How do I work out how much material I need?

Almost every quantity mistake is either a units error or no wastage allowance. Both are avoidable with two minutes of arithmetic, and both cost an afternoon when they go wrong.

Step by step

1. Measure carefully and write it down.Length and width of each area separately rather than one overall figure. Sketch it if the room is not a simple rectangle, and split awkward shapes into rectangles you can add together.
2. Work in one unit throughout.Metres, consistently. Mixing metres and centimetres is the single most common source of a tenfold error. Convert everything before you start calculating.
3. Calculate area for surfaces.Length times width, in metres, giving square metres. For walls, calculate each wall and add them. Subtract large openings — doors and windows — but only if they are significant.
4. Calculate volume for anything loose.Length times width times depth, in metres, giving cubic metres. Aggregate, topsoil, concrete and bark are sold this way, and depth is the number people forget to convert.
5. Add a wastage allowance.10% for most things. 15% for tiles laid diagonally, patterned wallpaper, or any area with many cuts and awkward corners. This is not optional — offcuts are unavoidable.
6. Check the coverage figure on the product.Paint states square metres per litre, adhesive per bag, aggregate per cubic metre. Coverage figures assume a smooth surface — porous or rough surfaces take considerably more.
7. Account for the number of coats.The most common paint miscalculation. Two coats means double the paint, and a mist coat on new plaster is a third. Multiply after calculating the area, not before.
8. Round up to whole units.You buy whole tins, bags and packs. Round up and keep the surplus for repairs — matching a batch later is often impossible.

Tips

• Keep the offcuts and any surplus, labelled, especially for tiles and flooring. Ranges are discontinued constantly and a repair later needs a match.
• Many merchants take back unopened surplus. Ask before buying — it changes how much you round up.
• Photograph your measurements and calculations. You will need them again when something needs replacing.

Common mistakes

• Mixing units mid-calculation — Centimetres and metres in the same sum produces an error of a factor of a hundred, and it is not always obvious in the answer.
• Forgetting the second coat — Area calculations give the surface, not the paint. Two coats is double, and a mist coat on new plaster adds a third.

Questions people ask