TryBuildCalcDuct Size Calculator (CFM to Round Duct Diameter)
Find your duct size instantly.
🕒 Last updated: September 11, 2026
Inputs
ℹ️Used only for the Suggested Total System CFM cross-check — never auto-applied to any run's own CFM.
ℹ️350 CFM/ton for humid climates, 400 standard, 450 for dry climates — used with tonnage for the cross-check above.
Duct Runs
Duct Runs Sized
1
run
Suggested Total System CFM
Not entered
Enter system tonnage above for a cross-check reference
Simplified velocity-based sizing — not a Manual D duct design. This calculator sizes duct diameter using a simplified velocity-only method — its velocity targets are common planning limits informed by ACCA Manual D guidance, not the full Manual D equal-friction duct-sizing procedure, which sizes ducts from design airflow, friction rate, duct material/roughness, and total system static pressure. It does not account for duct length, fitting/elbow friction loss, total effective length, or your system's available static pressure budget. Get a full Manual D duct design from a licensed HVAC contractor before fabrication or installation.
Run 1
Supply Trunk — 400 CFM
Target 700 FPM (max 900 FPM) — actual velocity at 12 in is 509 FPM
Assumptions Used
Each duct run is sized independently from its own entered CFM and duct category (Supply Trunk, Supply Branch, Return Trunk, or Return Branch) — this calculator does not attempt to auto-distribute a whole-system CFM across runs, since that's a genuine Manual D design step. Each category targets its own recommended (quieter, more conservative) velocity — Supply Trunk 700 FPM, Supply Branch 600 FPM, Return Trunk 600 FPM, Return Branch 400 FPM — planning limits informed by common ACCA Manual D velocity guidance, rather than the higher maximum each category still commonly allows. Required duct area is CFM ÷ target velocity, converted to a round diameter, then rounded UP to the nearest standard stock size (4 through 24 in). The velocity shown for the sized duct is recalculated at that actual standard size, which is usually a little under the target since sizes snap up in whole-inch steps. A run whose raw diameter would exceed 24 in is flagged rather than capped, since that's outside this calculator's modeled single-round-duct range and likely needs splitting into parallel runs or a professional design. "Suggested Total System CFM" (tonnage × CFM/ton) is an optional, informational cross-check only — it is never substituted into any individual run's own CFM. This is a simplified velocity-only sizing shortcut, not a full ACCA Manual D equal-friction design — a real Manual D calculation sizes duct from design airflow, friction rate, duct material/roughness, and total system static pressure, not velocity limits alone, and also accounts for duct length, fitting/elbow friction loss, and total effective length that this shortcut does not model. Get a full duct design from a licensed HVAC contractor before fabrication or installation.
What Is a Duct Size Calculator?
A duct size calculator estimates the round duct diameter needed to carry a given airflow (CFM) at a safe, quiet velocity — sizing each supply or return run independently from its own entered CFM and duct category (trunk vs. branch), since each targets a different planning velocity commonly informed by ACCA Manual D guidance.
This calculator computes a simplified velocity-only diameter — it is not a full ACCA Manual D duct design, which also sizes duct from design airflow, friction rate, duct material/roughness, and total system static pressure, not velocity limits alone. It does not account for duct length, fitting/elbow friction loss, or your system's available static pressure budget — always get a full duct design from a licensed HVAC contractor before fabrication or installation.
Why duct sizing needs its own calculator, not a single flat number:
- Each run — trunk vs. branch, supply vs. return — targets a genuinely different planning velocity, so the same CFM produces a different diameter depending on duct category
- A whole duct system is a mix of runs sized independently, not one number for the whole house
- Standard round duct is sold in fixed stock diameters — the raw calculated size must round up, not down, to avoid running faster (and noisier) than the target velocity
- A run whose raw diameter would exceed a single duct's practical capacity needs a different design approach, not a silently oversized single duct
Duct Size Calculator Formula: How Is It Determined?
Each run is sized independently from its own CFM and duct category, then rounded up to a standard stock size.
