Wallpaper calculator

Dividing the wall area by the area of one roll is the classic way to come up short. You hang wallpaper in strips: each roll only yields whole strips, the leftover end is usually waste, and with a patterned paper every strip has to be longer than the wall so the design lines up. So this wallpaper calculator works strip by strip and answers "how many rolls of wallpaper do I need" the way an installer would.

You get:

• the strip length, rounded up to whole pattern repeats,
• strips per roll,
• full strips plus the short pieces over doors and above and below windows,
• the number of rolls with an optional spare, paste and an estimated cost,
• a drawing to go with the result.

Roll presets include the common US double roll (20.5 in × 33 ft), 27 in × 27 ft, and metric rolls of 0.53, 0.70 and 1.06 m width. Every value is editable. US units are the default with a one-click switch to metric, there's nothing to sign up for, and the math runs in your browser. Export to PDF or Excel for free.

1. Strip length = wall height + trim allowance, rounded up to a whole number of pattern repeats. For a drop match, add half a repeat on average, because every other strip starts half a repeat lower.
2. Strips per roll = roll length ÷ strip length, rounded down.
3. Full strips = perimeter ÷ roll width, rounded up, minus the strips that doors and windows replace (opening width ÷ roll width, rounded down).
4. Short strips: each replaced strip becomes a shorter piece over or under the opening (wall height − opening height + trim), also rounded to the repeat. The tool packs these into rolls by length.
5. Rolls = full strips ÷ strips per roll (rounded up) + rolls for the short strips, then the spare percentage, rounded up.

Off-cuts from full-strip rolls are not counted toward short strips, so the estimate errs on the safe side.

Inputs: perimeter 48 ft, 8 ft ceilings, one 3 × 7 ft door, one window 3 ft wide and 4 ft tall. Double roll 20.5 in × 33 ft, straight match with a 21 in repeat, 4 in trim allowance, no spare.

1. Strip length: 96 + 4 = 100 in → 5 repeats = 105 in (8.75 ft)
2. Strips per roll: 33 ÷ 8.75 = 3.77 → 3
3. Full strips: 576 in ÷ 20.5 = 28.1 → 29; the door and the window each replace one → 27 strips
4. That's 27 ÷ 3 = 9 rolls
5. Short strips: one 63 in piece for the window (4 ft + 4 in, rounded to 3 repeats) and one 21 in piece over the door → 1 roll
6. Total: 10 double rolls.

With a drop match the average strip grows to 9.63 ft. Three strips still fit on a 33 ft roll, so the answer stays 10 here; with a larger repeat or a taller wall it can jump.

Compare the square-foot shortcut: 384 − 21 − 12 = 351 sq ft of wall ÷ 56.4 sq ft per double roll = 6.2 → 7 rolls. That's 3 rolls short.

• Random match (no repeat): strips can be cut anywhere; set the repeat to 0 and only the trim allowance is added.
• Straight match: the design continues at the same height on every strip. Strips are whole multiples of the repeat.
• Drop match (half-drop): the design continues half a repeat lower on the next strip. You lose half a repeat per strip on average, which adds up with large repeats.

The repeat length and the match symbol are printed on the roll label. If you're unsure, calculate it as a drop match; that's the safe side.

Single vs double rolls: in the US, wallpaper is often priced per single roll but sold in double-roll bolts. Check which one the price and the roll length on the label refer to before you enter them.

• Dividing square footage by roll size. As the example shows, it can leave you several rolls short.
• Buying from different batches. The run or batch number is on the label; buy all rolls from the same one, because shades can differ slightly.
• No spare roll. One mis-cut strip or a later repair is enough reason to keep a spare. The spare percentage is editable.
• Forgetting trim allowance. Leave a couple of inches at the top and bottom for a clean cut.
• Skipping wall prep. Prime the walls before hanging; the paint calculator helps with primer quantities.