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.
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 both m³/h and CFM without any unit juggling.
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 your target velocity. 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.
- Airflow is entered in m³/h and converted internally to m³/s.
- Rectangular width and height or circular diameter define the cross-sectional area.
- The selected standard changes the default sizing velocity and velocity assessment thresholds.
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 practical 10 mm increments for preliminary sizing.
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 does the same continuity and velocity math digitally and instantly, so you can use it as a digital ductulator without lookup tables.
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 rate and design velocity into duct area and dimensions (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
The equal-friction method (target 0.08–0.10 in. w.g./100 ft, or 0.8–1.0 Pa/m) produces balanced systems. Sizing purely to keep velocity under 1,000 FPM (5 m/s) can yield oversized ducts on short runs and undersized ones on long runs, causing unbalanced airflow across zones. Always check both velocity and friction rate.
Neglecting fitting losses — the "equivalent length" trap
Fittings (elbows, tees, transitions) can account for 50–70% of total system pressure loss in residential ductwork. A common shortcut is to add 50% to straight-run length as a fitting allowance — but ACCA Manual D requires using fitting equivalent lengths from ASHRAE tables. A single unvaned 90° rectangular elbow can equal 30–80 ft of straight duct.
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 — produces a duct roughly 4× too large or too small. Always confirm units before entering: a 400 CFM branch is 680 m³/h, not 400 m³/h.
Skipping aspect ratio limits on rectangular ducts
ASHRAE recommends rectangular duct aspect ratios ≤4:1 (width:height). Higher aspect ratios increase surface area per unit of flow, raising both friction losses and heat gain. A 48"×6" duct has more than twice the surface area of a 24"×12" duct carrying the same CFM. SMACNA caps aspect ratio at 8:1 as an absolute limit; above 4:1 a round or oval duct is almost always more efficient.