How to use this calculator
- Enter airflow. Use the design airflow through that duct section, not the whole air-handler flow unless the transition is on the trunk.
- Enter inlet and outlet openings. Set the rectangular inlet and choose either rectangular or round outlet dimensions.
- Set length and taper limit. Enter the available transition length and the maximum side taper angle you want to screen against.
- Check velocity and blank takeoff. Compare inlet and outlet velocity, the length margin, controlling taper angle, velocity pressure change and sheet-metal blank estimate.
How it works
A duct transition changes opening area, so the first airflow check is
simply:
v = Q / A
where Q is the design airflow and A is the inlet
or outlet free area. Rectangular openings use A = W x H, and a
round outlet uses A = pi x D^2 / 4.
The taper-length screen compares the half-change in width and height with the entered maximum taper angle: Lrec = max(|W1 - W2| / 2, |H1 - H2| / 2) / tan(theta max) The actual controlling taper angle is the larger of: thetaW = atan(|W1 - W2| / (2L)) thetaH = atan(|H1 - H2| / (2L))
The blank estimate treats the top/bottom and side faces as four trapezoids,
then adds one longitudinal seam allowance:
Ablank = Ashell + s x L
Shell mass is Ablank x thickness x density. The pressure output
is velocity-pressure change only:
Delta Pv = 0.5 x rho x (v2^2 - v1^2)
Pair this page with the duct size calculator, duct friction loss calculator, HVAC static pressure calculator, grille size calculator, sheet-metal box flat pattern calculator and sheet metal gauge chart.
Worked example
Verified against the live calculator
For 0.35 m3/s through a 400 x 250 mm inlet into a
300 x 200 mm outlet, the inlet velocity is
3.5 m/s and the outlet velocity is about 5.83 m/s.
With a 15 deg taper limit, the recommended length is about
187 mm; a 500 mm transition gives a controlling
taper angle near 5.7 deg.
Frequently asked questions
How long should a duct transition be?
A practical first-pass screen is to limit the taper angle. This calculator uses L >= max(|W1 - W2| / 2, |H1 - H2| / 2) / tan(theta max), where theta max is the entered side taper angle.
Does this calculate duct transition pressure loss?
It calculates inlet and outlet velocity pressure and the change between them, but not a full fitting-loss coefficient. Use the duct friction loss calculator for straight-run and fitting-equivalent duct loss, then add transition loss, elbows, dampers, coils, filters and terminals in the HVAC static pressure calculator or a full duct-design method.
Can this handle rectangular to round transitions?
Yes for airflow, velocity, taper length and a rough sheet-metal takeoff. For the blank estimate, a round outlet is screened by comparing the outlet diameter with the inlet width and height. A production square-to-round pattern still needs shop triangulation or CAD.
What taper angle should I use?
A lower taper angle makes a longer, smoother transition; a steeper transition saves space but can add noise and fitting loss. Use the project design standard, shop practice or engineer direction for the final limit.
Method & assumptions
- Inlet is rectangular. Outlet may be rectangular or round.
- Transition is centered; offset transitions, eccentric reducers and field obstructions are not modeled.
- Round outlet blank area is a rough face-area takeoff, not a triangulated square-to-round flat pattern.
- Pressure output is velocity-pressure change only, not a fitting loss coefficient.
- Final duct design should use the applicable ASHRAE, ACCA, SMACNA, manufacturer or engineer-approved method, including fittings, leakage, acoustics, balancing, fire/smoke dampers, access and local code requirements.