How to use the custom calculator
- Choose what you're measuring: an area (walls, floors, lawns), a volume (concrete, gravel, soil: an area times a depth) or a length (skirting, edging, cable).
- Add the pieces. Split awkward shapes into rectangles, triangles and circles. Tick Subtract for doors, windows and anything you're not covering. If you already know a figure, choose "I know the area".
- Say how the product is sold, using the numbers on its label: what one pack covers, how many pieces are in a pack, or its weight and how thickly it goes on.
- Add extra for cuts and waste, and the price if you want a cost.
Tap Save this calculator to keep it for next time (it's saved in this browser), Add to job to put it on a shopping list with your other calculations, or Share to send someone the whole thing.
The maths
Reading a product label
| The label says | Choose | Enter |
|---|---|---|
| "Covers up to 12m² per litre" on a 2.5L tin | Each pack covers an amount | 30 (12 × 2.5) m², and the number of coats |
| "1.44m² per box" | Each pack covers an amount | 1.44 |
| "60 bricks per m²", "500 per pack" | By the piece | 60 per m², 500 per pack |
| "25kg covers about 10m² at 2mm" | By weight | 1.25 kg per m² per mm (25 ÷ 10 ÷ 2), thickness 2mm, pack 25kg |
| "Approx. 3kg per m² at 3mm bed" | By weight | 1 kg per m² per mm (3 ÷ 3), thickness 3mm |
Coverage on labels is usually for smooth, ideal surfaces. On rough or porous surfaces, add more extra, or use the lower figure if the label gives a range.
Worked examples
Garage floor paint: the floor is 5m × 3m = 15m², and the label says a 2.5L tin covers 25m² per coat. Two coats is 30m², plus 5% = 31.5m², so 2 tins. Choose "Each pack covers an amount", enter 25 and set coats to 2.
Bark chippings in 70-litre bags: a 3m × 2m border at 75mm deep is 0.45m³, or 450 litres. With 5% extra that's 473 litres, so 7 bags. Measure a volume, enter a 75mm depth, and set the pack to cover 0.07m³ (70 litres).
Lawn edging in 1.2m lengths: 18m of edging needs 15 lengths exactly. With a few offcuts at corners (5%), order 16 lengths. Measure a length and choose "By the piece", with 0.83 pieces per metre (1 ÷ 1.2).
Awkward shapes
Split the shape into simple pieces: an L-shaped room is two rectangles, a curved border is part of a circle, and a gable end wall is a rectangle plus a triangle. Add each one, then add the doors, windows or a pond as pieces and tick Subtract. Sketch the shape on paper first and write each measurement on it, so you don't count any area twice. For areas only, the square metre calculator also handles common shapes and converts feet and inches.
When to use a dedicated calculator
Our specific calculators know things a label doesn't: brick and block joints, bulking of dug soil, grout depth, stock lengths of worktops and skirting, and the extra layers under a patio. If there's one for your job, it will usually be more accurate. Try the paint, tile or gravel calculators, or ask us for one we don't have yet.
Common mistakes
- Mixing up litres and m². Paint labels give m² per litre, so multiply by the tin size to get the coverage of one tin.
- Forgetting coats or thickness. Two coats needs twice the paint, and a 3mm bed needs three times the adhesive of a 1mm one.
- No extra. Rounding up to whole packs isn't the same as allowing for waste. Keep at least 5%.