Inputs
Results
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Required CFM
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Eq. duct length (ft)
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System static (in w.g.)
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Estimated delivered CFM
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Meets requirement
Fan Curve & System Point
The line represents your fan output between 0.10 and 0.25 in w.g. System dot indicates estimated static pressure from your duct layout.
Example Scenarios
| # | Room L×W×H (ft) | Method | Duct Ø (in) | Len (ft) | 90° | 45° | Wall cap | Flex | Fan 0.10 / 0.25 | Load |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 5×5×8 | Area | 4 | 10 | 1 | 0 | 30 | 1.25 | 80 / 60 | |
| 2 | 8×10×8 | ACH=8 | 6 | 20 | 2 | 0 | 30 | 1.25 | 110 / 90 | |
| 3 | 10×12×9 | Fixtures S+J | 6 | 35 | 3 | 1 | 40 | 1.4 | 150 / 120 | |
| 4 | 7×9×8 | Area | 8 | 45 | 2 | 2 | 30 | 1.15 | 200 / 160 |
How to Use
- Enter room dimensions. This calculates area and volume automatically.
- Choose a sizing method: Area, ACH, or Fixture adders. The calculator shows the highest requirement, respecting a 50 CFM minimum for intermittent ventilation.
- Describe the duct path: diameter, straight length, number of elbows and bends, wall cap style, and flex factor to model roughness.
- Enter your fan’s rated airflow at 0.10 and 0.25 inches of water. Most fan labels or spec sheets list both values.
- Click Calculate. Review the required CFM, equivalent duct length, system static pressure, and estimated delivered CFM. The status badge indicates pass or shortfall.
- Use the chart to visualize the fan curve and your system operating point. If short, increase duct size, shorten runs, reduce bends, or select a higher‑pressure fan.
- Export results to CSV or PDF for record keeping or project documentation.
Formulas Used
Required CFM
- Area method: CFM = max(50, Area(ft²) × 1)
- ACH method: CFM = Volume(ft³) × ACH ÷ 60
- Fixture adders: Base 50 CFM + 50 CFM for each: shower or bath, jetted tub, enclosed toilet
Equivalent Length
EqLen = Straight + 15×(90° elbows) + 9×(45° bends) + CapEq
Then EqLen × FlexFactor (1.00–1.40). Wall cap typical equivalents: 30–40 ft.
Duct Friction and Static Pressure
Using Darcy–Weisbach:
- Convert Q from CFM to m³/s, diameter (in) to meters. Area A = πD²/4, velocity V = Q/A.
- Reynolds: Re = ρVD/μ with ρ≈1.2 kg/m³ and μ≈1.8×10⁻⁵ Pa·s.
- Friction factor f = 64/Re (laminar) else f = 0.3164×Re⁻⁰.²⁵ (smooth turbulent).
- ΔP (Pa) = f × (L/D) × ½ρV². Convert to inches of water: in w.g. = Pa ÷ 249.0889.
Delivered CFM is estimated by linearly interpolating your fan’s rated points at 0.10 and 0.25 in w.g., then reading that at the computed system static.
This tool provides engineering estimates. Always consult local codes and manufacturer instructions. For complex layouts, consider HVI rated fans and dedicated performance curves.
FAQs
Many guidelines require at least 50 CFM intermittent exhaust for bathrooms with a toilet, bath, or shower. Larger rooms or continuous ventilation targets may differ. Check your local code.
Area is simple for small rooms. ACH offers better control for large volumes or moisture heavy uses. This calculator supports both and highlights the higher requirement.
Smaller diameters dramatically increase friction and static pressure, reducing delivered airflow. Upsizing to six inch ducting is often the easiest way to recover CFM.
Properly stretched flex can work, but kinks and tight bends add resistance. We model this with a flex factor; changing it shows how roughness reduces airflow.
Most bathroom fans publish airflow at 0.10 and 0.25 inches of water. Interpolating between them gives a reasonable estimate of delivered CFM at your system static pressure.
Consider a higher capacity or higher pressure rated fan, shorter duct runs, fewer sharp bends, smoother transitions, or a larger duct diameter. The chart helps visualize impact.