Step 1 — Required Area at Target Velocity
Required Area (sq ft) = Run CFM ÷ Target Velocity (FPM)
Target velocity is the selected duct category's recommended (not maximum) planning limit, informed by common ACCA Manual D velocity guidance — Supply Trunk 700 FPM, Supply Branch 600 FPM, Return Trunk 600 FPM, Return Branch 400 FPM. This is a velocity-only shortcut, not the full Manual D method, which also uses design airflow, friction rate, duct properties, and total system resistance.
Step 2 — Raw Diameter
Raw Diameter (in) = √(4 x Required Area ÷ π) x 12
This converts the required round cross-sectional area into an exact diameter in inches — before rounding to a stock size.
Step 3 — Nominal Size & Actual Velocity
Nominal Diameter = next standard size ≥ Raw Diameter, up to 24 in
Actual Velocity = Run CFM ÷ Area(Nominal Diameter)
The raw diameter always rounds UP to the next standard round duct size (4 through 24 in) — sizing down would run faster than the target velocity. The actual velocity at that standard size is shown for transparency, and is usually a little under the target since sizes snap up in whole-inch steps. A run whose raw diameter exceeds 24 in is flagged, not capped.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Run count | 1 | Each run is sized independently below |
| Suggested total system CFM | Not entered | Informational cross-check only, not applied to any run |
Step 1 — Required Area per Run
Required area = run CFM ÷ that duct category's target velocity.
| Run | Calculation | Raw Diameter |
|---|---|---|
| Run 1 (Supply Trunk) | 400 CFM ÷ 700 FPM | 10.24 in |
Step 2 — Nominal Size & Actual Velocity
| Run | Raw Diameter | Nominal Size | Actual Velocity |
|---|---|---|---|
| Run 1 | 10.24 in | 12 in | 509 FPM |
Therefore, this 1-run duct system sizes to the standard diameters shown above, each achieving a velocity at or under its category's recommended target. This is a simplified velocity-based sizing shortcut — get a full Manual D design from a licensed HVAC contractor before fabrication or installation.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Run Inputs & Design Basis+-
- Each run's CFM value pulled from an actual room-by-room ACCA Manual J load calculation, not guessed or evenly split across rooms
- Duct category selected correctly per run (Supply Trunk / Supply Branch / Return Trunk / Return Branch)
✓Before Fabrication / Design+-
- A full ACCA Manual D duct design obtained from a licensed HVAC contractor before fabrication or installation, not just this velocity shortcut
- Total effective length (straight duct length + fitting/elbow equivalent lengths) calculated for each run, not skipped
- Available static pressure budget confirmed against the air handler/furnace's rated external static pressure at the design CFM
✓Installation & Sealing+-
- All duct joints sealed with mastic or UL 181-rated tape, not cloth-backed "duct tape"
- Flexible duct runs pulled tight and supported, not left looped, kinked, or with excess slack
✓Airflow Verification+-
- Actual airflow at each register measured with a flow hood or anemometer after installation, not assumed from this calculator's target velocity
✓After Installation+-
- Final system static pressure tested against the equipment's rated external static pressure after installation is complete
Full QC Checklist+−
Verification checklist for a velocity-based duct sizing estimate — covering run input basis, pre-fabrication design decisions, installation and sealing, airflow verification, and post-installation checks. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Run Inputs & Design Basis+-
- Each run's CFM value pulled from an actual room-by-room ACCA Manual J load calculation, not guessed or evenly split across rooms
- Duct category selected correctly per run (Supply Trunk / Supply Branch / Return Trunk / Return Branch)
- Suggested Total System CFM (from tonnage x CFM/ton) compared against the sum of entered supply-trunk run CFM as a sanity check
- Airflow-per-ton selection (350 / 400 / 450 CFM/ton) matches your actual climate, not left at the default
✓Before Fabrication / Design+-
- A full ACCA Manual D duct design obtained from a licensed HVAC contractor before fabrication or installation, not just this velocity shortcut
- Total effective length (straight duct length + fitting/elbow equivalent lengths) calculated for each run, not skipped
- Available static pressure budget confirmed against the air handler/furnace's rated external static pressure at the design CFM
- Duct material and insulation R-value selected appropriately for its location (e.g. higher R-value for unconditioned attic runs vs. conditioned-space runs)
