How much window film do I need?
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The short answer: more than the area of your glass. How much more depends on three things you control, which are the margin you cut around each piece, the roll width you buy and how the pieces are laid out on the roll.
Step 1: add up the glass
Measure the width and height of each piece of glass (see how to measure windows for film). Multiply width by height for each pane and add them up. In inches, divide by 144 to get square feet.
Example: a small house job with two 30" × 40" windows and three 24" × 36" windows.
- 2 × (30 × 40) = 2,400 sq in
- 3 × (24 × 36) = 2,592 sq in
- Total glass: 4,992 sq in = 34.67 sq ft
Step 2: add the cutting margin
Film is cut larger than the glass and trimmed after it is applied, because frames are rarely perfectly square. With a margin of 1" per side, each piece grows by 2" in both directions:
- 30" × 40" glass → 32" × 42" cut piece
- 24" × 36" glass → 26" × 38" cut piece
The cut pieces now total 39.25 sq ft, about 13% more than the glass. Smaller panes grow proportionally more, because the margin is the same on every piece.
Step 3: fit the pieces on a roll
Film comes on rolls of a fixed width, and you pay for the full width of every length you pull. Pieces are usually cut in strips across the roll: pull a length, cut it across, then slit it into pieces. Whatever does not fill the width of each strip is waste.
That is why the film you actually use depends on the roll width. Here is the same job planned by the calculator (1" margin, rotation allowed):
| Roll width | Film length | Film used | Utilization |
|---|---|---|---|
| 36" | 16.50 ft | 49.50 sq ft | 79.3% |
| 48" | 11.83 ft | 47.33 sq ft | 82.9% |
| 60" | 10.17 ft | 50.83 sq ft | 77.2% |
Utilization is the area of the cut pieces divided by the film used. The best width for this job, 48", uses 47.33 sq ft of film for 34.67 sq ft of glass: about 37% more film than glass. The 60" roll needs the shortest length, but it is not the most efficient.
Step 4: decide what to buy
- Buy by length of the chosen width, not by glass area. In the example, you need at least 11.83 ft of 48" film.
- If the film is sold in fixed roll lengths, check that the pieces fit in the rolls you can buy. Enter the roll length in the calculator to get the number of rolls.
- Add a safety allowance for mistakes, according to your own experience. The calculator gives the plan with no extra allowance.
Watch out for pieces wider than the roll
If both sides of a cut piece are wider than the roll, it cannot be cut in one piece and would need a splice. In the example above, adding one 48" × 60" window with a 1" margin gives a 50" × 62" cut piece, which is too wide for a 48" roll in either direction. See choosing a window film roll width.
Do it for your own job
The window film cut calculator does all four steps for you: enter the glass sizes and the roll widths you can buy, and it shows the film length, area and step-by-step cuts for each width. Your measurements stay in your browser.
Planning aid only. Verify measurements and your film manufacturer's instructions before cutting. Figures are for the example shown and will differ for your job.