Introduction
Ductwork sizing generates the same questions from homeowners, DIYers, and apprentice installers — about CFM, duct diameters, flex vs. metal, friction rates, and what things cost. This page answers them directly, with numbers in both imperial and metric units. For the full derivations behind these answers, see the complete guide and the formula-explained guide; to get sizes for your own rooms, use the Ductwork Calculator.
Sizing Questions
How do I calculate what size duct I need?
Divide the room’s required CFM by your target velocity in FPM to get the duct area in square feet, then convert to diameter: D = 2 × √(area in in² ÷ π), rounding up to the next standard size. Example: 150 CFM ÷ 900 FPM = 24 in² → 5.53 in → 6-in (152 mm) round duct.
What size duct do I need for 100 CFM? 200 CFM? 400 CFM?
At 900 FPM (4.57 m/s) branch velocity: 100 CFM → 16 in² → 5-in calculated → 6-in round (152 mm); 200 CFM → 32 in² → 6.4 in → 8-in round (203 mm); 400 CFM → 64 in² → 9.0 in → 10-in round (254 mm). In metric: 100 CFM = 47 L/s → 125 mm; 200 CFM = 94 L/s → 175 mm; 400 CFM = 189 L/s → 250 mm standard sizes.
What is CFM and how do I find my room’s CFM?
CFM (cubic feet per minute; 1 CFM = 0.4719 L/s) is the airflow a room needs. The correct source is a room-by-room heating/cooling load calculation (ACCA Manual J), which accounts for windows, insulation, and orientation. Rough planning figures: bedrooms 100–150 CFM (47–71 L/s), living areas 150–250 CFM (71–118 L/s).
What is the 0.1 friction rate rule?
The equal-friction method (Manual D) designs residential ducts to roughly 0.08–0.10 inches of water column pressure drop per 100 ft of equivalent length (≈ 0.65–0.82 Pa/m). Compute your actual design friction rate as (available static pressure ÷ total equivalent length) × 100 rather than assuming 0.1.
How many CFM per ton of air conditioning?
About 400 CFM per ton is the standard residential figure — a 3-ton system moves roughly 1,200 CFM (566 L/s). Use it for rough planning only; actual airflow follows the equipment’s blower table and the Manual J load.
Material Questions
Is flex duct or sheet metal better?
Sheet metal has lower friction, lasts longer, and holds its shape; flex is cheaper, faster to install, and easier to route through tight spaces. The common professional compromise is sheet metal trunks with flex branches. Flex must be installed fully stretched and supported every 4 ft (1.2 m) — compressed flex performs far worse than its rated size. Full comparison in the material guide.
What is duct board?
Fiberglass duct board is a rigid, pre-insulated panel (typically R-4.2 to R-8) fabricated into rectangular ducts. It combines duct and insulation in one step and is common for trunks in some regions. Joints must be sealed and the interior liner kept intact; it’s less damage-tolerant than sheet metal.
What gauge sheet metal is used for residential ductwork?
Typically 26 gauge for smaller round duct and fittings, 24 gauge for rectangular trunks — per SMACNA standards. Heavier gauges (thicker metal) are used for larger commercial ducts. Rectangular ducts also need cross-breaking or beading to prevent panel drumming.
Can I mix flex and sheet metal in one system?
Yes — it’s standard practice. Size each section for the material actually installed, since friction characteristics differ. The usual pattern is metal trunk stepping down with flex branches of 25 ft (7.6 m) or less.
Installation Questions
How far can I run flex duct?
Keep individual flex runs under 25 ft (7.6 m) where possible, fully stretched, with no sharp bends (bend radius ≥ one duct diameter) and supports every 4 ft (1.2 m). Longer flex runs accumulate enough friction to starve the register — upsize the diameter or switch to metal for long runs.
Do supply ducts need to be insulated?
Any duct outside conditioned space (attics, crawlspaces, garages) must be insulated — energy codes typically require R-8 in attics and R-6 elsewhere (verify locally). Uninsulated ducts in a hot attic can lose a large share of their cooling before the air reaches the room.
Should I seal my ducts with mastic or tape?
Mastic (brush- or trowel-applied) is the durable standard for all joints and seams. If you use tape, it must be UL-181-listed foil or mastic tape — ordinary cloth duct tape fails within a few years. Typical leaky systems lose 20–30% of airflow; sealing is the highest-ROI duct improvement.
How often should ducts be supported?
Flex duct: every 4 ft (1.2 m) with wide straps that don’t constrict the duct. Round metal: typically every 10 ft (3.0 m); rectangular per SMACNA/code schedules. Sagging between supports raises friction and, in flex, effectively shrinks the duct.
Cost and Project Questions
How much does new ductwork cost?
Typical US planning ranges: $150–$300 per installed flex run, $25–$55 per linear foot ($82–$180/m) for installed sheet metal, and $4,000–$9,000 for a full whole-house replacement (1,500–2,500 sq ft / 140–230 m²). See the cost guide for detailed brackets and regional caveats.
Can I install ductwork myself?
Straightforward tasks — replacing a flex run, sealing ducts with mastic, adding a register boot — are within a careful DIYer’s reach. Full system design, sheet metal fabrication, and anything involving the equipment or refrigerant needs a licensed pro.
Does ductwork need a permit?
In most US jurisdictions, new or replacement ductwork requires a mechanical permit and inspection; simple repairs and sealing usually don’t. Requirements vary by locality — check with your AHJ (authority having jurisdiction) before starting. More in the code compliance guide.
Performance Questions
Why are some rooms hotter/colder than others?
The usual causes in order: missing or closed balancing dampers letting short runs steal air, undersized or missing return paths, crushed/sagging flex on long runs, and duct leakage. Equipment is rarely the culprit — distribution is. Check dampers, returns, flex condition, and leakage in that order.
Why is my ductwork noisy?
Air velocity above ~900 FPM (4.6 m/s) at grilles, undersized ducts forcing high velocity, or sharp fittings near registers. Fixes: upsize the noisy run, add turning vanes to square elbows, ensure dampers aren’t throttled nearly shut, and check that the blower speed matches the duct system.
How long does ductwork last?
Sheet metal ducts can last 25–50+ years; flex duct typically 15–25 years (the inner liner and insulation degrade). Replace flex that’s brittle, mold-affected, or crushed; metal that’s rusted through or chronically undersized.
Related Guides
- Complete Guide — full sizing walkthrough with worked example.
- Formula Explained — derivations of every calculation.
- Cost Guide — typical installed price ranges.
- Material Guide — sheet metal vs flex vs duct board.
- Pro Tips — installation practices from the trade.
- Installation Checklist — verify before close-up.
Get Your Answers as Numbers
Questions are useful; sized ducts are better. Enter your room CFM values into the Ductwork Calculator to get diameters in inches and millimeters for every run — then use the guides above to install them right.
Disclaimer: These answers are general planning information based on standard industry methods. Actual requirements vary with equipment, climate, and local code. Verify sizing per ACCA Manual D and consult a licensed HVAC professional for system design decisions.
