Here are a few tips and tricks on calculating how much fabric you need.
When calculating you must keep in mind, your size, your height, the fabric width and the grain line* (if there is one).
For a blouse, a dress or a coat you will need 1x the length of your garment plus the length of your sleeve. If you choose a wider style, then the fabric width is important. If you can still place your front piece and back piece next to each other, then 1x the length is enough.
If that is not possible then the front and back piece will be placed on top of each other on the fabric and you will need 2x the height. Don't forget some extra fabric for seam allowance and the hem, plus small patterns like the collar, ribbons, cuffs,...
When placing the front and back piece on top of each other, there may be room next to the front and back piece for your sleeve pattern. If this is not the case, then you
will need to add your sleeve length.
For skirts with a straight style, you will need 1x the length of the skirt.
If you choose a wider style or you have a fabric with a print then you have to take into account the fabric width. If the front piece and back piece don't fit next to each other,
then the pieces are placed on top of each other.
If you have a print, a big drawing and you want the side seams to match you will need 2x the length.
For Trousers with a straight style, you will need 1x the length. Don't forget some extra fabric for your waistband. If you want a wide style or if you have a print or a big drawing and you want the seems to match, then you will need 2x the length.
You can try and place your patterns so that you will only need 1,5x the length of the garment. It's looking for the right placement, which will take longer but is beneficial, try it with plain fabric first.
* The grain line is important when using a fabric with a pattern or a nap like with velvet. If this is the case you should place all your patterns in the same direction. All the hems (bottom of a pattern piece) should be facing the same side.