The first thing you need is an accurate measurement of the walls that are to be covered.  Use a yardstick or steel tape measure. Measure in feet rounding up to the next half or whole foot.  Below is a diagram showing a generic room to illustrate how this estimation takes place.  First, add together the length of every wall that will be covered then, multiply this number by your ceiling height.

Divide this figure by 25, the number of square feet in a single roll of wallpaper.  Round any fractions up.  This gives you the number of single rolls needed.  Ensure you specify either single or double rolls when ordering.  Generally speaking, wallpaper comes packaged in double rolls, so your wallcovering retailer will round your figures up to ensure you have more than enough wallpaper for your application.  In the example above, 12 double rolls would be ordered.

Keep in mind that wallpaper patterns with a significant drop match (24 inches or greater), rooms of unusual size or shape, greater than normal wastage during hanging, or other special circumstances may affect the amount needed.  In one of these cases, it may be best to contact a professional to calculate your exact wallpaper needs based on the pattern you select and your room geometry.  If you have significantly large windows or any other large openings (fireplaces, French doors, etc.) that will not require wallpaper, then some wallpaper may be deducted from the final total.  The deduction amount is best left to the professionals and in almost every case it is better to have leftover wallpaper for repairs than to come up short at the last minute.