HVAC · Air side

Duct Size Calculator

The round and rectangular duct size, velocity and friction loss for an airflow at a friction rate, by the equal-friction method (ACCA Manual D). Every size is computed from the Darcy-Colebrook duct model, never looked up from a hand-typed grid.

Field brief

400 / 800 CFMat 0.08 in. wc per 100 ft, 400 CFM needs 11 in round duct and 800 CFM needs 14 in. 1,200 CFM needs 16 in.11/14 in

Branch runouta 100 CFM room runout is 7 in round duct at the same rate. Step up a size for returns to keep velocity and noise down.7 in

Methodequal friction: the smallest standard duct whose friction loss at your CFM stays at or under the target rate. Rectangular equivalents share the same equivalent diameter.0.08 in. wc

Airflow (CFM)

Target friction rate (in. wc / 100 ft)

0.08 is the common residential design target (0.06-0.10 typical). Get your own rate from the static-pressure helper: friction rate = available static pressure × 100 / total effective length.

Duct role (for the velocity check)

Smallest standard round duct

14 in

the smallest stocked size that stays at or under your 0.08 in. wc/100 ft target

Round size14 in

Velocity748.4 fpm

Friction at this size0.059 in. wc/100 ft

Rectangular equivalents22×8, 18×10, 14×12

At 748.4 fpm this run is within the 700-900 fpm advisory band. Rectangular options carry the same airflow at the same friction (equal equivalent diameter).

Show the math

Equal-friction sizing: smallest standard round D with friction(D, 800 CFM) ≤ 0.08 in. wc/100 ft

Friction = (12 f L / D) × ρ × (V/1097)², f from Colebrook-White, ε = 0.0003 ft (galvanized), L = 100 ft

Velocity V = CFM / area = 800 / area(14 in) = 748.4 fpm

Rectangular equivalent: De = 1.30 (a b)^0.625 / (a + b)^0.25 (ASHRAE Fundamentals 21.3.1)

Where this number comes fromIntry Verified

Calculated from

ASHRAE / ACCA Manual D equal-friction method (galvanized, standard air). Computed by equal-friction sizing from the ASHRAE Darcy-Colebrook model (galvanized, 0.0003 ft, standard air), with Huebscher 21.3.1 rectangular equivalents against your advisory friction-rate and velocity targets. See the full receipt ↓

Run with

Airflow800 CFM
Target friction0.08 in. wc/100 ft
Duct roleSupply trunk / main
Round size14 in

Checked

Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 31163 automated checks, one suite for the whole site rather than a per-tool count, re-derive the numbers before each deploy from the tables this site transcribed from the ASHRAE Fundamentals and ACCA Manual D duct-design standards it was computed from. This is our own deterministic gate, not a third-party audit.

Final say

A full ACCA Manual D duct design and the equipment's blower data have final say.

What Intry Verified means

Intry Verified · Build 2731D30 · 2026-10-15

Standard air: sizes assume standard air (0.075 lb/ft³, about 70°F, sea level) and smooth galvanized steel duct. At altitude or high supply temperatures the air is thinner and both the friction and the airflow shift; flexible duct runs higher friction, so size it up. Velocity limits are an advisory noise/comfort band, not a code minimum. Fitting equivalent lengths come from ACCA Manual D Appendix 3, your input, not this tool.

800 CFM14 in

Round Duct Size by CFM (0.08 in. wc/100 ft)

Round duct size by airflow, equal-friction method at 0.08 in. wc per 100 ft (galvanized steel, standard air). Smallest standard round size at or under the target rate. Source: ASHRAE / ACCA Manual D equal-friction sizing, computed from the Darcy-Colebrook duct model.
Airflow (CFM)	Round size	Velocity (fpm)	Friction (in. wc/100 ft)
50	5 in	367	0.058
75	6 in	382	0.050
100	7 in	374	0.039
150	8 in	430	0.043
200	8 in	573	0.072
300	10 in	550	0.051
400	11 in	606	0.054
600	12 in	764	0.074
800	14 in	748	0.059
1,000	15 in	815	0.063
1,200	16 in	859	0.064
1,600	18 in	905	0.061
2,000	20 in	917	0.055

Each row is the smallest standard round size whose friction loss stays at or under 0.08 in. wc per 100 ft at that airflow, with the velocity it runs at. Sizes are galvanized steel in standard air. For rectangular duct, match the equivalent diameter with the calculator above.

The equal-friction formula

Friction loss per 100 ft = (12 × f × L / D) × ρ × (V / 1097)², where f is the Colebrook-White friction factor at galvanized absolute roughness ε = 0.0003 ft, D is the diameter in inches, ρ is standard air density 0.075 lb/ft³, and V is velocity in fpm (ASHRAE Fundamentals duct-friction chart). Velocity V = CFM / cross-sectional area. Rectangular to round: De = 1.30 × (a × b)^0.625 / (a + b)^0.25 (ASHRAE Fundamentals 21.3.1). The sizer steps up standard sizes until the friction loss meets your target.

Duct Size Calculator FAQs

What size duct do I need for 400 CFM?

A 11 in round duct carries 400 CFM at the common 0.08 in. wc per 100 ft friction rate (galvanized, standard air). That is roughly one ton of cooling airflow at 400 CFM per ton. At a looser 0.10 in. wc/100 ft you can drop a size; a rectangular duct with the same equivalent diameter carries the same airflow at the same friction.

What size duct for 800 CFM?

An 14 in round duct carries 800 CFM at 0.08 in. wc per 100 ft. Returns and trunks are often run a size larger to keep velocity and noise down. Use the calculator to see the exact velocity and to read off rectangular equivalents for a tight joist bay.

