Duct Calculator
Type the airflow and your friction rate or velocity limit: the sheet solves the round diameter, rounds it up to a size you can buy and recomputes the air at that size. At the example 1,000 cfm and 0.08 in. wg per 100 ft: exact 14.28 in, stock 15 in, 815 fpm, 0.063 in. wg per 100 ft.
Units available: US customary / metric.
Airflow, limit and duct
Order 15 in round
- Exact diameter
- 14.28 in
- Velocity at stock size
- 815 fpm
- Friction at stock size
- 0.0628 in. wg / 100 ft
- Velocity pressure
- 0.0414 in. wg
Drawings
M-101Takeoff
| Item | Net qty | Waste | Order |
|---|---|---|---|
| Round duct, 1,000 cfmExact 14.28 in at 0.08 in. wg / 100 ft (Darcy, Colebrook from Haaland). Rounded up by 1 in to 15 in: 815 fpm, VP 0.0414 in. wg, Re 107,556, f 0.019, 0.0628 in. wg / 100 ft. | 14.28 in | — | 15 in |
Assumptions used
- Standard air, 0.075 lb per cubic foot (McGill AirFlow), valid within 30 °F of 70 °F and below 1,500 ft of elevation.
- Velocity pressure VP = (V / 4005)², V in fpm, VP in in. wg (PNNL, standard air at 70 °F and 29.92 in Hg).
- Friction: loss per 100 ft = f × (100 / D in ft) × VP (Darcy), f by Colebrook iterated from a Haaland first guess (Purdue).
- Reynolds number with μ = 3.679 × 10⁻⁷ lb·s/ft² at 70 °F: NASA's 3.62 × 10⁻⁷ at 59 °F carried to 70 °F by Sutherland's law.
- Roughness 0.0003 ft (0.09 mm), galvanized steel as ASHRAE gives it (Chen et al., 2012). Another material takes its own roughness.
- The friction rate, 0.08 in. wg / 100 ft, is your design input: no rate is recommended here.
- Sizes round up by 1 in, then velocity and friction are computed again at the size you buy.
- Rectangular duct: friction through Huebscher's equivalent round, velocity through the actual area (McGill).
- McGill's chart reads 0.50 in. wg per 100 ft for 10,000 cfm in a 24 in round duct; this sheet gives 0.459 (-8 %). The chart's roughness basis sits in an appendix not opened here: type a higher roughness to test it.
- Straight duct only: fittings, takeoffs, grilles and filters add losses this sheet does not compute.
What the sheet computes, and on which physics
M-101 sizes straight galvanized duct in standard air, 0.075 lb per cubic foot, which McGill AirFlow holds within 30 °F of 70 °F and below 1,500 ft. Velocity pressure is VP = (V ÷ 4005)² (PNNL). NASA’s viscosity, 3.620 × 10⁻⁷ lb·s/ft², goes to 70 °F by Sutherland: × (529.67 ÷ 518.67)1.5 × (717.39 ÷ 728.39) = 3.679 × 10⁻⁷. Friction factor f: Colebrook from a Haaland guess (Purdue), roughness 0.0003 ft (0.09 mm), the galvanized value Chen et al. quote from ASHRAE.
Example: 1,000 cfm in 15 in moves at 815 fpm, VP 0.0414, Re 107,556, f 0.019; 0.019 × (100 ÷ 1.25) × 0.0414 = 0.063 in. wg per 100 ft.
Friction rate or velocity: your number
The sheet sizes to the limit you type. 0.08 in. wg per 100 ft and 900 fpm are examples, not recommendations: Manual D derives the friction rate from the blower’s available static pressure. By velocity, √(4 × 1,000 ÷ 900 ÷ π) × 12 = 14.27 in; by friction, 14.28 in.
Rounding up changes the air
A 14.28 in duct is not sold: the sheet rounds up by 1 or 2 in and recomputes at that size. At 15 in the loss is 0.063, not 0.08. At 16 in: 716 fpm, 0.0458 per 100 ft, and over 60 ft 0.0458 × 0.6 = 0.0275 in. wg, fittings excluded. 400 cfm: 10.13 in exact, 11 in stock.
