Category · HVAC
HVAC Calculators — Manual D Duct Sizing (Round, Rectangular, Oval, Flex)
Free HVAC duct sizing calculator built on ACCA Manual D equal friction — size round, rectangular, oval, and flex ducts from CFM, with CFM helpers, TEL friction-rate math, and altitude correction.
ACCA Manual D guide
HVAC calculators for Manual D duct design
The duct sizing calculator is a Manual D calculator for residential and light-commercial ductwork: enter CFM, pick a friction rate or target velocity, and get round, rectangular, oval, and flex sizes in one pass. It follows the ACCA Manual D equal friction method — every duct sized to the same pressure loss per 100 feet — with a TEL-based friction-rate finder, CFM helpers from tonnage and room loads, altitude correction to 10,000 ft, and velocity/noise checks on every result.
From CFM to duct size
Equipment CFM follows the 400-per-ton rule, while room CFM comes from the load split: (room load ÷ whole-house load) × equipment CFM, or about 1 CFM per square foot as a rule of thumb. The design friction rate comes from the pressure budget — FR = (available static pressure × 100) ÷ total equivalent length — with 0.08 in.wg/100 ft as the residential standard. For precomputed answers, the duct sizing chart — our CFM to duct size chart — covers 100–2,000 CFM at standard rates; walk through a full system in the or the room-by-room sizing walkthrough.
Round, rectangular, oval, and flex
Round duct wins wherever it fits: lowest friction per inch, cheapest to seal. Rectangular fits joist bays and soffits but must convert by Huebscher equivalent diameter — never raw area — with aspect ratio at or below 4:1. Oval splits the difference in shallow cavities. Flex needs about one size up when fully extended and pulled taut; compressed flex can need two or more. The math behind every conversion lives in the duct sizing formulas reference.
Quiet, balanced, and high-altitude systems
Keep supply branches at 600–900 FPM, trunks at or under 1,000 FPM, and returns at 400–600 FPM — when the friction answer runs loud, go one size up. Equal friction sizing means branches balance with damper trim instead of redesign. And at altitude, thinner air needs larger ducts for the same blower: Denver air at 5,000 ft is about 86% of sea-level density, and this calculator corrects automatically to 10,000 ft.
Guides
Go deeper
CFM to Duct Size Chart
100–2000 CFM at 0.08 friction — round, rect, flex.
How to Size Ductwork
Room CFM, TEL, friction rate, worked examples.
Duct Sizing Formulas
Darcy-Weisbach, Swamee-Jain, Huebscher equivalents.
FAQ
HVAC calculator questions, answered
What friction rate should I design to?
0.08 in.wg/100 ft is the residential standard. Use 0.06 for quiet bedrooms and long runs, 0.10 for short trunks, 0.10–0.12 for commercial.
How do I get room CFM?
Room CFM = (room load ÷ whole-house load) × equipment CFM, with equipment ≈ tons × 400. Rule of thumb: ~1 CFM per square foot.
Does flex duct need a bigger size than metal?
Yes — about one size up when fully extended and pulled taut. Compressed or sagging flex can need two or more sizes up.
What is ACCA Manual D?
The residential duct design standard. Its equal-friction method sizes every duct to the same pressure loss per 100 feet, so branches balance with damper trim instead of redesign.
How many CFM per ton of cooling?
About 400 CFM per ton — 350 in humid climates, up to 450 in dry ones. A 3-ton system moves roughly 1,200 CFM through the supply trunk.
Round vs rectangular duct — which is better?
Round wins on friction and sealing wherever it fits. Rectangular fits joist bays and soffits but must convert by Huebscher equivalent diameter, never raw area, with aspect ratio at or below 4:1.
What duct velocity keeps rooms quiet?
Supply branches 600–900 FPM (bedrooms 600–700), trunks at or under 1,000 FPM, returns 400–600 FPM. Above 900 FPM in living spaces you will hear the air.
Does altitude change duct sizing?
Yes — thinner mountain air carries less pressure per foot. Denver air at 5,000 ft is about 86% of sea-level density, so ducts run slightly larger. The calculator corrects to 10,000 ft.