If your fabric has a dominant pattern you may like to have the bottom of the blind on a particular part of the fabric so make an allowance for this when purchasing your fabric. For a blind wider than your chosen fabric - double the quantity of fabric e.g. for a blind measuring 170 cm wide x 130 cm length and the fabric is 137 cm wide, you will need: 130 cm + 30 cm = 160 cm x 2 = 320 cm.
If the fabric has a pattern you will have to check how many patterns repeats are needed to match the pattern e.g. based on the above blind and the pattern repeat is 64 cm you need 3 x 64 cm = 192 cm x 2 = 384 cm.
Sometimes on patterned fabric there is a half drop repeat, this amount needs to be added to cover the half drop. The retailer where you buy the fabric will be able to assist you. However, it does help for you to have an idea on calculating how much fabric you require and reduces the risk of not having enough or too much fabric. Any unused fabric will be returned to you with your blind.
It may sound complicated but most blinds don't have any joins so just buy the drop + extra as required. The only time you might need to use the pattern repeat is when you have several blinds, perhaps on a bay window. It looks better if the blinds have the same pattern on the bottom of each blind.
If you want a small blinds please send the whole width of the fabric, so the blind can be cut from the best position if the fabric is patterned. If you want two smaller blinds cut from the width of fabric on a roman blind you should allow 8 cm for the turnings on each blind, so please make sure you have enough fabric.