When calculating the square footage for a window film job, it's common to account for 10% to 20% waste. This percentage accounts for errors, trimming, and alignment issues that may arise during the installation process. The exact percentage may vary based on the complexity of the job, window shapes, and sizes. For standard rectangular windows with straightforward cuts, 10% might be sufficient, but for more intricate jobs with unusual shapes or multiple panes, 15% to 20% is safer to ensure you have enough material.