Input Parameters
About This Calculator
- Size mode: Given airflow, recommends duct dimensions (rectangular ratio ~2:1, or circular diameter)
- Velocity mode: Given airflow and duct dimensions, calculates actual velocity
- Flow mode: Given duct dimensions and design velocity, calculates total airflow
- Typical recommended velocity: 3–6 m/s for supply ducts, 2–4 m/s for return ducts
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.
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.
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.