How it works
The calculator turns your glass sizes into a cutting plan for each roll width you want to compare. Everything runs in your browser.
1. Cut size = glass + margin on every side
Window film is cut larger than the glass because frames are rarely perfectly square, and the edges are trimmed after the film is applied. The margin you enter is per side, so a 30" × 40" glass with a 1" margin becomes a 32" × 42" cut piece.
The default is 1" (25 mm) per side. Madico's flat-glass installation training recommends cutting film about an inch larger “all the way around”. 3M's flat-glass installation bulletin recommends cutting film 1" wider and longer, then trimming to about 1/8" from the frame. Set the margin to match your own practice.
All calculations use a grid of 1/16" (or 1 mm), and values are rounded in the safe direction:
- Glass sizes and the margin are rounded UP to the grid. Cutting too small ruins a piece.
- Roll width and roll length are rounded DOWN to the grid, so the optimizer never assumes more film than you entered. For example, a roll entered as 48.01" is used as 48". The calculator shows the usable value whenever it differs from what you typed.
2. Cuts you can make by hand
The plan uses 2-stage guillotine cuts, which can be made with a straight edge and a knife:
- Pull a length of film off the roll (a “shelf”) and make a cross-cut across the full roll width.
- Slit that shelf lengthwise into the piece widths.
- Trim any piece that is shorter than the shelf.
Each shelf is as long as its tallest piece. The optimizer tries a fixed set of strategies: piece orientations, sort orders, first-fit and best-fit shelf filling, and an exact width-filling step. It keeps the plan that uses the least film length. There is no randomness, so the same input always gives the same plan.
If rotation is allowed, a piece can be turned 90° when that saves film. If your film has a pattern or a direction, turn rotation off.
3. “Proven optimal” vs “Best plan found”
For every plan the calculator also computes a lower bound: the least film length that any plan could possibly use for those pieces on that roll width. It comes from the total area and the longest piece that has to run along the roll.
- Proven optimal: the plan uses exactly that minimum length and no piece was excluded, so no plan can use less film length.
- Best plan found: the best plan the optimizer found. A plan using less film might exist.
With a roll length set, “proven optimal” refers to the film length used, not the number of rolls.
4. Comparing roll widths
Each width gets its own plan. Widths are ranked by, in order:
- fewest excluded pieces;
- least film area used;
- fewest rolls, when every width has a roll length;
- narrowest width.
Cost, if you enter it, is shown but does not change the ranking. Only material is optimized: labor, roll availability and product-specific pricing are not considered.
5. Pieces that need a splice are excluded
When both sides of a cut piece are wider than the roll, the film has to be spliced. 3M's Safety & Security installation guidelines (Rev. C, January 2025) describe this case. Splicing rules differ by film type and manufacturer, so the calculator does not plan splices. Those pieces are listed separately, with a note to check your film manufacturer's splicing instructions, and they are not counted in the totals.
If the margin is what pushes a piece over the roll width, the calculator tells you the largest margin that would avoid the splice.
A piece is also excluded if it only fits rotated while rotation is off, or if it is longer than the roll in every allowed orientation.
Sources
- Madico U: Architectural Window Film Installation, Flat Glass (training).
- 3M Window Film Technical Bulletin: Flat Glass Installation.
- 3M Safety & Security Window Films Installation Guidelines, Rev. C (January 2025).
Planning aid only. Verify measurements and your film manufacturer's instructions before cutting. Savings are not guaranteed.