How the estimates are worked out
Every calculator here, and the room refresh planner built on them, follows the same four rules. The arithmetic for a particular calculator is printed on its own page, under “How the estimate is worked out”; a batch shows its working beside every line.
The four rules
- Show the working. Every answer carries the sum that produced it, with the numbers that went in. If you cannot check it, you should not trust it.
- Round once, at the end. All the arithmetic runs on unrounded figures. Rounding happens at the last step, when a quantity becomes whole packs, and it always rounds up. Rounding early is how estimates quietly drift.
- Different products are never added together. Wall paint and ceiling paint are usually different tins, so they are worked out and bought separately. Where you could combine them, the page says so and tells you how.
- Say what it cannot do. Each calculator lists what it does not measure and where it will read low or high, instead of hiding it in a wastage allowance.
Where the starting numbers come from
A calculator has to start somewhere, or you would have to fill in every box before it could tell you anything. So each assumption has a starting value — for paint: 2 coats, 10 m² per litre, 10% extra for waste and touch-ups, 2.5 litre tins.
These are examples, not manufacturer data and not a recommendation. They are labelled as examples on screen, they appear in the copied shopping list marked as examples, and they are all yours to change. Coverage in particular varies a lot between products and surfaces: the figure printed on the tin you are actually buying will beat our example every time.
We do not publish a manufacturer's figure as if it were ours, and we do not average figures across brands into a number nobody stands behind.
How a calculator is checked
Four different kinds of check, because they catch different things. The first two have happened and the third only in part. No person outside the build has done any of them:
- Worked examples written before the code. The expected answers were worked out step by step first, apart from the code, then the code was made to agree with them. They were worked out by the AI tools that built the site; a tradesperson has not checked them.
- Automated tests. Several hundred, run on every change: the arithmetic, every rounding boundary, every invalid entry, hostile and extreme values, the wording on screen and the colour contrast. They also run the real page in a browser and use it the way a person would.
- Code reviews by AI tools. AI tools other than the one that wrote the code have checked several releases and parts of the work, trying to break them. Two faults in the paint calculator found this way have been fixed and are recorded in the project's changelog. These are checks by software, not reviews by people.
- Review by people who do the work. A qualified painter or decorator checking that the assumptions match how the job really goes.
The fourth has not happened yet. Nor has testing with people who were not involved in building this. That is why every page still says “prototype”, and why no result here should be the only thing you rely on. The date the assumptions were “last reviewed”, shown beside a calculator, is when they were last gone over while the site was built. It is not a tradesperson's check.
Two version numbers
You will see two numbers at the foot of a calculator page, and they mean different things.
- The release — currently 0.14.0 — is the version of the website. Changing wording, colours or an information page changes it.
- The formula version — currently 0.1.1 for paint — is the arithmetic, the supported ranges and the assumptions that decide an answer. It only changes when a number that affects your result changes.
They are kept apart deliberately, so that a change to how the site looks can never be mistaken for a change to what it tells you to buy. Every change to a formula is written down, with the reason, in the project's changelog. The room refresh planner carries a third number, its template version, for the rules that decide which lines a job gets and how pack sizes are chosen; it uses the calculators' formulas and does not change them.
How pack sizes are chosen
Where a product comes in more than one size, the planner works out which sizes to buy. With a price for every size it finds the lowest total cost, and among equal costs the least left over, then the fewest packs. With any price missing it finds the least left over, then the fewest packs. It says which rule it used beside the answer, and offers the fewer-packs alternative when that differs.
The comparison is only between the sizes and prices you entered, or those recorded against one reviewed product. It says nothing about other shops or other products, and it never calls anything the cheapest. A page that compares products, rather than pack sizes, follows the separate rules on the page “How we compare products”.
If an estimate looks wrong
Trust the tin and the room in front of you before you trust this site, and then tell us. A result that disagrees with an experienced decorator is exactly what we need to hear about.