Duct CFM Calculator: Airflow & Sizing

This duct CFM calculator works out the airflow volume (in cubic feet per minute) for a round or rectangular duct, based on the duct’s cross-sectional area and the air velocity you enter. You type in the duct dimensions and the air speed, and it returns the CFM (cubic feet per minute) plus the equivalent velocity in feet per minute and meters per second. The result tells you how much air a given duct can carry at that speed, which is the first step in sizing ducts for HVAC systems, range hoods, or ventilation fans.

Formula

The calculator uses the standard airflow equation: CFM = Area (ft²) × Velocity (ft/min). For a round duct, the area is π × (diameter/2)², where the diameter is first converted from inches to feet. For a rectangular duct, the area is simply width × height, also in feet. The velocity is the speed of the air moving through the duct, measured in feet per minute (fpm).

Variables:

• d – round duct diameter, in inches (converted to feet by dividing by 12)
• w – rectangular duct width, in inches (converted to feet)
• h – rectangular duct height, in inches (converted to feet)
• v – air velocity, in feet per minute (fpm)
• CFM – airflow, in cubic feet per minute

If you measure velocity in meters per second (m/s), multiply by 196.85 to get fpm (since 1 m/s = 196.85 ft/min). The calculator includes a conversion output for that.

Worked Example

Using the default values: a round duct with an 8-inch diameter and air velocity of 800 fpm.

1. Convert diameter to feet: 8 in ÷ 12 = 0.6667 ft
2. Radius = 0.6667 / 2 = 0.3333 ft
3. Area = π × (0.3333)² = 3.14159 × 0.1111 = 0.3491 ft²
4. CFM = 0.3491 × 800 = 279.3 CFM

So an 8-inch round duct at 800 fpm carries about 279 CFM. The calculator rounds to one decimal place, giving 279.3. For the rectangular example (10 in × 6 in at 800 fpm): area = (10/12) × (6/12) = 0.8333 × 0.5 = 0.4167 ft², so CFM = 0.4167 × 800 = 333.3 CFM.

Why Duct CFM Matters

CFM is the volume flow rate of air. Every HVAC system, exhaust fan, or fresh-air ventilator is rated in CFM, and the ductwork must be sized to deliver that flow without excessive noise or pressure drop. If the duct is too small for the required CFM, air velocity rises, causing whistling, vibration, and higher energy use. If the duct is too large, it costs more and may not maintain proper air mixing. This calculator helps you check whether a proposed duct size can handle a target airflow at a chosen velocity.

Typical velocity guidelines (rules of thumb, not code):

• Main supply ducts: 700–1000 fpm
• Branch supply ducts: 500–700 fpm
• Return ducts: 400–700 fpm
• Range hood exhaust: 1000–1800 fpm

These are general ranges; always consult local codes and a qualified HVAC designer for real installations.

How to Choose a Duct Size

Start with the required CFM (from appliance ratings or room size calculations). Then decide on a target velocity, usually based on noise tolerance and duct type. Use the calculator in reverse: try different diameters or rectangular dimensions until the output CFM matches your target. For example, if you need 400 CFM and want 800 fpm, the required area is 400/800 = 0.5 ft². For a round duct, that means a diameter of about 9.6 inches (since area = π(d/2)²). You would choose a 10-inch round duct.

For rectangular ducts, many combinations work: 12×6 inches gives 0.5 ft², as does 10×7.2 inches. In practice, choose standard sizes (e.g., 12×6, 14×6, 10×8) and check the actual CFM at your velocity. Remember that friction losses increase with velocity, so lower velocity is often better for long runs.

Common Mistakes and Misconceptions

One frequent error is mixing units: using inches for area but feet for velocity. Always convert dimensions to feet before multiplying. Another mistake is assuming that a larger duct always means more CFM – actually, the fan or system pressure determines the flow; the duct size only limits it. If the fan is weak, a big duct won’t magically produce more CFM.

