Online Ductulator & Duct Size Calculator (CFM → Size)
Duct size from CFM: area = CFM ÷ velocity; round diameter (in) = 12 × √(4A/π). Example: 400 CFM at 900 fpm → 0.44 ft² → about 9 in round. Reverse check: 12×12 at 900 fpm ≈ 900 CFM. Then verify friction in duct static pressure.
Input parameters
Quick: CFM + velocity. Advanced: circular vs rectangular and fix one side.
Quick examples:
About this calculator
Screens duct cross-section once CFM is known — by design velocity or by friction rate (the equal-friction method a ductulator wheel uses). For room turnover CFM use ACH to CFM; for tons/BTU use the HVAC tonnage calculator. Browse all tools on the HVAC tools hub.
Calculation results
What you are seeing: free-area size if air moved uniformly at your design velocity. Not modeled: fittings, full friction chart, or aspect-ratio limits. Use as a screen before Manual D. Next: static pressure, HVAC size, or Industrial HVAC Design.
Engineering disclaimer
Velocity-method estimates only. Final duct design, acoustic limits, and code compliance require detailed friction/fitting analysis and a qualified professional when required. Not a substitute for ACCA Manual D.
Design velocity bands (screening)
| Application | Typical velocity (ft/min) |
|---|---|
| Main trunk (industrial) | 2,000–2,500 |
| Branch / takeoff | 1,500–2,000 |
| Low-noise occupied | 700–1,200 |
| Exhaust / hood (generic) | 2,500–3,500 |
Equal-friction rate chart (in.wg / 100 ft)
Ductulator-style friction bands for screening—pair with velocity above. Final SP still needs fittings + equipment.
| System | Typical friction (in.wg/100 ft) |
|---|---|
| Low-pressure supply | 0.08–0.10 |
| Medium-pressure / VAV | 0.15–0.25 |
| Industrial exhaust | 0.20–0.40 |
Round ↔ rectangular equivalent (same free area)
For the same area A: rectangular sides satisfy side₁ × side₂ = A × 144 (in²). Example: 12 in round ≈ 0.785 ft² → a 14×8 in rectangle is a common near-equivalent screen—confirm aspect ratio ≤4:1 for low noise.
Diffuser / grille face-velocity screen
After branch CFM is known, estimate free area: A (ft²) = CFM ÷ face velocity (fpm). Typical supply face velocity ~400–700 fpm; return grilles often ~300–600 fpm. Convert to in² (×144) and apply the manufacturer free-area ratio / Ak factor. Throw, NC, and pressure drop still require OEM catalogs—this is area screening only.
| Outlet CFM | Face V (fpm) | Free area (ft²) | Free area (in²) |
|---|---|---|---|
| 150 | 500 | 0.30 | 43 |
| 300 | 500 | 0.60 | 86 |
| 600 | 600 | 1.00 | 144 |
Formula & explanation
Free area A (ft²) = CFM ÷ V where V is design velocity in ft/min.
Round diameter D (in) = 12 × √(4A / π).
Rectangular: side₂ (in) = (A × 144) ÷ side₁.
Worked example: 12,000 CFM at 2,200 ft/min → A = 5.45 ft² → round D ≈ 31.6 in. Prefer standard sizes upward after acoustic and friction review.
Manual D vs this calculator
ACCA Manual D is the residential/light-commercial duct design standard using equal-friction (or static-regain) methods, fitting equivalent lengths, and pressure budgeting. Stamped Manual D is what many permits and final layouts require.
This tool is a velocity-method screen: CFM ÷ velocity → area → round/rectangular size. Best for early BOM and trunk checks; not for long runs with many fittings, permit drawings, or acoustic compliance alone.
How to size ducts: 5-step walkthrough
- Get supply CFM from ACH to CFM or the equipment schedule.
- Pick design velocity (residential ~600–900 fpm; commercial ~800–1200; industrial trunks often higher—see bands above).
- Compute area = CFM ÷ velocity (ft²).
