Introduction
Duct sizing raises the same questions on nearly every project: how much air each room needs, which friction rate to use, whether flex duct changes the answer, and what the numbers actually mean. This FAQ collects the questions asked most often about sizing HVAC ductwork — with direct, numeric answers you can verify. For the full walkthrough behind these answers, see the complete guide.
To run your own numbers as you read, open the Duct Size Calculator — enter CFM, friction rate, length, and fittings, and it returns the duct size instantly.
Sizing Basics
What is duct sizing?
Choosing the duct diameter (round) or dimensions (rectangular) that deliver a required airflow at an acceptable velocity and friction loss. The inputs are required CFM, friction rate (in. w.c. per 100 ft), duct length, and fittings — the output is the duct size that fits the system’s pressure budget.
How many CFM do I need per ton of cooling?
The standard planning rule is 400 CFM per ton: a 3-ton system moves about 1,200 CFM total, a 4-ton system about 1,600 CFM. Individual rooms get a share of that total based on their heating/cooling load — roughly 1 CFM per square foot is a crude planning rule of thumb, but a room-by-room load calculation is the correct basis.
What size duct do I need for 1,200 CFM?
At a typical residential friction rate of 0.08 in. w.c. per 100 ft: a 16-inch round duct (about 1,290 CFM capacity, 859 FPM) or a 24×10 in rectangular duct (about 1,410 CFM capacity, 720 FPM). Longer runs or lower friction rates need larger ducts for the same CFM — always compute your actual friction rate first.
What is a good friction rate for residential ducts?
Typically 0.08–0.10 in. w.c. per 100 ft (about 0.65–0.82 Pa/m). It’s computed, not guessed: (equipment external static pressure − component losses) × 100 ÷ total effective length of the longest run. The step-by-step derivation is in the formula-explained guide.
What air velocity should I design for?
Typical residential targets: supply trunks 700–900 FPM (3.6–4.6 m/s), branch runouts 600–700 FPM (3.0–3.6 m/s), return trunks 500–700 FPM (2.5–3.6 m/s), return grilles 300–500 FPM (1.5–2.5 m/s). Above these ranges you get noise; well below them, poor room air mixing.
Round vs Rectangular, Flex vs Metal
Is round or rectangular duct better?
Round wins on friction, cost per CFM, and air leakage (fewer seams). Rectangular wins when you must fit a shallow joist bay or soffit. A 24×10 in rectangular duct performs like a 16.6-in round duct — but keep aspect ratios at or below 3:1, because skinny rectangles lose capacity to corner friction and get noisier.
Does flexible duct need to be sized differently?
Yes. Flexible duct has higher friction than rigid metal — especially when compressed, sagging, or kinked — so the planning rule is to size flex one trade size larger than the rigid-metal result. Keep flex runs short, pull them taut, support every ~4 ft, and avoid sharp bends. Material comparisons are in the material guide.
How do I convert a round size to rectangular?
Use the equivalent-diameter formula: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25 (a, b in inches). Pick a rectangle whose equivalent diameter meets or exceeds the required round size. Example: 16-in round → 24×10 in rectangular (De ≈ 16.6 in).
Airflow Problems and Diagnostics
Why is one room always hotter or colder than the rest?
The usual suspects in order: the room’s branch is undersized for its load, its damper is closed or missing, the run has high-loss fittings that were never counted, the room has no return path (door closed, no jumper duct), or the duct is disconnected or crushed. Work through the sizing mistakes above before assuming the equipment is at fault.
Why are my ducts noisy?
Noise is a velocity report: whistling registers mean face velocity too high (register too small or damper over-throttled), rumbling trunks mean trunk velocity too high, and a roaring return grille means the return is undersized. Measure, then resize the offending section — adding return capacity is the most common fix.
Do I need a Manual D duct design?
Manual D is the industry-standard residential duct design procedure (friction rate from equipment data, TEL including fittings, room-by-room sizing). For a full system — new install or whole-house replacement — yes, it’s the correct procedure, and many jurisdictions expect it. For a single replacement runout, the calculator plus careful input measurement is proportionate — the Related Guides below break down when each level of rigor fits.
Costs, Permits, and Hiring
How much does ductwork cost?
Typical US planning ranges: full replacement $2,000–$7,500 (most homes $3,000–$6,000), new ductwork $3,000–$10,000+, flex $25–$50 per linear foot installed, rigid metal $35–$75 per linear foot. Manual sealing runs $300–$800, aerosol sealing $1,500–$2,500. Costs vary significantly by region and access — full detail in the cost guide.
Can I size and install ductwork myself?
Sizing a single replacement runout or planning a layout is well within DIY reach with the calculator and the estimating guide. Full system design, new installs requiring permits, and anything involving gas, refrigerant, or electrical connections calls for a licensed HVAC contractor. Sealing accessible ducts with mastic is one of the best DIY returns available.
Do I need a permit for ductwork?
In most jurisdictions, yes for new or substantially modified ductwork — and inspections typically check sealing, insulation, and support. Requirements vary by locality, so check with your local authority having jurisdiction before starting. More in the code compliance guide.
Related Guides
- Complete Duct Sizing Guide — the full walkthrough with worked examples.
- Pro Tips — design and installation judgment from the trade.
- Estimating Guide — how to measure CFM needs, lengths, and fittings.
- DIY vs Pro — when to do it yourself and when to hire.
- Duct Sizing Checklist — verify every step before you buy ductwork.
Get Your Number
Questions answered — now get your answer. Enter your required CFM, friction rate, longest run length, and fitting count in the Duct Size Calculator, check the velocity against the target ranges above, and verify the result against your equipment’s static pressure rating. For full-system designs, have a licensed HVAC professional review the layout.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual duct sizing depends on your system’s airflow requirements, equipment static pressure, duct material, and local code. Verify with a licensed HVAC professional, and consult your local building authority for code and permit decisions.
