I have a large hallway and no idea how many wallpaper rolls to buy

Yes , multiply the height by the length of both sides of the hallway and add those to sums together this will give you the square meterage, then divide this by the coverage stated on the wallpaper roll packaging, this will give you the amount of rolls needed. add another 10% for cuts and wastage. hope that helps

You will need to work out how many drops of paper are required. The critical part of this is the full drops ( ceiling to top of skirting) Start by measuring the width of each wall. If for example the width is 2 metres, then your average width of finishing paper is around 53cm, so a 2 metre run would require 4 drops. Then you need to know how many drops you will get from a roll. Wallpaper rolls are 10 metres long. So if the length of a drop( a few cms added for trimming) for example is 225 cm you can get 4 drops off a roll with some left over for small drops ( like the top of a doorway) This however would only apply if the pattern is a straight match. If the pattern has an offset repeat then there is more waste per drop and usually this will mean that only 3 drops per roll are achievable. So work out the full drops per wall, add the number of drops together. So if for example you need 21 drops and are looking at 4 drops per roll, purchase 6 rolls( should be spare available for any small drops etc) If youre looking at 3 drops per roll then purchase 7 rolls. This is only a guide to estimating wallpaper, and as with all estimation of building materials its better to err on the side of over estimating than under estimating. A lot of vendors will accept return of un-opened rolls, if purchased 'off the shelf' . But if the wallpaper is supplied to order then often they wont. But you will (or should) be advised by them to be sure to purchase sufficient rolls as wallpaper is batched, and in the situation where you may find you didnt order enough and try to order more, the same batch number may no longer be available. Hope this helps