Wallpaper calculation method and validation
This page records the exact model used by Wallometry, including where the estimate is conservative. It is a strip-counting estimator for walls with one shared maximum height, not a packing optimizer.
Inputs and units
All dimensions are converted to metres before arithmetic. One foot is 0.3048 m and one inch is 0.0254 m. Wall height and roll length are entered in metres or feet; roll width, trim and repeat use centimetres or inches. The optional price is an amount in your chosen currency, not a converted exchange-rate value.
The formulas
- For each wall i, strips_i = ceil(width_i / roll_width).
- Total strips = sum(strips_i).
- Minimum cut = maximum height + total trim.
- For free match, cut = minimum cut.
- For straight match, cut = ceil(minimum cut / repeat) × repeat.
- For half-drop allowance, cut = straight-match cut + repeat / 2.
- Yield = floor(roll_length / cut).
- Base rolls = ceil(total strips / yield). Order rolls = base rolls + selected spare rolls.
A zero yield is an error, not zero rolls. The calculator rejects non-positive wall and roll dimensions, negative trim and a zero repeat when a matched pattern is selected. A small 1e−9 rounding tolerance prevents ordinary floating-point conversion noise from adding a strip at an exact boundary. It is not a measurement allowance.
Why half-drop is an allowance
Pattern phases change as cuts are taken from a roll. An alternating A/B cutting plan may yield more strips than reserving half a repeat for every cut. We use the latter approach to provide a checkable, conservative quantity. We do not claim it is the minimum quantity, and we do not model quarter-drop or other offsets. Product instructions take priority.
What the model does not reuse
It pools full strips across walls at the common height. It does not pool narrow edge pieces, fill openings with arbitrary offcuts, optimize strips of different lengths, or reserve a specific starting motif. Tail length displayed is the remainder of a roll cut to its full calculated yield; it is not total waste across the whole order. The final roll may be only partly used.
Published validation cases
| Case | Expected |
|---|---|
| 4 m wall, 2.4 m height, free match | 8 strips; 4 per roll; 2 rolls |
| Same wall, 0.64 m straight repeat | 2.56 m cut; 3 per roll; 3 rolls |
| Same wall, half-drop allowance | 2.88 m cut; 3 rolls |
| 4.8 m wall, 2.8 m height, free match | 10 strips; 3 per roll; 4 rolls |
| Two 0.54 m walls, free match | 4 strips (each wall rounded separately) |
| Roll length shorter than required cut | Error; no order estimate |
Sources and review
Wells Interiors’ quantity guide illustrates full-strip yield calculation. Ethan Allen’s measuring guide explains match types and large-repeat ordering considerations. Our half-drop formula is an explicitly labelled planning choice rather than a universal manufacturer formula.
Content and calculation checks last reviewed: 13 October 2026. Examples are arithmetic test cases, not verified customer installations. Manufacturer instructions take priority over generic planning examples. Send reproducible issues through Contact.