Wallpaper Calculator

What the calculator does, step by step

Wallpaper is hung in vertical strips (called drops), so the real question is how many full-height strips you can cut from each roll. The calculator follows this chain:

1. Wall area. Perimeter = 2 × (length + width); gross area = perimeter × ceiling height. It then subtracts a fixed 21 sq ft for each door and 15 sq ft for each window. Those two values are the tool's assumptions, not measurements, so swap in a smaller or larger count if your openings are unusual.
2. Strip length = ceiling height + pattern repeat. With a repeat of 12 in on an 8 ft wall, every strip is cut 9 ft long so the pattern can line up at the top.
3. Strips per roll = roll length ÷ strip length, rounded down. A 33 ft roll gives 3 strips of 9 ft, and the 6 ft left over is waste.
4. Usable area per roll = strips per roll × strip length × roll width.
5. Rolls = wall area × 1.15 ÷ usable area, rounded up. The 15% allowance covers trimming, matching and mistakes.

The “estimated cost” is simply the roll count times a flat placeholder of $45 per roll. It is not a price lookup; multiply the roll count by the price on your own wallpaper instead.

Worked example: the default room

A 12 × 10 ft room with 8 ft ceilings, two doors, one window, 20.5 in wide double rolls (33 ft) and a 12 in repeat:

• Perimeter = 2 × (12 + 10) = 44 ft. Gross wall area = 44 × 8 = 352 sq ft.
• Openings = 2 × 21 + 15 = 57 sq ft, so net area = 295 sq ft.
• Strip length = 8 + 1 = 9 ft. Strips per roll = 33 ÷ 9 = 3.67, rounded down to 3.
• Usable area per roll = 3 × 9 × (20.5 ÷ 12) = 46.1 sq ft.
• Coverage needed = 295 × 1.15 = 339.3 sq ft; 339.3 ÷ 46.1 = 7.36, rounded up to 8 rolls (about $360 at the placeholder price).

Cross-check with a strip count

Because the tool works from area, it is worth checking it the way a paperhanger would. The 44 ft perimeter divided by the 20.5 in (1.708 ft) width is 25.8, so you need 26 strips. At 3 strips per roll, 26 ÷ 3 = 8.67, which means 9 rolls. The area method gave 8 because it treats the area over doors and windows as saved paper; in practice you still need short strips above and below openings, and those pieces often cannot be reused with a pattern match. When the two methods disagree, buy the larger number.

Why roll length matters so much

Choose the same room with single rolls (16.5 ft). A 9 ft strip fits once, and 7.5 ft of every roll is lost, so each roll yields only 1 strip and 15.4 sq ft usable. The same 339.3 sq ft would need 23 single rolls, which is 11.5 double rolls rather than 8. This is the main reason the roll-length selector exists: make sure it matches the length on your roll's label, not the length you assume. The same logic applies to ceiling height. An 11 ft ceiling with a 1 ft repeat needs 12 ft strips and drops the same 33 ft roll from 3 strips to 2.

Common roll sizes differ by region. One reference gives a European roll as 52 cm (about 20 in) wide by 10 m (about 33 ft) long, and an American roll as 27 in wide by 27 ft long. If your roll is 27 ft, note that for a 9 ft drop a 27 ft roll and a 33 ft roll both give exactly 3 strips, so the 33 ft setting gives the right strip count, but it will not for taller drops.

Pattern repeat and match types

The pattern repeat is the smallest rectangle that can be tiled to form the whole pattern, and it is printed on the roll label. With a straight match, the pattern lines up side to side at the same height, so each strip starts at the same point in the repeat. With an offset (drop or half-drop) match, the next strip begins part-way through the repeat, for example 12 in into a 24 in repeat, which usually costs more paper. A random or free match has no repeat to line up, so set the repeat field to 0 and the strip length becomes just the ceiling height.

One retailer's guide shows the effect clearly: on 2.4 m walls, a plain paper on a roll about 10 m long gives four drops, while a repeat of up to 64 cm gives only three. The calculator reproduces that pattern: every strip gets one full repeat added, which suits most straight matches. Large repeats on an offset match may need extra, so ask the manufacturer for the number of drops per roll.

Common mistakes

• Ignoring trimming. The tool adds no separate trim allowance beyond the 15% waste. For a plain random-match paper, enter a ceiling height a few inches taller if you want extra margin top and bottom.
• Using the wrong roll length. Single and double rolls change the strip count dramatically.
• Forgetting the ceiling, gable ends and sloped walls. The tool covers four straight walls of equal height only.
• Buying in two batches. Ask for all rolls from the same batch. A retailer guide recommends buying extra for complex rooms to keep batch continuity.
• Trusting the cost line. It is a flat $45 per roll placeholder.

What this calculator does not cover

It does not cover ceilings, borders, accent walls with different dimensions, adhesive, lining paper, wall preparation or price. Rough walls and thin papers may need cross-lining paper hung horizontally first, which is a separate quantity. For other finishes in the same room, use the paint calculator, the drywall calculator if you are repairing walls first, the wainscoting and board-and-batten spacing calculator for lower-wall panelling, and the curtain fabric calculator for matching window treatments.