Key Takeaways
- Measure the area or volume the job actually covers, in one consistent unit.
- Add a waste allowance you have chosen on purpose, and write it down so the number can be checked.
- Convert the total to the unit you buy, then round up to a whole purchase unit.
Measure the area or volume
Start with a plan of the space. For a flat area such as a floor, wall or bed, multiply length by width. Use the same unit for both sides: metres with metres, or feet with feet. For a material laid to a depth, such as gravel, mulch or concrete, multiply length, width and depth. The depth must be in the same unit as the other two measurements: 10 cm on a 3 m by 2.5 m patio is 0.1 m, not 10.
Measure each separate area and add the results. Subtract an opening only when the material does not go into it, such as a window in a wall you are tiling. Measure twice and write each figure down with its unit. Adding centimetres to metres is one of the easiest ways to get the total wrong.
Add a waste allowance
Every job uses up some material in cuts, breakage, spillage and offcuts that cannot be reused. Add an allowance on top of the measured quantity. The right percentage depends on the material and the layout: a straight, simple layout needs less than one with diagonal cuts or many small pieces. Where the manufacturer publishes a recommendation for the product, use it.
Write the allowance down as a percentage and keep it visible. For example, a 40 m² floor with a 10 per cent allowance needs 44 m² of tile. The allowance is a choice you make, not a property of the material, so state it in your order notes.
Convert to the unit you buy
Materials are sold in different units: tile by the box or by the square metre, gravel and mulch by the bag or by the cubic metre or cubic yard, concrete by the bag or by the cubic metre. Convert your total into that unit before any other step. One cubic foot is about 0.0283 cubic metres, and one cubic yard is about 0.765 cubic metres.
Check the coverage or yield printed on each bag or box. A label that states the area or volume covered at a given depth gives the count directly: divide the total by the yield and round up.
Round up and order
Round up to a whole number of purchase units. 4.2 boxes means an order for five boxes, not four. A shortfall halfway through a job means a second delivery and possibly a different production batch, which is harder to handle than one spare box. Keep the leftover, because a later repair may need it.
Before you order, check the supplier's minimum order and how it sells the product, for example by the pallet. Keep a record of the figures: the measurements, the allowance, the conversion and the order. That record makes a top-up or a return straightforward.
Check the result before you buy
Check the largest number first. A wrong depth, an area counted in the wrong unit or a forgotten room affects the order far more than a small rounding choice. Compare the result with a second, independent estimate, such as one from a calculator and the figure on the product label.
The following calculators apply these steps, one material each: Tile Calculator, Gravel Calculator, Mulch Calculator and Concrete Volume Calculator.