Window trim Calculator: measuring notes and method
Count the sides that receive trim
If a sill replaces the bottom trim, subtract that length or calculate the required pieces separately.
Prepare a cut schedule
Perimeter is a length estimate, not a stock count. Mitres and long uninterrupted pieces affect how stock lengths are consumed; use the required cuts before deciding what to buy.
CHECK A PRACTICAL EXAMPLE
Two windows with different heights
Both examples use two rectangular openings, each 1.2 m wide, with trim on all four sides and zero allowance.
1.2 m high
- Length including waste
- 9.6 m
- Net perimeter
- 9.6 m
1.8 m high
- Length including waste
- 12 m
- Net perimeter
- 12 m
If a sill replaces the lower trim, remove that side from the cut list. Convert individual required pieces into stock lengths only after checking mitres and joint positions.
Fixed comparison in metric units. Values are illustrative unless linked to a recorded project; changing the calculator above does not change this example.
Formula
Perimeter = 2 × (length + width) × number of openings. Apply waste.
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Length
- 6 m
- Width
- 4 m
- Quantity
- 1
- Waste allowance
- 10 %
Length including waste: 22 m
Imperial worked example
- Length
- 19.685 ft
- Width
- 13.1234 ft
- Quantity
- 1
- Waste allowance
- 10 %
Length including waste: 72.1785 ft
Follow the calculation
Lengths in this working are in metres.
- Net perimeter = 2 × (6 + 4) × 1 = 20 m.
- Allowance multiplier = 1 + 10 ÷ 100 = 1.1.
Reviewed 19 October 2026. How calculations are checked · Report a correction