Garden & Landscaping Calculators

What the garden and landscaping shelf answers

Outdoor jobs live or die on the same question as indoor ones: how much material does the space actually need? A lawn measured in square metres becomes rolls of turf, a fence line measured in metres becomes posts, panels and bags of postcrete, and a border measured as an area and a depth becomes tonnes of gravel or bags of mulch. This shelf covers the ground-up work: turf, fencing, decking, and the beds and borders that sit between them. Two calculators are live already, the Gravel Calculator and the Mulch Calculator, both built around area times depth, and a turf calculator, a fence calculator and a decking calculator are being built next to cover the rest of the outdoor list.

Turf and lawns

A new lawn is usually priced by the square metre of turf, sold in rolls that cover roughly one square metre each, so the starting figure is simply the lawn area with the beds, paths and trees cut out. The part people underestimate is wastage: a lawn is rarely a clean rectangle, and every curve, corner and obstacle produces offcuts, so budgeting 5 to 10% extra on top of the measured area is the difference between finishing the job and making a second trip for one more roll to cover a gap. Turf itself is perishable once it is cut, called sod once it is sold by the pallet rather than the roll in the US, and it should go down within 24 to 48 hours of delivery. Rolled turf left stacked in the sun starts yellowing and drying out from the inside within a day, so measure the area and order it before the lorry arrives, not after.

Fencing

Fence posts follow a rule of thumb worth knowing before you price anything: roughly a third of the post's total length should be below ground. A common 1.8m panel fence typically runs on 2.4m posts, which puts about 600mm to 750mm of each post in the hole, concreted in rather than just backfilled with soil, because a post that can rock in its hole is what eventually leans a fence line. Post spacing is set by the panel width you buy, most commonly 1.83m centre to centre for a standard 6ft panel, and end posts and corner posts usually want a slightly larger post section or extra concrete because they carry more load than a mid-run post. Once you know how many post holes the run needs, the Concrete Calculator on the building materials shelf will work out the volume and bag count for setting them, using its post hole option rather than the slab option.

Decking

Decking is bought two ways at once: boards for the surface, measured by area with a waste allowance for the gaps and cuts around the edges, and a substructure of joists underneath, spaced to suit the board and the load it needs to carry. The Square Footage Calculator on the inside-the-home shelf handles the area side of that sum for an L-shaped or multi-level deck the same way it handles an L-shaped room, added up section by section. Ground clearance and drainage fall matter as much as the area: a deck built flush to the soil with nowhere for water to run traps damp against the joists, which is a maintenance problem long before it is a materials one.

Beds, borders and ground cover

Gravel, mulch, sand, ballast, topsoil and bark are all bought the same way, area times depth converted into a weight or a volume, but they are not interchangeable in that sum because they weigh different amounts for the same volume. A cubic metre of sand is considerably heavier than a cubic metre of bark, so the same border needs a different tonnage depending on what is going into it, which is exactly what the Gravel Calculator accounts for by letting you choose the material before it converts your area into an order. The Mulch Calculator handles beds and borders the same way and rounds the answer up into the bag sizes garden centres actually stock, 2 cubic feet, 60 litre, 70 litre and 100 litre bags, or a bulk equivalent.

Small bags make sense for a border you're topping up by wheelbarrow, but the economics flip once the order gets large. Past roughly ten 25kg bags, which is about a quarter of a tonne, a bulk bag or a loose tipped load usually costs less per tonne and saves a stack of trips to the merchant. The trade-off is access and storage: a bulk bag needs somewhere flat to sit and often a crane offload, which a narrow side return or a shared driveway can rule out, so check access before the price alone decides it.

Getting the quantities right

Measure the space as a set of simple shapes rather than one clever figure. A lawn with a curved edge, a border that wraps round a patio, or a fence line with a dog-leg all break down into rectangles, triangles and circles you can measure with a tape and add together, and that is more reliable than pacing out one irregular outline and guessing. Decide the depth deliberately before pricing anything: a 50mm mulch layer looks similar to a 75mm layer on paper but is half again as much material, and depth is invisible once the job is done, so it is the easiest place to under-order without noticing until the bag runs out. Build wastage in rather than adding it as an afterthought, 5 to 10% for turf and gravel-type materials to cover trimming, cutting and settling, and buy perishable materials, turf and screened topsoil especially, close to the day you plan to lay them rather than storing them on site for a week. Finally, check delivery access for anything bought in bulk: a lorry with a crane arm or a tipper needs a way in, and finding that out after the load has been booked is a more expensive mistake than the extra cost of small bags would have been.

Where to go next

Once the ground is prepared, most gardens need something structural on top of it, a patio base, a shed base or a set of steps, which is the Building Materials shelf's job, starting with the Concrete Calculator. If the project also touches the inside of the house, new flooring leading out to a garden door, for instance, the Inside the Home shelf picks up from there. For anything that starts with a skip on the driveway or a full garden clearance, see Big Jobs & Projects. The Home & Garden hub lists every shelf if you're not sure which one your job belongs to.

Frequently asked questions

How much extra turf or gravel should I order for wastage?

Around 5 to 10% on top of the measured area covers the offcuts from curves, edges and obstacles on most gardens. Larger, simpler shapes need less; a garden full of borders, trees and odd corners needs more.

How deep should a fence post go in the ground?

Roughly a third of the post's total length is the standard rule of thumb, so a 2.4m post for a 1.8m panel fence wants about 600mm to 750mm below ground, set in concrete rather than just backfilled soil.

Should I buy aggregates in small bags or bulk?

Run the numbers with the Gravel Calculator, which shows both. As a rule, once you're past around ten 25kg bags, a bulk bag or loose delivery is usually cheaper per tonne, provided the site has room and access for it.