Home & DIY Calculators

The ideas these tools share

Measure the space, then add waste

Every material calculation starts with an area or volume and then adds an allowance for cuts, breakage and offcuts. The allowance is not padding; it is the difference between finishing the job and making a second trip. Complex layouts and patterned materials need more of it than simple rectangles do.

Area scales as the square, volume as the cube

Doubling the side of a room quadruples the floor area and multiplies the volume by eight. This is why estimates scaled by eye are wrong so reliably, and always in the direction of ordering too little.

Coverage rates assume ideal conditions

The coverage printed on a tin or bag assumes a prepared, even surface. Porous, textured or previously untreated surfaces absorb considerably more, and the first coat almost always covers less than the second.

Quote the number you will actually buy

Materials are sold in boxes, rolls, tins and pallets, not in exact quantities. The useful output is how many units to purchase, which means rounding up to the packaging, not the precise area.

Labour and materials scale differently

Doubling the area roughly doubles the material but rarely doubles the labour, because setup, mobilisation and clean-up are largely fixed. This is why small jobs cost disproportionately more per square foot.

Which one do you need?

You are measuring a room or a whole house

Square footage, which handles L-shapes, triangles and circles as well as rectangles.

You are buying paint

Paint — and note it needs wall area, not floor area.

You are tiling

Tile, which works in whole tiles per row rather than raw area.

You are pouring concrete

Concrete, which converts to cubic yards for ordering.

You are working out running costs

Electricity cost, from appliance wattage and your rate.

Where these go wrong

• Ordering the exact measured quantity

  Cuts, breakage and pattern matching all consume material. Ordering precisely what the area says is the reliable way to make a second trip.

• Scaling an estimate by eye

  Area grows as the square of the dimensions. A room that looks half again as big usually needs more than twice the material of the one you priced.

• Trusting the coverage on the tin

  Printed coverage assumes a prepared, even surface. Porous or textured surfaces absorb considerably more, and first coats always go further on paper than in a room.

• Forgetting materials come in packages

  You buy boxes, rolls and tins, not square feet. Round up to whole units, and check the unit size before comparing two prices.

Common questions