Duct Size Calculator (Free Online Ductulator)
Instant duct size / ductulator screen: Area = CFM ÷ velocity (ft/min); round Ø = 12 × √(4A/π). Example: 400 CFM at ~900 fpm ≈ 12×8 rectangular. Not a Trane wheel purchase page—planning estimate before Manual D.
Input parameters
Quick: CFM + velocity. Advanced: circular vs rectangular and fix one side.
Quick examples:
About this calculator
Screens duct cross-section with the velocity method after CFM is known. For room turnover CFM use ACH to CFM; for tons/BTU use the HVAC size calculator. Browse all tools on the HVAC Calculators 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.
Last updated: 2026-07-29. 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 Calculators Hub
Duct sizing chart by CFM (screening)
Illustrative velocity-method pairs for early planning—confirm friction rate and fittings on project tables. Rectangular sizes are approximate equivalents.
| CFM | Round dia (in) | Rectangular (in) | Velocity (fpm) |
|---|---|---|---|
| 100 | ~5 | 8×6 | ~700–800 |
| 200 | ~6 | 10×6 | ~800 |
| 400 | ~8 | 12×8 | ~900 |
| 600 | ~9–10 | 14×8 | ~950 |
| 1,000 | ~10–11 | 14×10 | ~1,000 |
| 1,500 | ~12 | 18×12 | ~1,050 |
| 2,000 | ~13 | 20×12 | ~1,100 |
| 12,000 | ~32 | 48×24 | ~2,200 |
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 |
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.
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 chart for 12×12 / 24×10 class ducts before buying sheet metal.