How to use this calculator
- Enter finished base size. Use outside mold-line base length and width between the side bends.
- Enter side height. Use the outside mold-line side height for all four tray sides.
- Set bend geometry. Enter material thickness, inside bend radius, bend angle and K-factor.
- Check blank and relief. Use the flat length, flat width, bend-line offset and corner-relief margin as the first-pass layout.
How it works
This calculator treats the box or tray as a rectangular base with four equal-height side flanges. Dimensions are outside mold-line dimensions, so the flat blank uses bend deduction: Lflat = Lbase + 2H - 2BD Wflat = Wbase + 2H - 2BD
The bend deduction comes from the same sheet-metal bend relation used by the bend allowance and press-brake pages: BA = (pi / 180) x A x (R + K x t) OSSB = (R + t) x tan(A / 2) BD = 2 x OSSB - BA
Bend-line placement is shown as the distance from each blank edge to the bend line: bend line = H - OSSB + BA / 2 The relief screen uses: minimum relief = R + t as a compact check for square corner notches. The same corner-relief workflow is available as a relief notch size calculator.
Pair this with the K-factor calculator, bend allowance calculator, press brake bend deduction calculator, press brake tonnage calculator and sheet metal gauge chart when moving from layout to tooling and material checks.
Worked example
Verified against the live calculator
For a tray with 300 mm outside base length,
200 mm outside base width, 50 mm sides,
1.5 mm thickness, 1.5 mm inside radius,
K = 0.33 and 90 deg bends, each bend deduction is
about 2.87 mm. The flat blank is about
394.3 mm x 294.3 mm, and the bend lines sit about
48.6 mm in from each edge before corner relief is applied.
Frequently asked questions
How do I calculate a sheet metal box flat pattern?
For an open tray with outside mold-line dimensions, calculate the bend deduction for each side bend, then use flat length = base length + 2 x side height - 2 x bend deduction. The flat width uses the same relation with the base width.
Where do I place the bend lines?
The bend-line offset from each blank edge is side height - outside setback + bend allowance/2. This places the bend line using the same bend allowance, outside setback and K-factor relation used for the flat size.
What corner relief should I use?
This page screens a simple square relief against inside radius plus material thickness. Production parts may need larger slots, round reliefs, tabs, miters, hems, weld gaps or shop-specific corner details.
Does this include press-brake capacity?
No. Use the press brake tonnage calculator for force and V-die checks. This box calculator is only a flat-pattern layout screen.
Are the dimensions inside or outside dimensions?
The calculator treats the entered base and side dimensions as outside mold-line dimensions because bend deduction is normally subtracted from summed outside dimensions.
Method & assumptions
- Open tray or box with four equal-height sides and one bend per side.
- Entered base and side dimensions are outside mold-line dimensions.
- Uses the standard bend allowance, outside setback and bend deduction formulas with an entered K-factor.
- Corner relief is modeled as four square cutouts; the relief screen is only radius plus thickness.
- Does not model tabs, hems, lock seams, joggles, closed corners, corner forming, bend sequence, punch/die clearance, grain direction, springback, coating, weld shrinkage, hole distortion or shop-specific bend compensation.