Wallpaper Calculator formula

The formula is short enough to write on one line:

Rolls = ceil(wall area ÷ coverage per roll)

Ceil means round up to the next whole number, always. It is not rounding to the nearest, and it is not rounding down: 6.1 becomes 7, 6.9 becomes 7, and only an exact 6.0 stays 6. The reason is physical. Wallpaper is sold by the roll, so a room that needs 6.4 rolls worth of paper still needs 7 rolls in the shop.

Where the two inputs come from

Wall area is the sum of length × height for every wall, minus doors and windows. That subtraction is part of the method, not an optional refinement: the tool's own guidance says to subtract the area of doors and windows before entering the total. Coverage per roll comes from the roll label, in square metres per roll. If the label gives a different unit, convert before dividing — never divide square metres by square feet.

One conversion the tool gives directly: square metres = square feet ÷ 10.764. A wall area of 300 square feet is therefore 300 ÷ 10.764 = 27.9 square metres, and at 5 square metres per roll that is 27.9 ÷ 5 = 5.58, which rounds up to 6 rolls. Enter the converted figure, not the raw square feet, or the answer will be far too large.

What the formula does not cover

The formula counts paper by area. It does not add anything for pattern repeat, which is why the tool warns separately that a large repeat wastes paper at every join and suggests one extra roll. It also does not decide your spare roll for you: the calculated number is the minimum, and a spare from the same batch is a buying decision on top of it. Knowing what the formula leaves out matters as much as knowing the formula itself, because both extras are added after the division, not inside it.

Frequently asked questions

What does ceil mean in the formula?

Round up to the next whole roll, always. 6.4 becomes 7, and only an exact 6.0 stays 6, because part of a roll cannot be bought.

How do I convert square feet to square metres?

Divide by 10.764. A 300 square foot wall area is about 27.9 square metres.

Does the formula allow for pattern repeat?

No. Pattern repeat is an extra roll on top of the calculated number when the repeat is large, because each strip must start at the same point in the pattern.

Wallpaper Calculator formula — how it actually works