A common misconception is that duct CFM is the same as the fan’s rated CFM. The fan rating is usually at zero static pressure; in a real system, duct friction reduces the actual flow. This calculator gives the theoretical maximum for a given velocity, not the actual system performance. To predict real flow, you need a duct friction chart or a pressure-loss calculator.

Also, be careful with round vs. rectangular equivalence: a 10-inch round duct has an area of about 78.5 in², while a 10×8 rectangular duct has 80 in² – similar but not identical. Always compute the actual area, not just compare one dimension.

Limitations of This Calculation

This calculator assumes uniform velocity across the duct cross-section, which is never true in practice. Air moves faster in the center and slower near the walls. The error is usually small for straight, smooth ducts, but bends, dampers, and transitions cause local velocity variations. The formula also ignores air density, which changes with altitude and temperature – for most HVAC applications at sea level and moderate temperatures, the error is under 5%.

For long ducts (more than 25–30 feet), friction losses become significant, and the velocity at the end will be lower than at the start. This calculator gives the flow at a single point, not the total system performance. For accurate system design, use a ductulator or friction-loss chart that accounts for duct length, roughness, and number of fittings.

When to Use a Professional

If you are designing a whole-house HVAC system, a commercial exhaust system, or any ductwork that must meet building codes, hire a licensed HVAC engineer or contractor. They use specialized software that models pressure losses, fan curves, and duct networks. This calculator is for quick estimates, educational purposes, and preliminary sizing. Never rely solely on this number for safety-critical ventilation, such as combustion exhaust or fume hoods – those require professional engineering and adherence to local codes.

Frequently Asked Questions

What is a good CFM for a bathroom exhaust fan? A common rule of thumb is 1 CFM per square foot of floor area, with a minimum of 50 CFM for a standard bathroom. For example, a 40 ft² bathroom needs at least 40–50 CFM. If the bathroom has a jetted tub or high ceiling, go higher. This calculator helps you size the duct for that fan, not the fan itself.

How many CFM do I need for a range hood? For electric stoves, a rule of thumb is 100 CFM per linear foot of cooktop. For gas stoves, 100 CFM per 10,000 BTU of burner output. So a 30-inch electric range needs about 250 CFM, while a 60,000 BTU gas range needs about 600 CFM. The duct must be sized to carry that flow at a reasonable velocity (1000–1800 fpm for exhaust).

Does duct length affect CFM? Yes, significantly. Every foot of duct adds friction, reducing the actual airflow. A 10-foot duct might deliver 90% of the fan’s rated CFM, while a 50-foot duct might deliver only 60%. This calculator gives the flow at a given velocity, but you must account for length separately using a friction chart or pressure-drop calculator.

What is the difference between CFM and FPM? CFM (cubic feet per minute) is a volume flow rate – how much air moves per minute. FPM (feet per minute) is a velocity – how fast the air moves. They are related by the duct area: CFM = area (ft²) × FPM. A large duct can move the same CFM as a small duct, but at a lower FPM.

Can I use this calculator for flexible duct? Yes, but with caution. Flexible duct has more friction than smooth sheet metal, so the actual CFM will be lower than calculated. As a rule of thumb, reduce the expected flow by 10–20% for flexible duct, or use a larger duct size to compensate. Always check the manufacturer’s friction data for accurate results.

How do I convert CFM to m³/h? Multiply CFM by 1.699 to get cubic meters per hour. For example, 400 CFM × 1.699 = 679.6 m³/h. Conversely, divide m³/h by 1.699 to get CFM. This is useful when comparing equipment from different regions.

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in d | Duct diameter (round) | in | 8 | 0 | 48
in w | Rectangular width | in | 10 | 0 | 120
in h | Rectangular height | in | 6 | 0 | 120
in v | Air velocity | ft/min | 800 | 0 | 5000
out cfm | Airflow | CFM | (3.14159 * (d/12)^2 / 4) * v | 1
out cfm_r | Rectangular airflow | CFM | (w/12 * h/12) * v | 1
out vel_mps | Velocity | m/s | v * 0.3048 / 60 | 2