How to use these guides
- They are built on the calculators, not beside them. Every density, allowance and coverage figure quoted here is the figure the matching calculator uses, and each guide shows a worked example you can check against the tool.
- The worked examples are real. Each one lists its inputs and the result the calculator returns for them, so you can repeat the job and get the same answer.
- They are guidance, not advice. A guide helps you plan an order. It does not design a structure, choose a specification or decide a building-control question.
- Every job is different. Ground conditions, existing construction and the material your supplier actually stocks all change the answer. Check your own measurements and the product data before ordering.
The guides
What the guides have in common
Each one follows the same three moves, which is really the whole method for ordering materials:
- Measure what you can actually measure. An area, a length, a room — the thing that is in front of you.
- Convert it to the unit the trade sells in. Tonnes, rolls, sheets, lengths, boxes of 1,000. Very little is sold by the square metre.
- Add the allowance that covers reality. Cutting allowance for turf, waste for slabs and sheets, a settlement allowance for topsoil, an allowance for uneven ground for aggregate.
Get those three right and the order is right. Get the third one wrong and you are making a second trip, which is the part that actually costs money.
Guides and the topic hubs
The guides sit alongside the five topic clusters, which group the calculators by the job they belong to:
Where a guide touches a rule rather than a quantity, it points at the regulations library rather than restating it: the drafting of a hard surface, surface-water discharge, ground level against a damp-proof course, or the linings and sound targets that apply to a boarded wall. Those pages name their primary sources; this one does not need to.
Where the figures come from
Every number in these guides comes from the calculator engines the site ships, and each guide states the inputs it used. If you want to check one, open the matching calculator, enter the same figures and compare — the two should agree exactly.
Guidance only. These guides are planning aids, not a quote, not a specification and not professional or structural advice. Always check your own measurements and your supplier's current product data before ordering, and where a job is structural, confirm it with a competent person.