa/30 m sized the duct for about 6 Pa/30 m and oversized every metric duct. At 472 l/s of metal duct, 20 Pa/30 m now returns 36.05 cm. US results were never affected.

The Swamee-Jain constant is written as 3.75 here where the published equation has 3.7. It is inherited from the original workbook and is applied in both directions, so the calculator remains self-consistent; it shifts diameters by a fraction of a percent against the textbook form.

References

• ASHRAE (2025). ASHRAE Handbook: Fundamentals, Chapter 21: Duct Design. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
• ACCA (2016). Manual D: Residential Duct Systems, Appendix 3: Circular Equivalents. Air Conditioning Contractors of America.
• Huebscher, R.G. (1948). "Friction Equivalents for Round, Square and Rectangular Ducts." ASHVE Transactions, 54: 101–118.
• Swamee, P.K., & Jain, A.K. (1976). "Explicit Equations for Pipe-Flow Problems." Journal of the Hydraulics Division, ASCE, 102(5): 657–664.
• Colebrook, C.F. (1939). "Turbulent Flow in Pipes." Journal of the Institution of Civil Engineers, 11(4): 133–156.
• SMACNA (2006). HVAC Systems Duct Design, 4th Edition. Sheet Metal and Air Conditioning Contractors' National Association.
• Moody, L.F. (1944). "Friction Factors for Pipe Flow." Transactions of the ASME, 66(8): 671–684.

This digital duct size calculator provides estimates comparable to traditional Ductulator tools, offering convenient online duct sizing for HVAC professionals and contractors. It is an estimating aid and does not replace engineering judgment.