- Choose round or rectangular and read diameter / sides from the calculator.
- Verify friction / ESP with the duct static pressure calculator and refine under Manual D when required.
Common duct sizing mistakes
- Velocity too high → noise and excessive pressure loss.
- Ignoring fitting equivalent length (elbows can add tens of feet of equivalent duct).
- Poor trunk/branch ratios or undersized returns.
- Forgetting lining / insulation free-area reduction on internal dimensions.
Sizing by friction rate (equal-friction / ductulator method)
A physical ductulator wheel answers the equal-friction question: for a given airflow and a target pressure loss per 100 ft, what diameter do I need? This page solves it too — set Sizing method to By friction rate. It inverts the ASHRAE duct friction relation used by our static pressure tool:
friction (in.wg/100 ft) = 0.109136 × CFM^1.9 ÷ D^5.02 → D (in) = (0.109136 × CFM^1.9 ÷ friction)^(1/5.02)
Assumes standard air (0.075 lb/ft³), galvanized steel, straight duct — the same basis as the duct static pressure calculator, so both pages agree. For rectangular duct, enter one known side and the other is solved to match the equivalent diameter. Screening only: round up to the next stock size and confirm the real run (length, fittings, ESP) before ordering.
| Airflow (CFM) | D @ 0.08 in.wg/100 ft | D @ 0.10 in.wg/100 ft | D @ 0.15 in.wg/100 ft | Velocity @ 0.10 (ft/min) |
|---|---|---|---|---|
| 200 | 7.9 | 7.6 | 7.0 | 642 |
| 400 | 10.3 | 9.8 | 9.1 | 759 |
| 800 | 13.4 | 12.8 | 11.8 | 899 |
| 1,200 | 15.6 | 14.9 | 13.7 | 992 |
| 2,000 | 18.9 | 18.1 | 16.7 | 1,123 |
| 4,000 | 24.6 | 23.5 | 21.7 | 1,329 |
| 8,000 | 31.9 | 30.5 | 28.2 | 1,573 |
| 12,000 | 37.2 | 35.6 | 32.8 | 1,736 |
Ductulator modes: a wheel switches between velocity and friction rate; this page now does both — choose the method under Sizing method. Round up to the next stock size, then confirm the real run (fittings, length, ESP) with the static pressure calculator. We do not sell a physical ductulator wheel.
Last updated: 2026-09-07. Screening estimate only—validate with Manual D or project friction charts.
References
- ACCA technical manuals (Manual D / related)
- ASHRAE technical resources
- SMACNA duct construction standards
Guides: Industrial HVAC Design — duct sizing · HVAC sizing · HVAC tools hub
Duct sizing chart CFM (round & rectangular)
Field-style CFM sizing chart / duct sizing chart: CFM → round or rectangular size at a design velocity (units: CFM, inches, fpm). Illustrative velocity-method pairs—confirm friction rate and fittings. Prefer the calculator above to verify any row with your exact CFM and velocity—there is no separate chart-only URL.
| CFM | Round dia (in) | Rectangular (in) | Velocity (fpm) |
|---|---|---|---|
| 100 | ~5 | 8×6 | ~700–800 |
| 200 | ~6 | 10×6 | ~800 |
| 300 | ~7 | 12×6 | ~850 |
| 400 | ~8 | 12×8 | ~900 |
| 500 | ~8–9 | 14×8 | ~900 |
| 600 | ~9–10 | 14×8 | ~950 |
| 800 | ~10 | 14×10 | ~1,000 |
| 1,000 | ~10–11 | 16×10 | ~1,000 |
| 1,200 | ~11–12 | 18×10 | ~1,050 |
| 1,500 | ~12 | 18×12 | ~1,050 |
| 2,000 | ~13 | 20×12 | ~1,100 |
| 2,500 | ~14–15 | 22×14 | ~1,200 |
| 3,000 | ~16 | 24×14 | ~1,250 |
| 5,000 | ~20 | 30×18 | ~1,500 |
| 12,000 | ~32 | 48×24 | ~2,200 |
How many CFM for 7" (and other) round ducts?
