How to use this calculator
- Enter or choose opening width between jambs.
- Enter or choose opening height.
- Enter or choose head housing into each jamb.
- Enter or choose extra length per jamb.
- Enter or choose number of frames.
- The stock lengths to buy updates as you type. Use Copy result or Share to keep it.
Method
Jamb = H + allowance; Head = W + 2 × housing; Net = Frames × (2 × Jamb + Head); Stock lengths by first-fit decreasing nesting of all cuts
- W, H
- opening width and height
- housing
- head extension into each jamb
Worked example
- Jamb = 2,100 + 50 = 2,150 mm; head = 900 + 2 × 32 = 964 mm.
- Net = 3 × (2 × 2,150 + 964) = 15,792 mm.
- Each 5.4 m length takes two jambs and a head (5,264 mm), so 3 lengths.
Three 900 × 2,100 mm frames need 3 stock lengths of 5.4 m, about 15.8 m of timber.
What the result means
Buy the stock-length count; the waste-allowance length is a check for when you buy by the metre. Add a spare length if the timber quality is variable.
Assumptions and limits
- Saw kerf is ignored.
- Each frame is two jambs and one head of the same section.
- Stops, architraves and thresholds are not included.
Questions
Why is the head longer than the opening?
When the head is housed into or runs over the jambs, each end gains the housing depth.
Why not just divide total length by the stock length?
Cuts cannot be joined, so off-cuts shorter than the next piece are wasted. Nesting the actual cuts gives the true count.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.