Measurement & project guide
Separate airflow geometry, pressure budget and equipment loads
Size or check round, rectangular and oval ducts against a stated airflow criterion. Use the equipment-load worksheet separately: a room's cooling load and a duct's airflow capacity are different questions.
- A Airflow Q
- B Cross-section A
- C Velocity Q / A
Before you calculate
- Airflow
- Use design airflow for the actual branch or system. Do not infer it solely from floor area or equipment nominal tonnage.
- Duct dimensions
- Use internal clear dimensions and the real shape. Insulation and lining can reduce the air passage.
- Pressure budget
- Use compatible equipment pressure data, component losses and total effective length for the design path. Avoid subtracting a loss already included in the rating.
Read the result before using it
- Continuous size
- The mathematical dimension meeting a criterion. It still needs to be checked against available fabrication sizes.
- Candidate size
- A size from the entered stock list that meets the selected constraint. It is not a guarantee of noise, balance or system performance.
- Friction rate
- A pressure-loss allowance per length, not a universal constant for every home or flexible duct installation.
Check before ordering or handing over
- Confirm room loads, design airflow and manufacturer blower data before selecting ducts.
- Include fittings, filters, coils and terminals on the same design path and pressure basis.
- Verify installed airflow, available static pressure and balancing after commissioning.
Specialist worksheet
Equipment load planning: cooling and heating checks
Use a measured design load when available. Area times an explicit load rate is only an early screening scenario; it cannot replace a room-by-room load calculation or equipment selection.
The nominal size is a comparison label, not a purchase recommendation. Verify minimum modulation, latent performance and low-temperature capacity using manufacturer data and the actual design conditions.
Detailed results and professional tools
View detailed results, reports and comparisons
Calculation report
A focused summary of the active inputs, results and assumptions. Changes are saved automatically in this browser; no account or cloud sync.
Duct cross-section & pressure budget
Selected shape and reference round option use the same drawing scale.
Bars compare magnitudes. Remaining pressure is labeled separately; they are not a signed waterfall.
Compare your options
Keep up to three scenarios while you explore. Each card retains its own inputs; export a calculation for a durable backup.
Worked example: 400 CFM at a 700 ft/min limit
Area = 400 ÷ 700 = 0.5714 ft². The theoretical round diameter is 10.24 in. With the illustrative candidate list, select 11 in; its actual velocity is 606.1 ft/min. This is a velocity selection, not a noise or code certification.
For the separate pressure example, 0.50 − (0.15 + 0.10 + 0.05) = 0.20 in. w.g. available. Across 150 ft effective length, the available design rate is 0.133 in. w.g./100 ft. The defaults are transparent examples; use your equipment data.
Inputs explained: what changes the answer?
- Airflow and velocity: more flow at the same velocity needs more cross-sectional area. Supply the airflow required by your system design; the calculator does not determine room loads.
- Friction and shape: perimeter matters as well as area. A long, narrow rectangle is not pressure-equivalent to a round duct simply because the areas match.
- Space and fabrication limits: these can prevent a feasible size even when a theoretical solution exists. Dimensions here are clear internal dimensions, not insulated outside dimensions.
- Air properties: temperature and altitude affect estimated density and pressure loss. Actual-volume airflow stays actual volume; no SCFM conversion is implied.
- Pressure budget: more external component loss leaves less pressure for the duct. A negative margin is a reason to review the assumptions/design, not increase a hidden multiplier.
Candidate sizes & reference calculations
The first table updates from the same engine using your current velocity/friction limits and rigid-duct assumptions. These are candidate diameters, not recommended airflow requirements.
| Airflow (CFM) | Theoretical diameter | Selected candidate | Actual velocity |
|---|---|---|---|
| 200 | 7.24 in | 8 in | 573.0 ft/min |
| 400 | 10.24 in | 11 in | 606.1 ft/min |
| 800 | 14.48 in | 16 in | 573.0 ft/min |
Changing units preserves the list. Changing the stock list is a separate choice. No automatic pressure-capacity table is extrapolated for flex duct.
| Setting | Initial value / basis | Applicability / override |
|---|---|---|
| Galvanized roughness | 0.09 mm; ASHRAE 2021 Table 1 friction-chart basis | Rigid duct approximation; editable 0–5 mm, with relative-roughness checks |
| Drawn tubing roughness | 0.00046 mm; ASHRAE 2021 Table 1 | Drawn tubing only; editable |
| Air properties | NASA approximate atmosphere and Sutherland viscosity | −40 to 80 °C; altitude −500 to 5,000 m; density override 0.2–2 kg/m³ |
| Velocity, pressures, dimensions | Illustrative scenario inputs | Not manufacturer recommendations; replace with project data |
| Stock list / aspect / increment | Illustrative purchasing/design constraints | Edit to actual supplier dimensions and project requirements |
Formula & methodology
Round: A = πd² / 4
Rectangle: A = width × height
Flat oval: A = (major − minor) × minor + π × minor² / 4
Hydraulic diameter: Dₕ = 4A / wetted perimeter
Re = ρVDₕ / μ
Pressure loss per length: R = fρV² / (2Dₕ)
Available pressure = blower ESP − external component losses
Design friction rate = available pressure / effective length
Maximum effective length = available pressure / chosen friction rate
The rigid-duct pressure model uses Darcy–Weisbach with a Colebrook friction-factor solution. Round laminar flow uses 64/Re; transition is interpolated and flagged. Nonround low-Reynolds-number estimates need independent verification. Equal-friction conversion numerically matches pressure loss per length at the same flow within this model; it is not a reproduction of a proprietary ductulator table.
