Last updated:
Why this exists
Buying material for a room is a small maths problem that people get wrong in an expensive direction. Some buy short and lose a weekend going back to the shop, and then discover the second tin is a slightly different batch. Most buy long: a tin and a half of emulsion nobody will ever open, two boxes of tile that live in a cellar for a decade.
The arithmetic is not hard, but doing it in a shop aisle on a phone, in units you half-remember, next to a product whose stated coverage assumes conditions your wall does not meet, is where it goes wrong. BuildSums does that arithmetic properly, shows its working, and says out loud what it assumed.
Who builds it
BuildSums is built and maintained by Fabio Festa, working alone, in Italy. It is not a company, not a front for a retailer and not a content farm. There is no team page because there is no team.
The relevant expertise here is software: the site's value is that the geometry and the unit conversions are implemented carefully and verified, rather than typed into a page and hoped for. Where the domain calls for a figure that is not mathematics — how much paint a litre covers, how much tile to allow for waste — the source is named below rather than invented.
Corrections are welcome and are acted on. If a formula, a default or a translation is wrong, write to lemonoakmont@outlook.com and it will be fixed and dated.
The method
All the room-based calculators share one geometry engine, so a room measured once behaves identically whichever tool you open. The chain is always the same:
- Wall area = 2 × (length + width) × height.
- Openings — each door and window, multiplied by how many there are — are subtracted from the wall area.
- The ceiling and, where the tool needs it, the floor are added only if you asked for them.
- The net area is multiplied by the number of coats and divided by the product's coverage.
- A waste allowance is applied as a percentage.
- The result is divided by the pack size and rounded up, because shops sell whole tins, whole boxes and whole rolls.
Imperial input is converted to metres and litres internally, the calculation is done once in metric, and the answer is converted back for display. Switching units therefore never changes the physical result — only how it is written. That property is worth more than it sounds: unit handling is where this kind of tool usually goes quietly wrong.
How the numbers are checked
Each calculation engine has an automated test suite that runs before anything is published — src/lib/calculators/__tests__/paint-engine.test.ts, tile-engine.test.ts and wallpaper-engine.test.ts in the source. The tests assert known worked examples, check that metric and imperial inputs describing the same room produce the same physical answer, and confirm that pack counts round up rather than down.
This is the ordinary discipline of software engineering rather than anything exotic, but it is not what most online calculators do, and it is the reason the formula on each page can be shown openly: you can reproduce any result by hand.
Our editorial rule is that a calculator is not published until it has a working tool, a visible formula, a verified example, a list of the mistakes people actually make and its own FAQ. A page that would only repeat another page is not published at all.
Default assumptions, and where they come from
Every default in the tools is editable, because your product is not the average product. They exist so the page is useful before you have found the tin. These are the figures and their basis:
- Paint coverage, around 10-12 m² per litre per coat
- The range printed by major manufacturers on interior emulsion for a smooth, sealed, previously painted surface. Always prefer the figure on your own tin: textured, porous, new or heavily filled surfaces can fall to 6-8 m² per litre, and the datasheet's figure assumes conditions a real wall rarely meets.
- Two coats as the starting point
- The standard manufacturer recommendation for opacity over an existing coating. One coat is realistic only for a refresh in the same colour; a strong colour change, and particularly a light finish over a dark base, commonly needs three.
- Primer where the substrate demands it
- Recommended by manufacturers on bare plaster, plasterboard, filler repairs, stains and glossy or previously oil-painted surfaces. Primer is calculated separately because its coverage differs from that of the finish coat.
- Tile waste, 10% as a baseline
- The allowance conventionally quoted by tile suppliers and trade bodies for a straight-set layout in a square room. Raise it to about 15% for a diagonal or herringbone layout, and further for rooms with many cuts, niches or non-parallel walls.
- Wallpaper: roll length and pattern repeat
- A standard European roll is about 10.05 m long and 0.53 m wide; the pattern repeat, printed on the roll's label, determines how much is lost at each drop. A straight match wastes least, an offset or drop match wastes most, and a plain paper wastes almost nothing.
Where a figure is a convention rather than a physical constant, we say so instead of dressing it up as precision. A waste percentage is a judgement about your room and your tiler, not a property of the tile.
What the calculators do not model
- Non-rectangular rooms, sloping ceilings, bay windows and alcoves — measure these in sections and add the results.
- Surface texture and absorbency, which are the largest single source of error in paint quantity.
- Colour change and hiding power, which can add a whole coat.
- The skill of the person cutting, which is the largest single source of error in tile and wallpaper waste.
- Adhesive, grout, filler, tape and the other consumables a job needs.
- Prices. Nothing on this site knows what anything costs.
When a tool cannot model something, the honest answer is to say so on the page rather than to produce a confident number. That is why each calculator carries its own accuracy note.
Independence and how this is funded
BuildSums carries advertising served by Google. Advertisements are labelled, are never placed above a calculation result, and are selected by Google rather than by us.
No manufacturer or retailer pays to be the default in any field, no result is weighted towards any brand, and no advertiser sees or influences what these pages say. If that ever changes — a sponsorship, an affiliate link, a commercial partnership — it will be disclosed on the page where it appears, before you act on anything.