DOOR SEALS / TOOL
Weatherstripping length calculator
Calculate the run length for two jambs and the top of a typical rectangular door opening; include an allowance for trimming and corners.
Worked example
For a top section of 36 inches and two 80-inch jambs: 36 + 80 + 80 = 196 inches, or about 16.33 feet. Adding a 10% allowance gives 215.6 inches, or about 17.97 feet. Round up to available packaging lengths after checking corner cuts.
When two rolls seem long enough but the cuts do not fit
Consider a head of 36 in and two jamb runs of 80 in each, plus 10% extra on each run. The three planning cuts are 39.6, 88 and 88 in, totaling 215.6 in. Two 108-in stock rolls total 216 in — yet no 108-in roll can hold an 88-in cut together with the 39.6-in head cut. The tool correctly gives three separate 108-in rolls for this conservative no-splice model, not two. One continuous 240-in roll can hold all three segments if the product allows the planned corner cuts. These are hypothetical lengths for teaching stock planning; the seal profile and allowable joints are separate decisions.
Measure the actual runs, not the nominal door size
A door described as “36 by 80 inches” does not necessarily need a 36-inch top strip and two 80-inch jamb strips. Trim, stops, mitred corners and the exact contact surface can change the cut lengths. With the door closed, identify the surface the weatherstrip must seal against; then record the actual top, hinge-side and latch-side runs separately. Enter those readings above rather than using the sales label on the door.
Practical shopping checklist
- Check whether the existing seal fits a kerf or uses an adhesive or nailed carrier; material length alone cannot establish compatibility.
- Confirm the manufacturer's allowed compression range and ensure the latch closes without excessive force.
- Compare the calculated total with available roll or kit lengths. If the kit contains pre-cut pieces, compare each required segment separately, not just the combined footage.
- Keep bottom-of-door sweep material separate. If the gap is at the threshold, use the door bottom gap checker instead of adding the width to a perimeter strip.
US, Canada and UK research-backed buying notes
US: the U.S. DOE job aid prioritizes preventing air leaks without impairing the door. Canada: Natural Resources Canada recommends weatherstripping that remains flexible and recovers its shape. UK: Energy Saving Trust distinguishes keyholes, letterboxes, bottom draught excluders and perimeter seals; none should block required ventilation.
Start with the country-aware door draught diagnostic and kerf profile compatibility. Do not interpret a purchase quantity as a safe seal type or approved door modification.
When the estimate is not enough
Unusually wide gaps, a warped door, water damage, or a rubbing door may require an alignment or threshold repair before any weatherstrip is installed. Use the draft source checklist to locate the leak and read door sweep versus weatherstripping before buying the wrong material. This is a length calculation, not a diagnosis or a guaranteed fit.