Round free area ≈ π×(D/24)² ft². At branch velocities:
| Round Ø (in) | Area (ft²) | ~CFM @ 700 fpm | ~CFM @ 900 fpm |
|---|---|---|---|
| 6" | 0.196 | ~140 | ~175 |
| 7" | 0.267 | ~190 | ~240 |
| 8" | 0.349 | ~245 | ~315 |
| 10" | 0.545 | ~380 | ~490 |
How many CFM for common rectangular ducts? (PAA reverse lookup)
CFM ≈ Area(ft²) × velocity(fpm). Area for W×H inches = (W×H)÷144. At 700 fpm (quiet branch) vs 900 fpm (typical branch screen):
| Duct (in) | Area (ft²) | ~CFM @ 700 fpm | ~CFM @ 900 fpm |
|---|---|---|---|
| 12×12 | 1.00 | ~700 | ~900 |
| 24×10 | 1.67 | ~1,170 | ~1,500 |
| 20×12 | 1.67 | ~1,170 | ~1,500 |
| 14×8 | 0.78 | ~545 | ~700 |
Flex duct often needs a lower design velocity (more friction) than metal—do not assume the same CFM for the same nominal size. Confirm with project friction tables.
CFM vs round diameter (at 2,200 ft/min)
| CFM | Area (ft²) | Round Ø (in) |
|---|---|---|
| 1,000 | 0.45 | ~9.1 |
| 2,000 | 0.91 | ~12.9 |
| 5,000 | 2.27 | ~20.4 |
| 12,000 | 5.45 | ~31.6 |
CFM Requirements by Room Type (Residential Screening)
Quick reference for supply air CFM by room type, based on typical residential cooling loads (1 CFM per sq ft for average rooms, higher for kitchens and high-solar-gain areas). Use as a starting point for duct sizing — actual values depend on Manual J room-by-room load.
| Room Type | Typical Size | CFM per sq ft | Total CFM | Round Duct (≈900 fpm) |
|---|---|---|---|---|
| Bedroom (small) | 100–150 sq ft | 1.0 | 100–150 | 5"–6" round |
| Bedroom (master) | 200–300 sq ft | 1.0 | 200–300 | 6"–7" round |
| Living room / Great room | 300–500 sq ft | 1.0–1.2 | 300–600 | 7"–9" round |
| Kitchen | 150–250 sq ft | 1.5–2.0 | 225–500 | 7"–8" round |
| Bathroom | 50–100 sq ft | 1.0–1.5 | 50–150 | 4"–6" round |
| Home office / Study | 100–200 sq ft | 1.2–1.5 | 120–300 | 6"–7" round |
| Garage (conditioned) | 400–600 sq ft | 0.5–0.8 | 200–480 | 7"–8" round |
| Open-plan whole floor (trunk) | 1,000–2,000 sq ft | 1.0 | 1,000–2,000 | 12"–16" round or 10×20" rect |
CFM values are screening estimates for average insulation and standard ceiling heights. High-solar-gain rooms, vaulted ceilings, or rooms with many windows may need +20–30% CFM. Always confirm with room-by-room Manual J / Manual D calculations.
Worked Duct Sizing Examples (2 Real Scenarios)
Example 1 — 2,000 sq ft Single-Family Home (3-ton system, 1,200 CFM)
System: 3-ton AC = 1,200 CFM (400 CFM/ton) · supply trunk + 6 branch runs
Step 1 — Main supply trunk: 1,200 CFM at 900 fpm → duct area = 1,200 ÷ 900 = 1.33 ft² = 192 in² → 14" round or 10×20" rectangular (200 in², velocity 864 fpm)
Step 2 — Branch ducts (per room):
- Master bedroom (250 CFM) → 7" round (velocity 935 fpm)
- 2× bedrooms (150 CFM each) → 6" round each (velocity 764 fpm)
- Living room (400 CFM) → 8" round (velocity 1,146 fpm — slightly high, consider 9" or 8×10" rect)
- Kitchen (250 CFM) → 7" round
Step 3 — Return air: 1,200 CFM return at 800 fpm → 16" round or 12×20" rectangular (often split into two 12×12" returns for better air distribution)
Total estimated friction loss: ~0.08–0.10 in WG per 100 ft for main trunk, ~0.05–0.08 for branches. Total external static pressure (ESP) target: ≤0.5 in WG for standard blower.
