Answers

The questions people ask before they reach for a calculator, answered with the figure first and the working underneath. Every table on these pages is generated by running this site's own calculators across a grid of inputs, so a table can never state a quantity the tool would not produce.

What size screws for plasterboard?

38 mm for 12.5 mm board on timber, not the 32 mm the merchant hands you. Board thickness plus the specified penetration, for every thickness and both frames.

How many tiles do I need?

Whole tiles at the areas people actually tile, for four common formats in straight, diagonal and herringbone, with the waste that produced each figure.

How much tile adhesive do I need?

Twelve tile sizes, the trowel notch each one calls for, and the bags that follow. Grout too — which moves the opposite way as tiles get bigger.

How much paint per square metre?

Coverage by surface, why the first coat costs more than the second, and the litres and tins for seven room sizes with and without the ceiling.

How many bags of concrete do I need?

Bags per cubic metre for every bag size sold, common slabs and post holes — and the weight, which is what decides between mixing and ordering.

Why these pages exist

A calculator answers a question you already know how to ask. Most people arrive at a merchant's counter with a different problem: they do not know that screw length is specified rather than chosen, or that a 600 mm tile needs nearly four times the adhesive of a mosaic, or that two bags a fence post is right for one hole size and wrong for the rest.

So these pages lead with the number, show where it came from, and name the assumption that would change it. If your situation is one of the ones the table cannot cover — and most real rooms are — the calculator behind each page takes your own measurements.

How the figures are produced

Each table is harvested: a script drives the live calculator across a grid of inputs and records what it outputs, and a test re-runs that grid on every build and fails if any published figure has drifted from the tool. It is the only arrangement under which a lookup table and an interactive calculator on the same site can be relied on to agree. Where a figure comes from somewhere other than the calculator — a manufacturer's specification, or trade practice with no single authority behind it — the page says so, by name, in its own sources table.

Reference data — the underlying tables: waste factors, coverage rates, fixings by substrate
Worked examples — whole jobs, every item on the order

Last reviewed 2026-10-17.