Mulch coverage

Beds, bags and the depth a dressing settles into

The arithmetic is a prism: bed area times depth, in metric throughout. Because the depth is entered in metres, an ordinary dressing is a small decimal — the default corresponds to seventy-five millimetres, a conventional starting point the pack supplies rather than a rule. A layer spread for looks and a layer spread thickly for weed suppression are different decisions, and the depth input is where that difference is made.

One arithmetic serves several materials. Mulch, topsoil, decorative gravel and sand all obey the same area-times-depth honesty; what changes between them is the depth the job calls for and the package the merchant sells. The calculator holds the geometry constant and lets those two inputs carry the material’s identity.

The bag volume is the merchant speaking. The default stands in for the common fifty-litre bag of the garden-centre shelf, but the same input stretches to bulk sacks of a cubic metre or more — enter the volume printed on whatever the material actually ships in, because the count divides by exactly this figure.

The count rounds up, always. No merchant splits a bag, so the loose volume is taken to the next whole one — and the part-bag left over at the end is not an error but the top-up every bed wants after the first settling. Buying in larger sacks shrinks the count but enlarges that remainder; the arithmetic makes the trade visible.

What the figures leave out is the bed’s preparation and its future. Edging, membrane beneath a gravel dressing, and the slow settling that thins a mulch layer over a season are outside the take-off; when the layer has sunk, the same calculation runs again at the top-up depth rather than the original one.

When this calculation is used

• Buying mulch, soil, gravel or sand for a bed once its area is measured and a dressing depth chosen.
• Comparing bagged against bulk-sack supply by changing only the package volume over the same bed.
• Topping up a settled bed by running the same area at the thin depth the refresh actually needs.
• Checking whether a deeper weed-suppressing layer fits the budget by watching the count respond to depth alone.

Worked example

The pack’s golden vector: a bed of thirty square metres dressed at seventy-five millimetres, bought in fifty-litre bags — with a companion vector that prices a larger bed in bulk sacks of two cubic metres.

The bed comes back as two and a quarter cubic metres of loose material, and forty-five bags to carry it home in. In the companion vector the same arithmetic meets bulk sacks instead, and the count collapses to a handful — the volume is the bed’s truth, the count merely the packaging’s echo of it.

Every figure in this example is computed by the certified engine at the moment the calculator loads and checked against the signed pack’s declared test vectors; the page keeps no answers of its own. Set the depth to zero and the engine refuses — a dressing with no thickness is not an order — and the pack declares that refusal as a vector too.

What each input represents

Assumptions and limits

• The dressing is one material at one depth across the whole bed; areas dressed differently are separate runs of the same arithmetic.
• The depth is the reader’s horticultural judgement, entered as a value; the page multiplies it and asserts nothing about what a given plant or purpose requires.
• The volume is loose, as-poured material; settling over a season thins the layer, and the top-up is a fresh run at the top-up depth.
• The result is material only: membrane, edging, delivery and the labour of barrowing sit outside it, and bagged and bulk supply differ in price per volume in ways the count alone does not show.

What the guards protect against

• Depth must be strictly positive, and the pack ships the zero-depth refusal as a declared test vector: a layer with no thickness covers nothing, and the engine refuses to dress a bed in it rather than returning an empty volume as if it were advice. The depth is also capped at two metres — past that the entry describes earthmoving, not a dressing.
• The bag volume must be strictly positive and is capped at five cubic metres: the count divides by this figure, so a zero would be a division the purchase cannot mean, and anything larger than the cap is a delivery vehicle, not a bag.
• The bed area must be strictly positive and is capped at a hundred thousand square metres, so an empty measurement or a whole-park total is turned away before it becomes a haulage contract.

Provenance

Standard landscaping take-off arithmetic (loose-volume convention)

Loose volume as the bed area times the dressing depth; the bag count as that volume divided by the volume of one bag and rounded up to whole bags.

Quantity reference to be verified against the bed and the product actually bought; planting advice and material suitability are never determined here. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.