How to use this calculator
- Enter the rectangular opening. Use the finished width and height of the square or rectangular end.
- Enter the round end and height. Set the round diameter and the straight transition height between the opening planes.
- Add offset and stations. If the round outlet is not centered, enter X and Y offset. Choose a station count for hand layout detail.
- Check true lengths and blank area. Use the max true length, station pitches, developed area and seam allowance to plan the pattern before shop verification.
How it works
The rectangular end and round end are both divided into N
stations. Rectangular station pitch is:
p_rect = 2(W + H) / N
and round station pitch is:
p_round = pi D / N
For each station, the plan offset is measured between the rectangular
perimeter point and matching round point. True length is:
L_i = sqrt(h^2 + e_i^2)
where h is transition height and e_i is station
plan offset. The developed-area estimate sums the station strip using the
average rectangular and round station pitch, then adds
seam allowance x average true length.
Use this beside the duct transition calculator, cone flat pattern calculator, box flat pattern calculator, K-factor calculator and powder coating coverage calculator.
Worked example
Verified against the live calculator
For an 18 in x 18 in square base, 14 in round
outlet, 12 in transition height, 24 stations
and 1/2 in seam allowance, the rectangle perimeter is
72 in and the round circumference is about
44.0 in. The calculator reports the true-length range,
max skew angle, station spacing and a seam-aware blank area for layout
planning.
Frequently asked questions
How do you lay out a square to round transition?
Divide the rectangular perimeter and the round circumference into matching stations. For each station, measure the plan offset between the rectangular point and the round point, then use true length: sqrt(height squared plus plan offset squared).
Does this produce a production-ready flat pattern?
No. It is a triangulation worksheet and material takeoff screen. Final production patterns still need shop seam details, thickness allowance, hems, locks, relief notches, forming stretch and CAD or shop verification.
Can this handle an offset round outlet?
Yes. Enter X and Y offsets for the round center. The calculator updates true-length spread, max skew angle and the preview so the offset side is visible.
How many layout stations should I use?
Use enough stations to keep the hand layout smooth. Sixteen is a coarse screen, 24 to 48 is a practical hand-layout range, and very high station counts are usually better done in CAD or CAM.
Method & assumptions
- Station zero starts at the selected side midpoint and follows the rectangular perimeter and round circumference in the same direction.
- Round offset is measured from the rectangular centerline in the same plane view.
- True lengths are straight station lines between opening planes.
- Developed area is a station-strip approximation for layout and takeoff, not a certified CAD flat pattern.
- Material thickness is used only for shell mass. Thickness allowance, K-factor, bend/stretch allowance and forming method are not applied.
- Does not include seams, locks, hems, relief notches, gore layout preferences, fitting loss coefficients, leakage, reinforcement, duct pressure class, code approval or shop-specific fabrication rules.