Example 2 — 5,000 sq ft Small Office (10-ton RTU, 4,000 CFM)
System: 10-ton rooftop unit = 4,000 CFM · VAV system with 8 zones
Step 1 — Main supply duct: 4,000 CFM at 1,200 fpm (commercial design velocity) → area = 4,000 ÷ 1,200 = 3.33 ft² = 480 in² → 24" round or 16×32" rectangular (512 in², velocity 1,125 fpm)
Step 2 — Zone branches (average 500 CFM each): 500 CFM at 1,000 fpm → 10" round or 8×12" rectangular
Step 3 — Diffusers: Each zone served by 2–4 ceiling diffusers at 100–150 CFM each. 150 CFM diffuser → 6" round neck, face velocity ~500 fpm
Step 4 — Return air: 4,000 CFM at 1,000 fpm → 28" round or 20×32" rectangular (often uses ceiling plenum return instead of ducted return in office buildings)
Commercial duct note: Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) guidelines recommend maximum duct velocities of 1,500 fpm for supply mains in noise-sensitive areas, 2,000 fpm for industrial. Rectangular ducts should have aspect ratio ≤4:1 (ideally ≤3:1) to minimize material and friction loss.
Sizing the return air duct
The return air duct size calculator uses the same velocity-area method as supply, but returns are usually screened at a lower velocity because the air also passes through the filter and return grille. A practical band is 600–800 ft/min for return trunks versus 900–1,200 ft/min for supply.
Rule of thumb: make the return equal to or one nominal size larger than the supply—many contractors size the return at 1.5× the supply cross-sectional area to keep filter and grille face velocity down and reduce noise. For a 1,200 CFM system at 800 ft/min return velocity, that is 1.5 ft² → a 16" round or 12×20" rectangular return (see Example 1).
Enter your return CFM and a return velocity in the calculator above, or derive return airflow from room volume with ACH to CFM.
Frequently asked questions
How do you calculate HVAC duct size from CFM?
Divide CFM by design velocity (ft/min) to get area in ft². For round duct, diameter in inches is 12 × √(4A/π). Enter values above for an instant result.
What velocity should I use for an HVAC trunk size calculator?
Industrial trunks often screen near 2,000–2,500 ft/min; branches 1,500–2,000. Lower velocity for quieter spaces. Always confirm owner and code criteria.
Is this Manual D duct design?
This page is a velocity-area screen before Manual D. Equal-friction Manual D, fittings, and static regain need detailed design—see Manual D vs this calculator.
How is this different from the HVAC size calculator?
The HVAC size calculator estimates tons/BTU/h. This page sizes duct after you know CFM—use ACH to CFM if you only have room volume and air changes.
How do I size a diffuser or grille from CFM?
Free area ≈ CFM ÷ face velocity (fpm). Supply outlets often screen at 400–700 fpm; returns 300–600 fpm. Apply OEM free-area / Ak factors, then confirm throw and NC from catalog data—see the diffuser table above.
How do I calculate duct size for HVAC step by step?
(1) Get CFM, (2) pick velocity, (3) compute area = CFM ÷ V, (4) convert to round or rectangular size, (5) bump to a standard catalog size and check noise/static in design software.
How many CFM can a 12×12 duct handle?
