I am putting up a new pole, i am unsure how much the pole should extend past the window and if that should also include the architrave. is there a rule of thumb? I assume it depends also on the width of the window - ours is 142cm or 166cm including architrave.
For windows that size we would look to extend the pole 20-30cm either
side. Poles tend to come in standard sizes, 180cm, 220cm. You will
probably just get away with a 180cm pole for the 142cm window. We'd make
the second window proportionate.
Generally when you go up a pole size you need more fabric widths.
I am making curtains and the pole is not fitted yet. How do I calculate the hem? I have the height the pole will be fitted at and the diameter of pole and rings. Can I calculate it from this?
We would always wait for the pole to go up then measure and make. (it's far simpler)
The Hem is at the bottom so is just dependent on the length of the curtain. 20cm allowance full length, 16cm allowance sill length.
However I think you are asking about the allowance for the top. You could estimate the hook drop then add on extra amount to take into acount any error, then calculate fabric etc as per the instructions. You could then make the curtains up to heading stage, then finish them off to the correct length when you have the pole up and have measured the actual hook drop.
Note: Making the curtains to an estimated length then trying to hang the pole at the correct height is difficult to get right.