Practical tools for measured work
Duct Seam & Lock Allowance Calculator
Estimate rectangular or round duct blank girth with a shop-specified seam allowance, in millimetres or inches, plus strips per stock width.
Switching shape or unit resets all inputs to defaults for that configuration.
How it works
This calculator adds your shop-specified total longitudinal seam allowanceto the nominal duct perimeter to estimate flat blank girth. The seam allowance accounts for both edges of the sheet that are joined (lock or lap seam) along the full length of the duct.
Rectangular duct: Perimeter = 2 × (width + height). Total girth = perimeter + seam allowance. Example: 12 in × 8 in + 1 in seam = 41 in total girth.
Round duct: Perimeter = π × diameter. Total girth = π × d + seam allowance. Example (metric): 200 mm diameter → 200 × π + 25 = 653.319 mm total girth; on a 1220 mm sheet, 1 piece fits with 566.681 mm leftover.
The pieces per sheet figure is a simple floor division of sheet width by total girth. It excludes kerf, trim loss, and any additional handling margins. A result of zero pieces means the sheet is too narrow to accommodate even one full girth.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Width
- 12 in
- Height
- 8 in
- Seam allowance (both edges)
- 1 in
- Sheet / stock width
- 48 in
Primary result: Flat blank girth: 41.000 in.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
- This is a nominal thin-sheet girth calculation. It does not account for thick-plate neutral-axis shift, bend deductions, or springback corrections.
- Pieces-per-sheet is a theoretical fit count. Real fabrication requires additional allowance for kerf, trim, and handling.
- Use the combined allowance required by your lockformer and seam profile. Confirm it with a test piece.
- Changing shape or unit resets all field values to sensible defaults for that configuration.
Frequently asked questions
Why is the seam allowance added once, not twice?
The seam allowance represents the total material consumed by joining both edges of the flat blank into a closed duct. It is a single combined value covering the lap or lock or lap at both longitudinal edges. You enter the total, not a per-edge value.
What does 'pieces per sheet' actually mean?
It is the maximum number of full-girth strips that fit within the sheet width, computed as ⌊sheet width ÷ total girth⌋. It does not include kerf, trim, or any other fabrication losses.
Can I use this for thick plate or structural sections?
No. This nominal perimeter estimate omits thickness corrections. Thick rolled plate needs a neutral-axis developed length based on your inside or outside diameter and forming process.
What if the result shows zero pieces?
The sheet width is smaller than the total girth. You need a wider sheet, a smaller duct cross-section, or a different approved fabrication layout. Do not reduce the seam below your equipment requirement.
Does this account for the length of the duct?
No. This calculator only addresses the girth (cross-sectional perimeter plus seam). The duct length is independent and determined by your layout or order quantity.
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