At a free-area velocity of 700 fpm, a 12×12 duct (1.0 ft²) carries about 700 CFM; at 900 fpm about 900 CFM. See the rectangular CFM reverse table. Real capacity drops with flex, fittings, and friction limits.
How much CFM is a 7 inch duct?
A 7" round duct has about 0.267 ft² free area. At 700 fpm ≈ 190 CFM; at 900 fpm ≈ 240 CFM. See the round CFM table, then confirm friction with static pressure.
What is the rule of thumb for HVAC duct sizing?
Common branch screens use about 700–900 fpm supply velocity (quieter toward 700); trunks often higher. Compute area = CFM ÷ velocity, pick the next standard size up, then check noise and ESP. There is no universal “one size fits all” inch rule without CFM.
Is this the same as a Trane ductulator or online ductulator wheel?
Same job (CFM ↔ size), different product: this is a free velocity-method web screen. Plastic/Trane ductulator wheels and OEM apps add friction-rate modes—use them for final selections. Keywords like ductulator and online ductulator land here for planning only.
Flex duct vs metal—same CFM for the same size?
Usually no. Flexible duct has higher friction; many shops derate velocity or upsize one nominal diameter versus metal. Enter a conservative velocity above, then confirm on the project friction chart.
How do I figure out what size ductwork I need?
Start from required CFM (ACH to CFM or equipment schedule), pick design velocity, then solve size here. Cross-check the duct sizing chart CFM and 12×12 / 24×10 reverse table before buying sheet metal.
What is a duct sizing chart CFM used for?
A duct sizing chart maps CFM ↔ duct size at a design velocity (or reverse: known size → CFM). Use the chart and calculator on this page as a field/desk screen—not a substitute for friction tables or Manual D.
How many CFM is a 24×10 duct?
Free area ≈ (24×10)÷144 ≈ 1.67 ft². At 700 fpm ≈ 1,170 CFM; at 900 fpm ≈ 1,500 CFM. See rectangular reverse table.
Where is the CFM sizing chart on this page?
Use the duct sizing chart CFM block (CFM → round/rect size) plus the rectangular reverse table. Then enter the same CFM and design velocity in the calculator to verify—do not open a separate chart-only page.
What velocity should a residential CFM sizing chart assume?
Quiet branch screens often use about 700–900 fpm; trunks run higher. The chart rows above list a typical velocity band for each CFM—always re-check with your project noise and friction criteria in the calculator.
Is picking a larger duct from the chart always safer?
Not always. Oversizing drops velocity and can hurt throw at outlets; undersizing raises noise and static. Match CFM to the design velocity, then confirm ESP with the static pressure calculator.
How do I size a return air duct (return air duct size calculator)?
Return ducts use the same velocity-area method as supply, but at a lower design velocity. A common screen is 600–800 fpm for return trunks (vs 900–1,200 fpm for supply) because returns also pull air through filters and grilles. Compute area = return CFM ÷ return velocity, then convert to round or rectangular size. Rule of thumb: size the return equal to or one nominal size larger than the supply—many contractors make the return 1.5× the supply area to lower filter and grille face velocity. Example: 1,200 CFM return at 800 fpm → 1.5 ft² → 16" round or 12×20" rectangular (see the worked Example 1 above). Enter your return CFM and a return velocity in the calculator, or start from ACH to CFM.
Can I size a duct by friction rate instead of velocity (online ductulator)?
Yes — switch Sizing method to “By friction rate”. That is the equal-friction method a physical ductulator wheel uses: you enter airflow plus a target pressure loss per 100 ft, and the tool back-solves the diameter. Typical design targets are 0.08–0.10 in.wg/100 ft for supply trunks and 0.06–0.08 for noise-sensitive branch runs. For example, 1,200 CFM at 0.10 in.wg/100 ft solves to about a 14.9 in round duct running near 990 fpm. The velocity method is the better pick when you already have a noise or velocity limit; the friction method is better when you are budgeting pressure loss across a long run. Both use standard air on galvanized steel, and both agree with the duct static pressure calculator.