Outdoor project calculators

Outdoor jobs are measured differently from rooms. Beds and paths are areas with a depth, a lawn is an area with a seeding rate, and a deck or fence is a set of boards, posts and rails. Each outdoor calculator asks for the measurements that job needs and the values on the bag, the seed box or the supplier's sheet, then works out the volume or quantity, adds the allowance you choose and rounds up to whole bags, cans or bulk loads.

Bulk materials such as mulch, soil and gravel can be bought in bags or in bulk units; enter the sizes you can buy and the planner shows how many of each you need and which option costs less. Stain and exterior paint use the coverage on the can and the number of coats, like the indoor paint calculator.

The numbers are planning estimates. Check the supplier's figures for your product, and ask a professional for anything that bears weight, drains water or needs a permit.

Which outdoor calculator to start with

For beds, borders, paths and raised beds, the mulch, soil and gravel calculator turns an area and a depth into a volume and then into bags or bulk loads. For a lawn, the lawn seed, turf and feed calculator works from the area and the rate printed on the seed box or feed bag. For wood that needs staining or sealing, such as decks, railings, fences and patios, use the deck, fence and patio stain calculator, and for the walls of a house use the exterior paint calculator.

Measuring a garden

Split an irregular shape into rectangles and circles, work out each area, and add them. A rectangle is length × width. A circle is 3.14 × radius × radius, so a round bed with a radius of 1.5 m covers about 3.14 × 1.5 × 1.5 = 7.07 m². Measure the depth you actually want to spread, because the depth is multiplied by the area: 5 cm over 10 m² is a very different load from 10 cm. Keep the units straight: depth in centimeters has to be converted to meters (5 cm is 0.05 m) before it is multiplied by an area in square meters.

Worked examples with plain arithmetic

Mulch: a bed of 5 m × 2 m is 10 m². At a depth of 5 cm the volume is 10 × 0.05 = 0.5 m³. With a 10% allowance that is 0.55 m³. If a bag holds 60 liters (0.06 m³), then 0.55 / 0.06 = 9.17, so you buy 10 bags, which is 0.6 m³, leaving 0.05 m³.

Stain: a deck surface of 4 m × 3 m is 12 m². With an example coverage of 8 m² per liter per coat and two coats, you need 12 × 2 / 8 = 3 liters, or 3.3 liters with a 10% allowance. In 2.5 liter cans that is 1.32 cans, so 2 cans (5 liters). Rough or weathered wood usually takes more than a smooth board, so use the lower figure from the label if it gives a range.

Lawn: for 80 m² and an example seeding rate of 35 g per m², the seed is 80 × 35 = 2,800 g. With a 10% allowance it is 3,080 g, so with 1 kg boxes you buy 4. Always use the rate printed for your own seed mixture.

Common mistakes outdoors

The most frequent errors are mixing up centimeters and meters in the depth, reading a bag size in liters as if it were cubic feet or cubic meters, and forgetting that bulk material is sold by volume while bagged material is sold by the bag. Compare both on the same volume, as the mulch calculator does, before you choose. Another is trusting an "up to" coverage figure for stain or paint on rough surfaces, where a best-case number rarely holds. The results here are quantity estimates; for anything that carries weight, drains water or needs a permit, ask a qualified professional.