Wallpaper calculator: rolls for a room

What this calculator does

Wallpaper is hung in vertical strips, and every strip is cut from a roll in one piece. The calculator therefore counts strips instead of dividing areas: it works out how many strips the walls need and how many full strips come out of one roll.

A pattern repeat makes every strip longer, because strips have to be cut so that the pattern lines up. The calculator takes this into account.

How to use it

1. Choose metric or US units.
2. Add up the widths of all walls you want to paper and enter the total. Include doors and windows unless they are very large.
3. Enter the height of the walls.
4. Enter the width and length of one roll and the pattern repeat. All three are printed on the label. Enter 0 as the repeat for plain paper or paper with a free match.
5. Set the extra length for trimming each strip at the ceiling and the floor.
6. Select Calculate.

How the result is calculated

1. Strips needed = total width of the walls ÷ roll width, rounded up.
2. Length of one strip = wall height + extra length for trimming. With a pattern repeat, this length is rounded up to the next full repeat.
3. Strips from one roll = roll length ÷ length of one strip, rounded down, because a partial strip cannot be used.
4. Rolls = strips needed ÷ strips from one roll, rounded up.

The calculation assumes a straight match, where the pattern lines up at the same height on every strip. A half-drop or offset match can use more paper; see the questions below.

Worked example

The walls of a room are 12 m wide in total and 2.4 m high. A roll is 52 cm wide and 10.05 m long, and the paper has no pattern repeat. Strips needed: 1200 ÷ 52 = 23.1, rounded up to 24.

Each strip is 240 cm plus 10 cm for trimming, which is 250 cm. One roll gives 1005 ÷ 250 = 4.02, so 4 strips. Rolls: 24 ÷ 4 = 6 rolls.

With a pattern repeat of 64 cm, each strip is cut as 4 repeats, which is 256 cm. One roll then gives 1005 ÷ 256 = 3.9, so only 3 strips, and you need 24 ÷ 3 = 8 rolls.

In US units: walls of 40 ft in total and 8 ft high, with double rolls of 21 in by 33 ft. Strips needed: 480 ÷ 21 = 22.9, rounded up to 23. Each strip is 96 + 4 = 100 in. One roll gives 396 ÷ 100 = 3.96, so 3 strips. Rolls: 23 ÷ 3 = 7.7, rounded up to 8 double rolls.

About the example values

Roll sizes differ between countries and manufacturers. The metric example of 52 cm by 10.05 m is the size the British manufacturer Sanderson states for its standard rolls. Other widths are on sale, and one centimetre can change the number of strips, so use the width on your label.

The US example of 21 in by 33 ft is the size that the wallpaper brand Milton & King describes as the usual American double roll, with a single roll being half as long. Check whether the price and the label refer to a single roll or a double roll.

The extra 10 cm or 4 in for each strip is an example of the trimming allowance at the top and bottom. Increase it if the ceiling is not level.

The wallpaper maker Graham & Brown advises adding about 10 percent for waste when you calculate by area. This calculator counts whole strips instead, which already discards the rest of every roll. If the result fits exactly, consider one roll more as a reserve, because a later batch can differ in colour.

Common mistakes

• Dividing the wall area by the area of a roll. This ignores that the end of every roll is too short for another strip.
• Ignoring the pattern repeat. A large repeat can cost one strip per roll.
• Confusing single and double rolls when ordering in the United States.
• Subtracting doors and windows. The strips above and below them still have to be cut from full-length pieces in most cases.
• Buying rolls with different batch numbers. Check that the batch or lot number on every roll is the same.