Introduction
Ductwork is the quiet half of every forced-air system: a network of trunks, branches, and fittings that must carry exactly the right amount of air to every room. Size a run too small and the room starves — weak airflow, hot and cold spots, a whistling register. Size it too big and you waste material and lose register throw. Duct sizing replaces both mistakes with arithmetic: room airflow (CFM) divided by a target velocity gives the duct area, and the Ductwork Calculator runs that math, returning round and rectangular sizes in inches and millimeters with CFM converted to L/s.
This guide explains everything behind those sizes — the terminology, the equal-friction method, a full worked example, and the judgment calls the calculator can’t make for you.
What Ductwork Sizing Is and Where It’s Used
A forced-air system moves heat by moving air. The equipment produces a fixed total airflow — about 400 CFM per ton of cooling, so a 3-ton system moves roughly 1,200 CFM (566 L/s) — and the duct system divides it among rooms in proportion to each room’s heating and cooling load. The people who need this math:
- Homeowners and DIYers adding a run, replacing crushed flex, or checking a contractor’s proposal.
- HVAC installers laying out trunk-and-branch systems and selecting fittings.
- Estimators and contractors counting takeoffs, elbows, and transitions to price jobs.
- Designers doing preliminary layouts before a full Manual D calculation.
The principle never changes: every duct is sized from airflow (CFM) and a target velocity, then checked against the friction rate the blower can overcome. Undersized ducts raise static pressure and cut airflow; oversized ducts cost more and deliver air too slowly to mix the room properly.
Key Terms and Variables
CFM (cubic feet per minute)
The airflow a duct must carry — 1 CFM = 0.4719 L/s. Typical bedrooms need 100–150 CFM (47–71 L/s); large living areas 150–250 CFM (71–118 L/s). Room CFM comes from a load calculation (Manual J); “1 CFM per square foot” is only a rough check.
Friction rate
The pressure budget per 100 feet of equivalent duct length. The equal-friction method (ACCA Manual D) designs every section to the same rate — typically 0.08–0.10 in w.g./100 ft (≈ 0.65–0.82 Pa/m) residential. Higher rates mean smaller, cheaper ducts but a harder-working blower.
Equivalent length
Each fitting converted to the straight-duct length causing equal pressure drop. Typical planning values: 90° smooth elbow 10 ft (3.0 m), pleated elbow 15 ft (4.6 m), 45° elbow 5 ft (1.5 m), supply takeoff with damper 35 ft (10.7 m), register boot 15 ft (4.6 m), gradual transition 10 ft (3.0 m). Compressed flex adds far more — keep it taut. Full table and derivation in the formula-explained guide.
Air velocity (FPM)
Feet per minute — 900 FPM = 4.57 m/s. Velocity is the noise lever: fast air is noisy air. Residential caps: branch runs 900 FPM (4.6 m/s) max, supply trunks 700–900 FPM (3.6–4.6 m/s), returns 600–700 FPM (3.0–3.6 m/s).
Trunk, branch, takeoff
The trunk is the large main duct leaving the plenum; branches split off toward registers; the takeoff is the fitting (usually with a balancing damper) where a branch leaves the trunk. Trunks step down in size as branches peel airflow off — e.g., 14×8 in (356×203 mm) reducing to 12×8, then 10×8.
The Formulas Involved
Three calculations in sequence:
1. Area = CFM ÷ Velocity (FPM). A 150 CFM branch at 900 FPM needs 150 ÷ 900 = 0.1667 ft² = 24 in². This division is the heart of the calculator.
2. Diameter = 2 × √(Area ÷ π). For 24 in²: 5.53 in → round up to 6 in (152 mm) standard. Always round up, never down.
3. Friction rate = (Available static ÷ Total equivalent length) × 100. With 0.25 in w.g. available and 300 ft equivalent: (0.25 ÷ 300) × 100 = 0.083 in w.g./100 ft — inside the 0.08–0.10 band, confirming a workable layout.
Rectangular equivalents follow standard tables (6-in round ≈ 8×4 or 6×6 in), and material choice shifts the friction math — see the material guide for sheet metal vs flex vs duct board.
Worked Example: Bedroom Branch
A bedroom needs 150 CFM (71 L/s); the branch is round sheet metal.
