Reference tables
The figures behind every calculator on this site, published in full with the reasoning for each one. If a calculator gives you a number you were not expecting, the explanation is on one of these pages.
Waste factor table
How much extra to order, by laying pattern, room shape and room size. Covers tile, plank flooring, board, concrete and trim, with three worked examples.
Paint coverage rates
Practical coverage per litre by surface, first-coat absorption factors, how many coats each situation really needs, and why the tin's figure is a ceiling.
Adhesive and grout coverage
Consumption per trowel notch, the geometry that derives grout quantity from any tile size, and a live table you can set to your own tile thickness.
How to measure a room properly
Why no room is square, where to take each measurement, how to check the diagonals, and the measuring errors behind most material shortfalls.
Fixings by substrate
What holds in brick, block, aerated block, concrete, plasterboard and tile, with indicative safe loads, drilling technique and how to find studs.
Answers
The same data read the other way round: one question, the figure first, and a table at the quantities people actually need rather than a tool to work them out in. All five answer pages.
What size screws for plasterboard?
38 mm for 12.5 mm board on timber, 25 mm on metal, and why the 32 mm screw everyone buys is short on every thickness sold.
How much tile adhesive do I need?
Twelve tile sizes, the notch each one calls for, and the bags that follow — from 1.9 to 8.3 kg per square metre.
How much paint per square metre?
Twelve square metres a litre on a repaint, six on bare masonry, and what the first coat takes that the second does not.
How many bags of concrete do I need?
108 bags of 20 kg to the cubic metre, common slabs and post holes — and the tonnage that decides between mixing and ordering.
Worked examples
Two real rooms taken from measurement through to a complete order list, showing which figure each quantity came from — and the items people leave off.
Tiling a 2.4 × 1.8 m bathroom
Floor and walls, two different trowel notches, and the fourteen items on the order list that are not tiles.
Boarding out a 4.8 × 3.6 m room
Twenty-two sheets, and the screw length British Gypsum specify that almost nobody buys.
A 4 × 3 m garden deck
Boards, joists and the span check that settles the frame before anything is ordered.
Painting a 4.2 × 3.4 m bedroom after plastering
New skim drinks the first coat. What that does to the litres, and why one tin beats six.
Laminate in a 5 × 3.7 m living room
Eleven packs, two layers underneath, and the first-row rip that has to be worked out before a single plank goes down.
How these figures are put together
Each number falls into one of three categories, and we label which:
- Derived. Calculated from geometry, like grout quantity or the number of screws a sheet needs at a given stud spacing. These are the most reliable, because you can check the arithmetic yourself — the formulas are printed on the page.
- Published. Manufacturer or standards figures, like adhesive consumption per trowel notch or bag yields for concrete mix. These vary between brands, usually by 10–20%, so we use a mid-range figure and say so.
- Working estimates. Waste factors and practical paint coverage. These come from merchant estimating practice and installer guidance rather than a single authority. They are deliberately rounded up, because under-ordering costs far more than over-ordering.
Corrections
If one of these figures does not match your experience on site, or your supplier's data sheet contradicts it, tell us. Reference data is only worth publishing if it gets corrected, and a trade figure from someone who fits this material every week is more useful than another chart. We name the change and the date on the page when we make one.
Index last reviewed 2026-10-17.