Duct Size & CFM Calculator
Round and rectangular sizing, fitting losses and endpoint static pressure.
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- Free preview: the rules, dates and checks that apply to your figures, with every computed amount hidden.
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Working
Assumptions and sources
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Duct Size & CFM Calculator
Work from actual airflow in CFM, litres per second or cubic metres per hour, or calculate airflow from room volume and an entered air-change rate. Size a round duct and a rectangular alternative by maximum velocity or equal friction; check an existing round or rectangular section and see how the two geometries differ. A series-run schedule includes fittings, entered equipment losses and the static-pressure change between its endpoints. The free preview masks computed figures and keeps exports locked; a Premium pass opens the calculation and files.
How to use it
- Choose sizing, a section check or a series-run schedule, then enter actual airflow or room volume and your design air-change rate.
- Enter density, dynamic viscosity and duct roughness for the operating condition; the starting values are examples.
- For sizing, choose maximum velocity or friction rate and a rectangular aspect ratio; for a check, enter inside dimensions and length.
- For a series schedule, enter name, length, round diameter and total fitting K for every run on one CSV line, plus equipment loss outside those K values.
- Inspect the free preview and warnings, then unlock with a Premium pass to copy the working or download CSV, PDF and an available chart.
Examples
1000 m3/h, maximum velocity 8 m/s, rectangular width/height 2
Round inside diameter about 210.26 mm; rectangular dimensions about 263.52 by 131.76 mm. Each shape has its own friction loss.
Equations and assumptions
Q = room volume × ACH / 3600 in m3/s. Round D = √(4Q/(πV)); rectangular area = Q/V. Darcy loss per length is fρV²/(2D). The Altshul–Tsal estimate uses f0 = 0.11(ε/D + 68/Re)^0.25, with f = 0.85f0 + 0.0028 when f0 < 0.018; a laminar round duct uses 64/Re. Huebscher equivalent diameter is 1.3(ab)^0.625/(a+b)^0.25. A fitting loses KρVactual²/2. Static-pressure drop equals total-pressure loss plus outlet velocity pressure minus inlet velocity pressure at equal endpoint elevations.
Sources
Limitations
- Estimate — not a professional design. Verify noise, leakage, fire requirements, installation and the fan operating point.
- Series rows share one airflow; parallel branches and a complete fan critical path require separate engineering analysis. Rectangular equivalence is approximate and is not an exact laminar model.
- ACH is an entered design condition, not a universal ASHRAE requirement. Residential whole-building ventilation and local exhaust depend on the applicable standard and building details. No compliance verdict is produced.
- Equal-velocity round and rectangular alternatives need not have equal friction; the tool reports the rectangular loss separately.
- PDF supports Latin, Greek, Cyrillic and the supplied Indian scripts; unsupported characters trigger a clear error. CSV and copied text preserve the original labels.
Privacy
On this device; its files download once
All entered dimensions, temperatures, labels and schedules stay on your device. PDF fonts and calculation code can download from MySmartCoPilot; inputs are never sent with those downloads.
Frequently asked questions
What do I get without a pass?
Without a pass, Duct Size & CFM Calculator shows the rules, dates and checks that apply to your figures, with every computed amount hidden. Until you unlock it, the result can’t be downloaded or copied. A Premium or Ultimate pass, a one-time payment that never renews, unlocks the full result. The pricing page lists the passes and their prices.
Does CFM mean standard or actual airflow here?
Use actual volumetric airflow at the density you enter. A flow quoted at a standard reference condition must be converted to the operating condition before sizing.
Is total pressure loss the same as static pressure drop?
They are equal when endpoint velocity pressures are equal. The schedule includes their difference; fan external static pressure also depends on its discharge and connections.
Can six air changes per hour prove residential compliance?
No. It is an editable example. Apply the relevant whole-building ventilation, local exhaust and intermittent-operation rules for the building.