Duct Sizing Calculator — Manual D Equal Friction (Round, Rect, Oval, Flex)

This free HVAC duct sizing calculator is a Manual D calculator for the field: enter airflow in CFM, pick a friction rate or target velocity, and get round, rectangular, oval, and flex sizes in one pass. It works like the ductulator online tools contractors grew up with — a slide rule rebuilt for the browser — but adds what paper cannot: TEL-based friction-rate math, altitude correction, velocity and noise checks, shareable links, and print-ready reports. No signup, no install, and it keeps working offline in attics and crawlspaces.

ACCA Manual D and the equal friction method

ACCA Manual D is the residential duct design standard, and the equal friction method is its workhorse: every duct in the system is sized to the same pressure loss per 100 feet, so branches balance with damper trim instead of redesign. The design friction rate comes from your pressure budget: FR = (available static pressure × 100) ÷ total equivalent length. Standard residential practice lands at 0.08 in.wg per 100 ft; use 0.06 for quiet bedrooms and long runs, 0.10 for short trunks, and 0.10–0.12 for commercial work. The calculator solves Darcy–Weisbach for the exact diameter at your rate, reports the resulting velocity alongside, and rounds up to the nearest standard size. For the underlying math, see the friction and Huebscher formulas.

CFM to duct size: from load to diameter

Going from CFM to duct size starts with knowing the airflow each run must carry. Equipment CFM follows the 400-CFM-per-ton rule — a 3-ton system moves about 1,200 CFM — while room CFM comes from the load split: (room load ÷ whole-house load) × equipment CFM. A 150 sq ft bedroom typically needs about 150 CFM, the classic ~1 CFM per square foot rule of thumb. Enter that CFM with your friction rate and the duct size falls out: 150 CFM at 0.08 needs a 7-inch round, 400 CFM needs a 12-inch round, 1,200 CFM needs a 16-inch trunk. For precomputed answers, use the duct sizing chart covering 100–2,000 CFM at standard rates, or the room-by-room sizing walkthrough for full worked examples.

Round duct sizing: the efficient default

Round duct sizing is the starting point for every layout because round carries more air per inch of material with less friction than any other shape — lower surface area per volume means lower pressure loss per foot, tighter-sealing joints, and less sheet metal per CFM. Use round wherever it physically fits: exposed basement trunks, attic runs, and straight branch takeoffs. As a duct size calculator for round work, this page returns the exact Darcy–Weisbach diameter plus the next standard size up, with velocity reported so you can confirm the run stays quiet.

Rectangular duct sizing: Huebscher, not raw area

Rectangular duct sizing belongs where round will not fit — joist bays, soffits, and tight chases — but it must never be done by matching cross-sectional area. A rectangle has more wall per square inch than a circle, so equal area means higher friction and an undersized duct. The honest conversion is the Huebscher equivalent diameter: De = 1.30 × (a×b)^0.625 ÷ (a+b)^0.25, the round size with identical friction loss. A 12×8 in rectangular duct equals about a 10.7 in round — area-matching would have told you roughly 30% less. This calculator converts every result both ways and caps aspect ratio at 4:1, since flat, wide rectangles waste energy and rumble.

Oval, flex, and the rough-wall penalty

Oval duct splits the difference for shallow cavities: flatter than round, smoother-flowing than a wide rectangle. Flex duct is the convenience option with a friction cost — fully-extended flex runs rougher than smooth metal and needs about one size up at the same friction rate. Compressed, sagging, or kinked flex can need two or more sizes up, which is why the only acceptable flex install is pulled taut, strapped every 4 ft, with no kinks. Size flex honestly here, then install it honestly on site.

Velocity and noise: when to size up

Friction-correct is not always quiet-correct. Keep supply branches at 600–900 FPM — 600–700 for bedrooms and living areas where ears notice — trunks at or under 1,000 FPM, and returns at 400–600 FPM since return grilles sit near people. Above 900 FPM in living spaces you will hear whistling registers and fitting rumble. When the equal-friction answer runs loud, go one standard size up: velocity falls faster than friction rises, and the system gets quieter for the price of one duct size.

Altitude correction, built in

Thinner mountain air carries less pressure per foot, so Denver installers cannot trust sea-level charts: at 5,000 ft, air density is about 86% of sea level, and the same blower needs slightly larger ducts for the same CFM. This calculator corrects for altitude up to 10,000 ft automatically — enter elevation and the Darcy–Weisbach solution adjusts. For the full precomputed reference including metric sizes and flex columns, open the full CFM chart.

Related duct sizing resources

For lookups, use the duct sizing chart. For method, use the duct sizing formulas and sizing walkthrough. For the full collection, browse all HVAC calculators.