- Return air pathway confirmed (dedicated return ducts per room, or transfer grilles/jump ducts) rather than sizing supply runs alone
✓Installation & Sealing+-
- All duct joints sealed with mastic or UL 181-rated tape, not cloth-backed "duct tape"
- Duct support spacing meets manufacturer/code requirements to prevent sagging over the run's length
- Flexible duct runs pulled tight and supported, not left looped, kinked, or with excess slack
- Trunk-to-branch takeoffs installed with proper conical/manufactured fittings, not sharp 90-degree taps cut directly into the trunk
- Ducts routed through unconditioned spaces (attic, crawlspace, garage) insulated to at least the local code-minimum R-value
✓Airflow Verification+-
- Actual airflow at each register measured with a flow hood or anemometer after installation, not assumed from this calculator's target velocity
- Total system airflow verified against the equipment's rated CFM at the actual installed duct static pressure, not just its nameplate rating
- Balancing dampers installed on branches feeding rooms far from the trunk, and adjusted during commissioning
- Noise levels checked at any run sized close to or at its category's maximum velocity, especially return branches
✓After Installation+-
- Register/grille free-area size matches the actual duct branch feeding it, not left as whatever was pre-installed
- Final system static pressure tested against the equipment's rated external static pressure after installation is complete
- All accessible duct joints visually inspected during final walkthrough for missed mastic or tape coverage
Duct Sizing Reference Tables
Planning velocity limits by duct category, commonly cited from ACCA Manual D guidance, and standard stock duct sizes.
| Duct Category | Recommended Velocity | Maximum Velocity |
|---|---|---|
| Supply Trunk | 700 FPM | 900 FPM |
| Supply Branch | 600 FPM | 700 FPM |
| Return Trunk | 600 FPM | 700 FPM |
| Return Branch | 400 FPM | 700 FPM (allowed but not recommended, for noise control) |
Standard Round Duct Sizes
| Nominal Diameters (in) |
|---|
| 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24 |
When should you use this duct size calculator?
- Getting a ballpark duct diameter for a single supply or return run before requesting contractor quotes.
- Planning a full trunk-and-branch system and sanity-checking each run's diameter against its target velocity.
- Converting a known CFM value (from a load calculation) into a standard duct size to shop for material.
- Cross-checking whether a proposed duct size seems roughly reasonable before committing.
Quick Duct Sizing Tips
- Always get a full Manual D design from a licensed HVAC contractor before fabrication — this calculator is a planning estimate, not a final sizing number.
- Resist undersizing to save material cost or ceiling space — a duct running above its recommended velocity gets noticeably noisier.
- Seal every joint with mastic or UL 181 tape, not cloth-backed duct tape — leaky joints waste conditioned air regardless of how correctly the duct was sized.
- Avoid kinking or looping flexible duct — a kink chokes airflow well below what its nominal diameter suggests.
- Size return air pathways too, not just supply runs — a system starved for return air underperforms even with perfectly sized supply ducts.
Common Mistakes
- Sizing every run to the same flat diameter instead of each run's own CFM and duct category.
- Sizing to the maximum allowed velocity instead of the recommended (quieter) velocity by default.
- Rounding a raw diameter DOWN to the nearest stock size instead of up, which runs the duct faster and noisier than intended.
- Skipping total effective length and fitting friction loss, then being surprised the installed system underperforms this shortcut's estimate.
- Treating this velocity-based estimate as a final Manual D design and skipping a licensed contractor's review before fabrication.
Limitations
- This is a simplified velocity-based sizing shortcut, not an ACCA Manual D equal-friction duct design.
- Does not account for duct length, fitting/elbow friction loss, total effective length, or your system's available static pressure budget.
- Sizes round duct only — does not convert to rectangular/flat duct equivalents.
- Does not calculate the CFM each run should carry — that value must come from your own load calculation and system design.
- Does not check duct material, insulation R-value, or code-specific requirements — all site-specific and jurisdiction-specific.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.