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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.