Wallpaper rolls calculator

Non-positive dimensions are rejected with a clear invalid_input error instead of inventing a roll count from zero or negative lengths.

How wallpaper quantity is counted with pattern repeat

Wallpaper is hung in vertical strips, not painted as a continuous area, so a naïve wall-area divided by roll-area estimate undercounts whenever the pattern forces each strip to be cut longer than the wall height. The standard decorator method starts with drop length. Strips_per_roll is how many full drops fit into the roll length, using floor division so a leftover stub shorter than one drop does not count as a hangable strip. All five length fields must use the same unit—metres with metres, feet with feet—because the calculator never converts units; it only enforces positive wall and roll dimensions and a non-negative pattern repeat. When pattern_repeat is zero—plain paper, texture, or random match—the drop equals the wall height. When pattern_repeat is positive, every drop is rounded up to the next whole multiple of that repeat so neighbouring strips can be matched at the same motif height; that extra length is intentional waste, not a bug. Next, strips_needed is the wall width divided by roll width, always rounded up so a partial strip still consumes a full width of paper. Finally rolls_needed is strips_needed divided by strips_per_roll, rounded up. If the computed drop is longer than the roll, the call fails: no whole strip can be cut, and inventing a fractional roll would mislead a purchase. The response also reports rolls_by_area_only so you can see how much pure area math would understate the job once pattern waste and strip geometry are included.

How to call the API and what comes back

Send a JSON body with four required positive finite numbers—wall_width (aliases width, wall_w, room_width), wall_height (aliases height, wall_h, room_height), roll_length (aliases roll_len, length, paper_length), and roll_width (aliases paper_width, strip_width, roll_w)—plus optional pattern_repeat (aliases repeat, pattern, match, rapport), which defaults to zero when omitted. Absolute values must stay within the published range so intermediate products remain finite in IEEE double precision on the Worker and in the browser. On success the response echoes rounded inputs, sets drop_length and pattern_multiples from the height-and-repeat rule, strips_needed and strips_per_roll from the ceil and floor steps, rolls_needed as the purchase count, wall_area and roll_area as width times height products, covered_area as strips times drop times roll width, pattern_waste_per_drop and total_pattern_waste_length for the extra length forced by matching, rolls_by_area_only as the optimistic area-only ceil, message as a short English summary, and steps as an ordered array of substitution lines. Failed validation—missing fields, non-finite numbers, non-positive wall or roll dimensions, a negative pattern repeat, or a drop longer than the roll—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price.

Practical tips, limits, and related searches

Prefer sending numbers rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern. Keep dimensions inside the published absolute range so ceil and floor steps cannot overflow. Prefer this capability when you already know wall and roll sizes and only need the strip-based roll count; if your source is a photo of a room, measure first. Round-trip checks are easy: recompute ceil of width over roll width, floor of roll length over drop length, and ceil of those strip counts by hand, and confirm they match the published fields within the same twelve-decimal rounding used for lengths and areas. Measure wall width as the sum of all surfaces you will hang, or call once per wall if doors and windows change the strip plan; this tool does not subtract openings, so large windows should be handled by reducing wall_width or by a separate strip plan. Read pattern_repeat from the product label—often printed in centimetres or inches next to “repeat” or “match”—and convert it into the same unit as your wall measurements before calling. When teaching or writing tests, pair the same wall with pattern_repeat zero and with a positive repeat so rolls_needed rises only when drop length grows. This landing targets people searching for a wallpaper rolls calculator, how many rolls of wallpaper, or pattern repeat wall coverage online; it is not a paint-coverage estimator, not a tiling calculator, and not a 3D room visualizer. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when you deliberately send a zero wall height to verify error handling.