Rectangles: Huebscher for friction, the real area for speed
Friction uses Huebscher’s Deq = 1.30 (ab)0.625 ÷ (a + b)0.25;
velocity, McGill says, uses the actual cross-sectional area
. At 8 in high the example
needs 22.77 in, rounded to 23: Deq 14.34 in, 0.078 per
100 ft, 1,000 ÷ (8 × 23 ÷ 144) = 783 fpm. At 900 fpm the
width is 20 in. Aspect 2.875, printed with no limit.
A trunk that steps down after each branch
Trunk mode subtracts each branch takeoff and resizes. 1,200 cfm less 200, 300, 250, 150 cfm carries 1,200, 1,000, 700, 450, 300 cfm through 16, 15, 13, 11, 10 in. A branch that takes the last air ends the trunk.
The McGill cross-check
McGill: From the friction loss chart, the loss of 10,000 cfm air flowing in a 24-inch round duct is 0.50 inch wg per 100 feet.
This model gives 0.459, -8 % against the chart (3,183 fpm, VP
0.632, f 0.0145). The chart’s roughness basis is in an appendix we
did not open; type a higher roughness and the loss rises. McGill’s
12 × 45 in Deq: 24 in; ours
24.14. Open the advisory.
What the ranking pages do
Adicot solves Darcy with Huebscher; ductulator.com attributes its pressure drop to Hazen–Williams, an equation for water; oceanhvac and servicetitan take an equivalent length. None of the six reduces a trunk, rounds up and recomputes, draws to scale or exports CSV.
Frequently asked
- How many CFM will a 12x12 duct carry?
- At 0.08 in. wg per 100 ft, 798 cfm (Deq 13.12 in, 798 fpm). At a 900 fpm limit, 900 × 1 sq ft = 900 cfm.
- What size duct is needed for 6000 CFM?
- At 0.08: 28.07 in, 29 in stock, 1,308 fpm. Another rate gives another size.
- How to calculate duct size for exhaust fan?
- Type the fan’s rated cfm and your limit. 1,200 cfm at 0.08: 15.29 in, 16 in stock.
- What size trunk line do I need for a 3 ton unit?
- The blower data gives the cfm, not the tons. Type it in trunk mode: 1,200 cfm at 0.08 starts at 16 in.
- How many CFM will a 14 inch pipe carry?
- At 0.08, 949 cfm at 888 fpm. At 900 fpm, 900 × π × (14 ÷ 12)² ÷ 4 = 962 cfm.
Sheet M-101 prepared by TakeoffMetric Editorial; no author, credential or field measurement is claimed.
Sources checked
The documents the numbers on this sheet were checked against. Nothing here is attributed to an author or a credential we do not have.
Where this page compares itself with other calculators, it describes the pages ranking on Google in the US when they were read, on September 26, 2026. Rankings change.
- McGill AirFlow Engineering Design Advisory #1 - Duct design, §1.3.2, §1.4.5 and Equation 1.10 Standard air at 0.075 lb per cubic foot and its window (within 30 °F of 70 °F, below 1,500 ft); the Huebscher equivalent round for friction in a rectangle, with velocity on the actual area; and the one cross-check printed: its chart reads 0.50 in. wg per 100 ft for 10,000 cfm in a 24 in round duct. Opened 2026-09-26.
- Pacific Northwest National Laboratory / U.S. DOE Building America Solution Center Measuring Mechanical Ventilation Airflow Velocity pressure: V = 4005 √VP, V in feet per minute, for standard air at 70 °F and 29.92 in Hg. Opened 2026-09-26.
- Purdue University, ME 308 (Wassgren) Pipe flows - losses, Eq. 11.37 to 11.39 The Darcy friction factor: Colebrook, solved by iteration from a Haaland first guess, as the notes describe. Opened 2026-09-26.
- Purdue University (Chen et al., 2012) Duct fittings study - surface roughness of galvanized steel as provided by ASHRAE The default roughness, 0.09 mm (0.0003 ft) for galvanized steel. Any other material takes its roughness from you. Opened 2026-09-26.
- NASA Glenn Research Center Viscosity of air - Sutherland's formula Viscosity for the Reynolds number: 3.62 × 10⁻⁷ lb·s/ft² carried to 70 °F by Sutherland's law, 3.679 × 10⁻⁷. Opened 2026-09-26.
- NASA Glenn Research Center Air properties definitions The temperature the viscosity is quoted at: the sea-level standard day, 59 °F. Opened 2026-09-26.