Step 1 — Area: 150 ÷ 900 FPM = 0.1667 ft² = 24.0 in².
Step 2 — Diameter: 2 × √(24 ÷ π) = 5.53 in → 6 in (152 mm) round.
Step 3 — Velocity check: 6-in duct = 0.1963 ft²; 150 ÷ 0.1963 = 764 FPM (3.88 m/s) — under the 900 FPM cap.
Step 4 — Friction check: 25 ft duct + takeoff (35) + elbow (10) + boot (15) = 85 ft (25.9 m) equivalent. At 0.08 in w.g./100 ft that’s 0.068 in w.g. — a modest slice of a typical 0.5 in w.g. blower budget.
Metric cross-check: 150 CFM = 70.8 L/s; 0.0708 m³/s ÷ 4.57 m/s = 0.0155 m²; diameter = 140 mm → 150 mm standard. Same answer either system. Pricing this run is covered in the cost guide.
Common Variations
- Rectangular trunk: same area math; pick the rectangle fitting the framing bay — 12×8 in (305×203 mm) suits 16-in joist spacing.
- Flex branch: same diameters, but keep runs under 25 ft (7.6 m), stretched taut, supported every 4 ft (1.2 m).
- Multiple branches: size each branch from its room CFM; size each trunk segment from the remaining CFM downstream.
- Returns: lower velocity (600–700 FPM / 3.0–3.6 m/s), never smaller in aggregate than the supply served.
Mistakes That Distort Results
- Sizing from square footage instead of a load calculation — garbage CFM in, garbage duct out.
- Ignoring equivalent length — fittings can triple a run’s resistance versus its tape length.
- Forgetting the return path — a choked return raises static pressure system-wide.
- Crushed or sagging flex — performs like a duct one to two sizes smaller.
- No balancing dampers — short runs permanently steal air from long ones.
- Rounding down — sizes step up to the next standard size, never down.
Full catalogue with fixes in the common mistakes guide; pre-install verification in the installation checklist.
Related Guides
- Ductwork Calculator FAQ — quick answers to the most asked sizing questions.
- Pro Tips — layout and installation advice from the trade.
- Estimating Guide — counting fittings and pricing a ductwork job.
- DIY vs Pro — which jobs suit a homeowner and which need a contractor.
- Code Compliance — sealing, insulation, and support rules to check locally.
- Case Study — a hypothetical full-house sizing walkthrough.
Frequently Asked Questions
What size duct do I need for 150 CFM?
At 900 FPM (4.57 m/s): 150 ÷ 900 = 24 in² → 6-in (152 mm) round. In metric: 70.8 L/s ÷ 4.57 m/s gives 140 mm calculated → 150 mm standard.
What is the 0.1 friction rate rule?
The equal-friction method designs residential ducts to ~0.08–0.10 in w.g. per 100 ft (0.65–0.82 Pa/m) of equivalent length — the Manual D starting point balancing duct size, cost, and blower load.
How do you convert CFM to duct size?
Area = CFM ÷ velocity (FPM); diameter = 2 × √(area ÷ π), round up. Example: 200 CFM ÷ 900 = 32 in² → 6.4 in → 8-in round (203 mm).
Can I use flex duct for the whole system?
You can, but keep runs short (under 25 ft / 7.6 m), fully stretched, supported every 4 ft. Compressed or sagging flex has far higher friction than the math assumes.
How many CFM per ton of cooling?
About 400 CFM per ton residential (3 tons ≈ 1,200 CFM / 566 L/s) — rough planning only; actual airflow follows equipment specs and the Manual J load.
Should trunk ducts reduce along their length?
Yes. Each takeoff removes airflow, so downstream segments carry less CFM — stepping down keeps velocity up and material cost down.
Get Your Number
Duct sizing is straightforward arithmetic on honest inputs: real room-by-room CFM, a sensible velocity target, and full equivalent length including every fitting. The Ductwork Calculator does the arithmetic in imperial and metric — your job is the tape measure and the load numbers.
Disclaimer: Calculator results are planning estimates based on the values you enter. Actual duct performance depends on installation quality, sealing, and equipment static pressure. Verify sizing against ACCA Manual D methods and your local mechanical code, and consult a licensed HVAC professional for system design decisions.
