Introduction
Every forced-air heating and cooling system lives or dies by its ductwork. Undersize the ducts and rooms starve for air while the blower strains against high static pressure; oversize them and air drifts too slowly to mix, leaving hot and cold spots — plus you pay for metal you didn’t need. Duct sizing matches each duct run to the airflow (CFM) it must carry, at a friction rate the equipment can push against.
The Duct Size Calculator does the core arithmetic: enter required airflow, friction rate, duct length, and fittings, and it returns the duct diameter or dimensions you need. This guide explains everything behind that number — what the inputs mean, where they come from, and a fully worked example in imperial and metric units.
What Duct Sizing Is and Where It’s Used
Duct sizing selects a duct diameter (round) or width-by-height (rectangular) that delivers a required airflow at an acceptable velocity and friction loss. The two main families are supply ducts (conditioned air to rooms) and return ducts (room air back to the equipment). Returns deserve special respect: an undersized return is the single most common cause of noisy systems and weak airflow, because the blower can’t move air it can’t pull back.
Homeowners use this math when replacing ducts or diagnosing a chronically uncomfortable room; contractors use it to lay out trunk-and-branch systems and verify existing ductwork against equipment specs. For the step-by-step measuring procedure, see the estimating guide; for the math behind the results, the formula-explained guide.
Key Terms
CFM — the airflow currency
CFM (cubic feet per minute) is the air volume a duct must carry. The standard planning rule: 1 ton of cooling ≈ 400 CFM — a 3-ton system needs about 1,200 CFM total. Rooms get their share based on heating/cooling load (a proper room-by-room load calculation is the gold standard; ~1 CFM per square foot is a crude planning rule of thumb).
Friction rate — the pressure budget per 100 feet
Friction rate is the allowable pressure drop per 100 feet of duct, in inches of water column (in. w.c.). Take the blower’s rated external static pressure, subtract pressure lost to the coil, filter, grilles, and dampers, and divide the remainder by the total effective length of the longest run. Residential systems typically design around 0.08–0.10 in. w.c. per 100 ft (≈ 0.65–0.82 Pa/m).
FPM — air velocity, and equivalent length
FPM (feet per minute) is air speed in the duct — the noise and comfort constraint. 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). Equivalent length converts each fitting (elbows, tees, boots) into feet of straight duct with equal friction; straight length plus fitting equivalents is the total effective length (TEL) — and TEL, not tape length, goes into the friction rate.
Worked Example: Sizing a 3-Ton System’s Trunk
System: 3-ton air conditioner → 3 × 400 = 1,200 CFM total.
Step 1 — Friction rate. Air handler rated 0.50 in. w.c. external static pressure. Component losses: coil 0.15 + filter 0.06 + grilles 0.03 = 0.24 in. w.c. Available static pressure = 0.50 − 0.24 = 0.26 in. w.c. Longest run TEL = 325 ft. Friction rate = (0.26 ÷ 325) × 100 = 0.08 in. w.c. per 100 ft.
Step 2 — Diameter. At 0.08 friction rate, 1,200 CFM needs a duct with at least 1,200 CFM of capacity. A 16-inch round duct carries about 1,290 CFM at 0.08 (exact computed diameter: 15.6 in → round up to the standard 16-in size).
Step 3 — Velocity check. 16-in duct area = 1.396 ft²; 1,200 ÷ 1.396 = 859 FPM ✓ — inside the 700–900 FPM trunk target.
Step 4 — Rectangular option. A 24×10 in rectangular duct has an equivalent round diameter of ≈ 16.6 in, carrying roughly 1,410 CFM at 0.08 with air at 720 FPM ✓. (Keep rectangular aspect ratios under 3:1 — skinny shapes lose capacity to corner friction.)
Metric cross-check — 1,200 CFM × 0.000471947 = 0.566 m³/s; 16 in = 406 mm; 859 FPM = 4.36 m/s; 0.08 in. w.c./100 ft = 0.65 Pa/m. A 400 mm round duct (0.126 m²) at 0.566 m³/s moves air at 4.51 m/s — the same duct in metric units, confirming both forms agree.
CFM to Round Duct Diameter at Common Friction Rates
Required round duct diameter (inches) for a given CFM, rounded up to standard sizes. For flexible duct, treat these as minimums and go one size up — flex has higher friction than rigid metal:
| CFM | 0.06 in. w.c./100 ft | 0.08 in. w.c./100 ft | 0.10 in. w.c./100 ft |
|---|---|---|---|
| 100 | 7 in | 7 in | 6 in |
| 200 | 9 in | 8 in | 8 in |
| 300 | 10 in | 10 in | 9 in |
| 400 | 12 in | 12 in | 10 in |
| 650 | 14 in | 14 in | 12 in |
| 800 | 14 in | 14 in | 14 in |
| 1,000 | 16 in | 16 in | 14 in |
| 1,200 | 16 in | 16 in | 16 in |
| 1,500 | 18 in | 18 in | 16 in |
| 2,000 | 20 in | 20 in | 20 in |
Branch example: a bedroom needing 200 CFM at the system friction rate of 0.08 needs exactly 7.9 in → round up to 8-inch round (207 CFM capacity ✓, 573 FPM ✓). Metric: 200 CFM = 0.094 m³/s; 8 in = 203 mm; 573 FPM = 2.91 m/s.
Mistakes That Distort Results
- Undersizing the return. A starved blower can’t deliver the supply CFM no matter how good the supply ducts are.
- Guessing the friction rate. Compute it from static pressure and TEL — “0.10 because everyone does” undersizes long, fitting-heavy runs.
- Ignoring fittings. Elbows, takeoffs, and boots add 30–50% effective length on typical runs.
- Long, sagging flex runs. Compressed flexible duct can lose several times the friction of rigid metal.
- Skipping the load calculation. Sizing from square footage alone misallocates CFM room to room — the classic hot-bedroom pattern.
The full catalogue with fixes is in the common mistakes guide.
Related Guides
- Duct Size Calculator FAQ — quick answers to the most asked duct sizing questions.
- Pro Tips — duct design and installation judgment from the trade.
- DIY vs Pro — when you can size ducts yourself, and when to hire an HVAC contractor.
- Code Compliance — permits, duct sealing, and insulation requirements.
- Duct Sizing Checklist — every step to verify before you buy ductwork.
- Hypothetical Case Study — a worked undersized-duct scenario with the math shown.
Frequently Asked Questions
What size duct do I need for 1,200 CFM?
At 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. Always compute your actual friction rate first — longer runs need larger ducts for the same CFM.
How many CFM per ton of cooling?
The standard planning rule is 400 CFM per ton: 3 tons ≈ 1,200 CFM. Follow the equipment manufacturer’s data for high-efficiency or dehumidification-focused setups, which sometimes run 350 CFM/ton.
What is a good friction rate for residential ducts?
Typically 0.08–0.10 in. w.c. per 100 ft — computed as (external static pressure − component losses) × 100 ÷ total effective length. The derivation is in the formula-explained guide.
Round or rectangular duct — which is better?
Round wins on friction, cost per CFM, and leakage (fewer seams). Rectangular wins when you must fit a shallow joist bay. Keep aspect ratios under 3:1.
Get Your Number
Duct sizing is honest arithmetic on honest inputs: room-by-room CFM, a friction rate computed from your equipment and longest run, fittings counted as equivalent length, and a velocity check at the end. The Duct Size Calculator runs the numbers — your job is the tape measure, the fitting count, and the equipment nameplate. Verify the final layout against your local mechanical code before you buy ductwork.
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.
