How these calculators are built

Anyone can print a formula. What decides whether a calculator is worth trusting is what happens around it: which constants it carries, when it rounds, what it does when the honest answer is “it depends”. These are the rules Make It Exact follows, written down so they can be held against us.

1. Sourced constants, or no constant

Geometry needs no citation — a cylinder’s volume is not a matter of opinion. Product characteristics do. A bag yield, a coverage rate, a density: each is a claim about somebody’s manufactured product, and it belongs to that manufacturer’s documentation, with the date it was read.

Where sources disagree, we do not average them and we do not pick the popular one. The concrete calculator dropped a widely used bag size because published yields for it differ by about 4% — enough to change what you buy. The source note next to each result distinguishes a figure read at the manufacturer from one taken second-hand.

2. Convert at the edges, round at the end

Inputs are converted to one internal unit on the way in and back on the way out. Nothing is rounded in between. Area and volume factors are derived from the linear factor rather than stored separately, so a converter cannot disagree with itself about how long a foot is.

Rounding up is reserved for things that cannot be bought in fractions — bags, tiles, boards. Everything else keeps its decimals, and the page says which figure is the rounded one.

3. Assumptions are visible and adjustable

Every assumption that changes the answer sits next to the answer as a field you can edit. The waste allowance defaults to 10% and says so; it is not folded into the total. Net quantity and ordered quantity are shown as two numbers, because they are two decisions.

4. What the calculation subtracts

Most calculators of this kind multiply three dimensions and stop. Real work has holes in it: a wall has openings, a column may be hollow, a post occupies part of the hole it is set in. Where a subtraction changes the order materially, this site makes it — and tells you what it removed, so you can put it back if your supplier bills the gross.

5. Where this goes wrong

Every templated calculator carries a section naming the mistake people actually make with that particular calculation — not a disclaimer, a specific error with a worked figure. A calculation whose pitfall nobody can name probably does not need its own page. Two automated tests enforce this, and a third checks that every worked example printed on a page is genuinely what the calculator produces from its own defaults, so the prose cannot drift away from the code.

6. What we do not publish

When a number cannot be sourced, the feature is left out rather than estimated. Reinforcing steel is missing from the concrete calculator for that reason, and a competitor’s estimate of it is not a good enough source to copy. Health and diagnostic instruments are out of scope entirely — see about.

7. No model, no guessing

There is no language model in these pages and no expression evaluator. Every formula is ordinary typed code with unit tests, including the normal distribution, which is tested against published tables to the precision its method actually guarantees rather than the precision it happens to reach. Nothing is computed on a server: the arithmetic runs in your browser, so no input you type is ever transmitted.