How do I calculate duct size from CFM?

Duct size comes from the equal-friction method: pick a friction rate (0.08 in. wc per 100 ft is the residential default), then find the smallest standard duct whose friction loss at your CFM stays at or under that rate. Friction loss is (12 f L / D) x density x (velocity/1097) squared, with the friction factor f from the Colebrook-White equation at galvanized roughness 0.0003 ft. This calculator solves it for you and gives the round size, the velocity, and the rectangular equivalent.

What is a good friction rate for residential duct design?

0.08 in. wc per 100 ft is the standard residential design target, and ACCA Manual D Appendix 3 tables are built on it. Most residential designs land at about 0.06 to 0.10 in. wc per 100 ft: lower means larger, quieter ducts, higher means smaller ducts and more noise. Your actual friction rate is available static pressure times 100 divided by total effective length.

What CFM does a 6 inch round duct carry?

A 6 in round duct carries roughly 90 CFM at 0.08 in. wc per 100 ft before it needs to step up to 7 in. Push more air through it and the velocity and friction climb past the residential target, which is where duct noise starts. The chart on this page lists each round size against the airflow it carries.

Is round or rectangular duct better, and how do I convert between them?

Round duct moves air with less friction and material for the same airflow, but rectangular fits flat in joist bays and above ceilings. To convert, match the equivalent diameter: De = 1.30 times (a times b) to the 0.625 power, divided by (a plus b) to the 0.25 power (ASHRAE Fundamentals 21.3.1). The calculator prints the rectangular sizes whose equivalent diameter matches the round size it recommends.

Convert a shape, or jump to a common size

Have a size already and need the other shape? The round-to-rectangular converter matches equivalent diameters both ways. Or open the reference page for a common airflow or duct size:

What size duct for 400 CFM?

400 CFM needs a 11 in round at 0.08 in. wc/100 ft.

What size duct for 600 CFM?

600 CFM needs a 12 in round at 0.08 in. wc/100 ft.

What size duct for 800 CFM?

800 CFM needs a 14 in round at 0.08 in. wc/100 ft.

What size duct for 1000 CFM?

1000 CFM needs a 15 in round at 0.08 in. wc/100 ft.

What size duct for 1200 CFM?

1200 CFM needs a 16 in round at 0.08 in. wc/100 ft.

6 inch duct CFM

A 6 in round carries about 98 CFM at 0.08 in. wc/100 ft.

8 inch duct CFM

A 8 in round carries about 212 CFM at 0.08 in. wc/100 ft.

10 inch duct CFM

A 10 in round carries about 386 CFM at 0.08 in. wc/100 ft.

12 inch duct CFM

A 12 in round carries about 628 CFM at 0.08 in. wc/100 ft.

Related HVAC Tools

CFM Calculator

Airflow from the load: CFM = sensible BTU / (1.08 × ΔT), or 400 CFM per ton. Feeds this sizer.

Static Pressure & Friction Rate

Turn available static pressure and total effective length into the friction rate this tool needs.

BTU / Load Calculator

Size the equipment first, then the airflow, then the duct. The start of the air-side job.

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receipt

Where these duct sizes come from

The smallest standard round size that holds your target friction rate, the velocity at that size, and its rectangular equivalents are computed by the locked duct-airflow engine on every build, from the equal-friction model below. The friction factor is solved from Colebrook-White, not read off a chart, so no sizing number is hand-typed.

ASHRAE Fundamentals duct-friction model (Darcy-Colebrook): Friction loss per 100 ft from the ASHRAE IP duct form Dpf = (12 f L / D)(rho)(V/1097)^2, with the Colebrook-White friction factor at a galvanized absolute roughness of 0.0003 ft (ASHRAE base value) and the standard-air Reynolds shortcut Re = 8.56 x D x V. These are the canonical ASHRAE Fundamentals duct-friction forms; ASHRAE is the standards body that publishes them. We have not read the ASHRAE Handbook directly, because it is not freely published: the equations were read from a secondary that reproduces them. The standard is named because it is the origin, and the reading is secondary because that is what it was.
ASHRAE Fundamentals 21.3.1 equivalent diameter (Huebscher): The rectangular equivalents use De = 1.30 (a b)^0.625 / (a + b)^0.250, the round size with the same friction at the same airflow. The Huebscher equation, published by ASHRAE in Fundamentals 21.3.1. We have not read the ASHRAE Handbook directly, because it is not freely published: the form was read from a secondary reproducing it. Published for aspect ratios up to about 8:1.
Standard manufactured duct sizes (McGill AirFlow / SMACNA): The size menu the engine selects from is the common whole-inch stocked set from the McGill AirFlow single-wall round duct catalog and SMACNA Round Industrial Duct Construction Standards. Presented as common stocked sizes, not an exhaustive list; the engine returns the smallest of these that meets your target, and rounding up is conservative.
Advisory design targets (Manual D basis, a secondary's velocity bands): The 0.08 in. wc/100 ft default is the residential friction-rate target that HVAC School, a secondary, reports the ACCA Manual D Appendix 3 tables assume (Energy Vanguard, a secondary: 0.08 to 0.10 typical). The velocity check compares against a secondary's published noise/comfort bands. Both are advisory design guidance, not a code number, and vary by source.

The sizes are computed by the equal-friction engine from the ASHRAE friction model, not measured, and assume standard air (0.075 lb/ft^3, about 70F, sea level) in smooth galvanized steel duct. At altitude or on flexible duct the friction is higher, so size up. The friction-rate target and the velocity bands are advisory design guidance, not a code minimum. Fitting equivalent lengths (ACCA Manual D Appendix 3) are your input, not published here. A full Manual D design and the equipment's blower data govern the install.