Q = A × V to connect airflow, duct area, and velocity. Enter any two inputs, choose round or rectangular duct, and use the result as a preliminary size before checking friction rate, fitting losses, noise, balancing, and the full room load.Input Parameters
CALCULATION RESULT
Recommendation
Your selected calculation inputs.
Recommended duct size
--
Use this as a preliminary design input, then verify the full duct run.
SUPPORTING RESULTS
- Cross-sectional area
- -- m²
- Airflow
- -- m³/h
- Air velocity
- -- m/s
Formula, assumptions and boundary
Q = A × V. The calculator converts airflow to m³/s, then solves for duct area, velocity, or airflow according to the selected mode.
This is a preliminary screen. Check friction rate, fittings, noise criteria, material gauge, balancing, and local code before final duct selection.
Check the pressure budget next
A size that meets the target velocity still needs a pressure-loss check. Use the available static pressure, total effective length and friction-rate worksheet before selecting a final duct size.
How this duct sizing calculator works
This calculator uses the airflow continuity relationship between flow rate, duct area, and velocity. It can recommend a duct size from airflow, calculate velocity from dimensions and airflow, or calculate airflow from dimensions and velocity.
Converting air velocity to airflow (field measurement)
The most common field task is turning an anemometer reading into a flow rate: Q = A × V, where Q is airflow (m³/s), A is the duct cross-section area (m²), and V is the measured velocity (m/s). Multiply by 3,600 to get m³/h, or by 2,119 to get CFM. For example, a 128 mm duct (A = 0.0129 m²) with a 4.94 m/s reading gives 0.064 m³/s = 228.8 m³/h ≈ 134.7 CFM.
Use the Flow from velocity mode above: enter the duct diameter or width/height plus the measured velocity, and it returns airflow in the selected units. Switching units converts the inputs and displayed result.
From Load to Duct Size
Start with the room or whole-home cooling load, convert that load to design airflow, then use this page to select a preliminary duct area at the calculator’s fixed preliminary screening velocity of 7 m/s (about 1,378 FPM). For a transparent whole-home estimate that includes climate, ceiling height, insulation, windows, and occupants, use the Manual J Calculator (Simplified) first. Final duct layouts still need friction-rate, fitting-loss, balancing, and equipment data checks.
Inputs explained
- Calculation mode determines which value is solved: size, velocity, or airflow.
- Duct type selects rectangular or circular area formulas.
- Choose US (CFM, FPM, inches) or metric (m³/h, m/s, mm) units. Calculations use SI internally.
- Rectangular width and height or circular diameter define the cross-sectional area.
- Size mode uses a fixed preliminary screening velocity of 7 m/s (about 1,378 FPM); the same screening velocity applies in commercial mode.
FAQ
Why does the calculator warn about high velocity?
High air velocity can increase pressure drop and noise, so the result includes a design suggestion.
Can I use the recommended size directly?
Use it as a starting point. Final duct design should also check pressure drop, noise criteria, fittings, branches, and balancing.
Why are rectangular sizes rounded?
Recommended dimensions are rounded to 10 mm increments internally, then converted to inches for US display. These are preliminary dimensions, not nominal US stock sizes.
Is this a free online duct calculator?
Yes. It runs in your browser at no cost — enter any two of airflow, duct dimensions, or velocity and it solves the third for rectangular and circular ducts, no sign-up required.
What is a ductulator?
A ductulator is the traditional handheld slide rule used to size ducts by friction rate and velocity. This calculator solves continuity and velocity only. It does not calculate friction rate, fitting losses, or a complete Manual D duct design.
Is this a free duct size online calculator?
Yes. This is a free duct size online calculator for rectangular and round ducts: it converts airflow into duct area and dimensions at a fixed screening velocity of 7 m/s (Q ÷ V = A). Use the result as a starting point; final design should also check friction rate and fitting losses.
How do I convert air velocity to airflow (m³/h or CFM)?
Use Q = A × V. Measure the duct diameter (or width/height), compute the cross-section area A, and multiply by the measured air velocity V from an anemometer. Multiply the result by 3,600 for m³/h or by 2,119 for CFM. Select Flow from velocity mode above and it does this automatically.
Common Duct Sizing Mistakes — What Engineers Get Wrong
Using velocity alone — ignoring friction rate
Velocity alone does not determine the pressure a fan must overcome. Check the available static pressure, straight-duct friction and fitting losses for each run. Equal-friction sizing still requires attention to run lengths and balancing; it does not guarantee equal airflow at every outlet.
Neglecting fitting losses — the "equivalent length" trap
Elbows, tees and transitions add resistance that a straight-duct calculation omits. Use loss coefficients or equivalent lengths for the actual fitting geometry and airflow. A fixed percentage added to every run cannot represent all fitting arrangements.
cfm vs. m³/h unit confusion
US airflow is in CFM (cubic feet per minute); metric is in m³/h or L/s. 1 CFM = 1.699 m³/h = 0.472 L/s. Entering a value in CFM into a metric calculator — or vice versa — changes calculated area by a factor of about 1.699 at the same velocity (round diameter by about 1.303). Always confirm units before entering: a 400 CFM branch is 680 m³/h, not 400 m³/h.
Skipping aspect ratio limits on rectangular ducts
Long, narrow ducts have more perimeter than compact ducts of the same cross-sectional area. For example, 48"×6" and 24"×12" ducts each have 288 in² of area, but their perimeters are 108 and 72 inches: the narrower duct needs 50% more wall area per unit length. Check friction, space and construction requirements before selecting the shape.