Door Threshold Length Calculator

Example data table

Scenario	Inputs (summary)	Output (summary)
Shed door	Opening 36 in, clearances 0.125/0.125, end caps 0.25 each, 1 piece, waste 7%	Cut each ≈ 36.75 in; total with waste ≈ 39.32 in
Wide patio entry	Opening 72 in, 2 pieces, seam gap 0.06, end caps 0.25, waste 10%	Cut each ≈ 36.78 in; total with waste ≈ 80.99 in
Mitered ends	Slab 35.75 in, miter allowance 0.5 each end, waste 8%	Cut each ≈ 37.13 in; total with waste ≈ 40.11 in

Numbers are illustrative; your results depend on settings.

Formula used

BaseWidth = Opening width or door slab width (based on selection).
PieceLength = BaseWidth + LeftClearance + RightClearance + 2×EndCapEach + (Miter? 2×MiterAllowEach : 0).
DoorRun = PieceLength + SeamGap×(PiecesPerDoor − 1).
CutEachPiece = DoorRun ÷ PiecesPerDoor.
TotalCut = CutEachPiece × (PiecesPerDoor × Doors).
TotalWithWaste = TotalCut × (1 + Waste%/100).

All calculations run in millimeters internally, then convert back to your chosen unit.

How to use this calculator

Select your unit system and measurement type (opening or slab).
Enter the matching width value, then add left and right clearances.
Add end-cap allowance if your threshold has caps or wrap trims.
If you will join pieces, set pieces per door and seam gap.
Choose cut style; add miter allowance if you use returns.
Set the number of doors and waste factor for extra material.
Press Calculate, then download CSV or PDF for records.

Accurate measuring for thresholds in outdoor areas

Outdoor doorways often vary because slabs swell, jambs shift, and flooring transitions are uneven. Measure at the left, center, and right, then use the largest value to avoid short cuts. Record your selected unit, because mixing inches and millimeters creates costly errors. When working on garden sheds, potting rooms, or patio entries, also note weather exposure, since moisture can change the final fit after installation.

Allowances that protect the finished edge

Clearance allowances cover expansion gaps and minor framing irregularities. End-cap allowance adds length for caps, wrapped trims, or slightly overhanging profiles. If the threshold has decorative returns or mitered ends, the miter allowance accounts for the extra material needed to form clean corners without undercutting. These options let you build a realistic cut plan before you open packaging or cut metal and hardwood stock.

Planning multi-piece runs and seam gaps

Wide openings sometimes require joined pieces when stock lengths are limited. The seam gap setting helps you plan movement space between sections, especially across long spans exposed to sun. This calculator adds seam gaps to the total run and distributes that run across the selected number of pieces, producing a practical cut length for each segment while keeping the full doorway coverage intact.

Waste factor and buying the right stock length

A waste factor compensates for trimming, test fitting, and small mistakes. Typical values range from 5% to 12%, but increase it for mitered work or rough openings. The stock length suggestion helps you choose common retail lengths that exceed your required cut length, reducing the risk of coming up short and limiting unnecessary offcuts on site.

Using the exports for a clean install record

CSV and PDF exports capture inputs and outputs for repeatable work. Keep them with your materials list to match thresholds to door locations, sealant choices, and fastener patterns. This documentation supports consistent results across multiple doors, simplifies handoffs to installers, and makes it easier to re-order the same profile for future garden upgrades.

FAQs

1) Should I measure the opening or the door slab?

Use the opening when you want full coverage between jamb faces. Use the slab width when the threshold aligns to the door edge and the opening varies due to trim or casing.

2) What clearance values are typical?

Many installs use 1/8 inch per side for minor movement and fitting. Outdoor projects may need slightly more if wood expands or the floor transition is irregular.

3) When should I use end-cap allowance?

Add it when your threshold has end caps, wrap trims, or you want a small overhang beyond the jamb line. If ends are flush, set it to zero.

4) How do seam gaps affect the cut list?

Seam gaps increase the total run length for a multi-piece threshold. The calculator then divides the run across pieces, giving a cut length that preserves doorway coverage with planned separation.

5) Why add a miter allowance?

Mitered ends and returns consume extra material for the angled cut and clean corner. The allowance helps prevent a short final piece after trimming to the exact fit.

6) What waste factor should I choose?

Use 5% for simple square cuts and consistent openings. Use 8–12% for multiple doors, miters, or unfamiliar materials. Increase it when access and re-cuts are difficult.