When you mitre a moulding you lose the moulding's width in wastage.
When you mitre a picture frame moulding on both ends you lose twice the the width of the moulding from the rebate to the back edge.
If the frame has four sides we can then calculate a total length of moulding required to make a complete frame.
2 x (length + width) +
8 x width of moulding +
an allowance for cutting say 200mm
The frame size is obtained by measuring the external size of the work to be framed.
When you calculate how much moulding you need you must also make an allowance for the width of the cutting equipment that you are using and in case you make a mistake. I've suggested 200mm above.
If you were to cut the moulding to exactly the size of the work to be framed (frame size), the work would not fit the joined frame.
You should add an allowance of + 2mm when you calculate the size to cut your moulding.
We can now arrive at a formula for calculating the external size of each piece we wish to cut.