Continuous dimensions are solved first, then rounded up to a candidate size or fabrication increment and checked again. Equal-area conversion does not automatically meet a separate velocity/friction target; the result reports those checks. Calculations use SI internally and retain precision until display.
Common mistakes to avoid
- Using outside dimensions where the calculation needs clear internal dimensions.
- Treating equal area, hydraulic diameter and equal-friction diameter as interchangeable.
- Entering a fixed friction target without checking the actual available pressure.
- Subtracting a component already included in the blower rating, or counting a fitting twice.
- Applying a single flex-duct percentage to different products, compression and installation conditions.
- Using one uniform-run estimate as a complete supply/return network design.
Assumptions, limitations & sources
This is a browser-based sizing and checking aid for ordinary low-speed air ducts. It does not certify installation safety, Manual D compliance, acoustics, leakage, fan selection, thermal performance or a balanced network. Manufacturer data and the complete design take precedence.
- ASHRAE 2021 Fundamentals, Chapter 21 — duct friction, hydraulic diameter and roughness basis.
- ACCA: calculating friction rate — available pressure and effective length.
- ACCA Manual D scope — full-system design and flex installation considerations.
- NASA atmosphere model and NASA Sutherland constants — approximate air properties.
- NIST SP 811 — unit conventions.
Method/data reviewed September 2, 2026. Calculation engine hvac-1.0.0. Referenced organizations do not endorse Efficienco.
Cross-section and sheet-metal surface area
Cross-sectional area is the air passage perpendicular to flow. For a 12 × 8 in rectangle it is 96 in², or 0.667 ft². A 12 in round passage is π × 12² ÷ 4 = 113.097 in², or 0.785 ft². At 800 CFM and a chosen 900 ft/min, the preliminary area is 800 ÷ 900 = 0.889 ft²; this is a candidate geometry, not a completed duct design.
Sheet-metal or insulation surface area is different. A straight rectangular section uses 2 × (width + height) × length. A 24 × 12 in duct, 20 ft long, has 2 × (2 + 1) × 20 = 120 ft² of long-face surface. A straight round section uses π × diameter × length. These formulas exclude end caps, fittings, seams, connectors and fabrication allowance. Fittings and transitions require their own developed-area takeoff.
Cooling load, nominal size and cold-weather heating
The equipment worksheet accepts an explicit cooling design load; that value overrides its area-based screening calculation. A 300 ft² zone at an illustrative 20 BTU/h per ft² yields 6,000 BTU/h. The same area at 25 yields 7,500 BTU/h. Those rates are scenarios, not climate-specific load calculations.
For a supplied 11,500 BTU/h cooling design load, 12,000 BTU/h is the next nominal label in the worksheet's comparison list. It is not automatically the right machine. Check actual sensible and latent capacity, minimum modulation and manufacturer performance at the project's conditions. If the design heating load is 14,000 BTU/h and the unit delivers 12,000 BTU/h at that outdoor temperature, the worksheet shows a 2,000 BTU/h heating shortfall.
ACCA Manual J addresses residential loads; Manual S addresses equipment selection from those loads and manufacturer data; Manual D addresses the duct system. Floor area alone does not complete any of those procedures.
Frequently asked questions
Does equal area mean equal friction?
No. Equal area gives the same average velocity at the same airflow, but wetted perimeter and hydraulic diameter can differ. Choose modeled equal friction when pressure loss per length is the criterion.
Is the suggested size guaranteed to be stocked?
No. Selection uses the editable candidate diameter list or fabrication increment. Replace these examples with your supplier’s internal dimensions and verify insulation, reinforcement and installation clearances.
Can this design a complete HVAC system?
No. It analyzes a uniform run or a simplified equivalent-length path. It does not solve a branched network, choose equipment, calculate room loads, predict sound levels or certify Manual D compliance.
Why is there no automatic flex-duct penalty?
Compression, sag, bends and product construction affect flex losses. Enter a manufacturer or measured friction rate for the exact size, airflow and installation. Friction-based flex sizing and conversion require a product curve and are not inferred here.
Which pressure drops should I enter?
Use blower data at the intended airflow and subtract only components external to that rating boundary. Do not subtract a coil or filter twice if it is already included in the stated available pressure.
Does changing units change my calculation?
No. Unit controls convert entered measurements, including candidate stock dimensions. Selecting a different candidate list or fabrication increment is a separate physical change.
Can the equipment worksheet size a mini-split from floor area?
It can show an explicitly labeled screening scenario or compare a supplied design load with nominal catalog sizes. It does not perform Manual J or select equipment. Verify sensible and latent performance, minimum modulation and low-temperature heating capacity with the design and manufacturer data.