Mulch calculator

Ask a mulch calculator how much you need and it asks for an area and a depth. Three inches, it says, and multiplies. That is the right answer exactly once — the first year, on a bare bed.

Every year after that, there is still mulch in the bed. An organic mulch loses roughly an inch a year to decomposition, which means a bed mulched to three inches last spring has about two inches left this spring. It wants one more inch, not three.

That is not a rounding difference. It is the difference between fifteen bags and forty-five, every year, on an ordinary garden — and it compounds in the wrong direction, because mulch that keeps being added without being measured ends up four, five, six inches deep, at which point it has stopped helping and started causing harm.

And the decomposition is the point, not a defect. A bark mulch breaking down into the soil is doing the job — feeding soil life, improving structure, holding moisture. Something that never breaks down never does any of that, which is the honest case against rubber.

Mulch has the tidiest arithmetic on this site, and the whole of it comes from one number: a cubic yard is 27 cubic feet, so at one inch deep it covers 324 square feet.

Three conversions cover essentially every question asked about mulch quantities. 13.5 bags make a cubic yard, because 27 divided by two is thirteen and a half. About 65 bags make a pallet, which is around 4.8 cubic yards. And one yard covers 324 square feet an inch deep, from which every other depth follows by division.

Bagged mulch costs roughly half as much again per cubic yard as bulk. That sounds decisive, and for a small garden it is decisive the other way — because bulk arrives on a truck and the truck charges a fee that does not care how much is on it.

So there is a break-even volume, and it is simple arithmetic: the delivery charge divided by what a yard costs extra in bags. For ordinary hardwood mulch that lands around 4.1 cubic yards — below it, bags; above it, bulk, and the gap widens fast.

Mulch works by covering soil: it shades out germinating weed seed, slows evaporation, and evens out the temperature swing at the surface. All three of those jobs are done by about three inches.

Below two inches, light reaches the soil and weeds come through. Above about 4, three separate things go wrong. Water stops soaking through and starts running off the surface, especially once a crust forms. The soil beneath goes anaerobic, because gas exchange through six inches of packed bark is poor. And roots — which follow air and moisture — grow upward into the mulch layer itself, where they dry out and die the first hot fortnight.

Deep mulch is not generous. It is a slow way of suffocating a bed.

The practical choice is usually between shredded and nugget, and it is about your ground rather than your taste. Shredded bark knits together as it settles and stays where it is put, including on a slope. Nuggets last longer and look tidier, and they float — after a heavy storm they are at the bottom of the garden.

Stone is the other family entirely, and it belongs to the gravel calculator: it never breaks down, it is five times the weight, and it is a permanent decision rather than an annual one.

Landscape fabric under bark mulch is one of those things sold alongside the mulch that makes the bed worse. It is worth being specific about why, because it does work under stone.

Weeds in a mulched bed do not mostly come up through the mulch from below. They germinate from seed that lands on top, in the layer of decomposed mulch that builds up over a couple of years — which is, by then, quite good compost sitting on a sheet of plastic. The fabric is underneath all of that and stops nothing.

Meanwhile it does three things you did not want. It separates the decomposing mulch from the soil it was supposed to be feeding. It makes the bed effectively undiggable, so planting anything means cutting the fabric. And after a few years it starts working its way to the surface in ragged grey strips that cannot be removed without lifting everything.

Under stone, none of that applies: stone does not decompose, so the fabric stays clean and does its actual job of keeping stone out of soil.

Mulch is light for its volume, which is the one thing that makes it a realistic DIY material. A cubic yard of shredded hardwood weighs about 450 pounds dry — against something like 2,800 for a yard of gravel, and over 4,000 for a yard of concrete.

The catch is rain. Soaked hardwood runs to around 800 pounds a yard, close to double, and mulch yards store it outside. A yard that a half-ton pickup handles comfortably in August is over the axle rating after a wet week, and nobody at the yard will tell you so.

Depths, the decomposition rate and the trunk clearance come from university extension horticulture guidance, taken at the middle of the published ranges. Bag and pallet counts, bulk pricing and the delivery charge come from a survey of what yards and big-box stores actually charge, with both packagings priced so the crossover can be worked out rather than guessed. The volume arithmetic is exact — 27 cubic feet to the yard, twelve inches to the foot — and everything is on the sources page, dated.

Two assumptions worth stating. The depth you enter as already present is subtracted straight from the target, which is the whole point of the page and is why the answer here is usually smaller than elsewhere. And the bag count rounds up to whole bags while the bulk figure does not, because a yard will sell you a part yard and a shop will not sell you part of a bag.

Nothing you type is sent anywhere — the whole calculation runs in your browser.

Cubic yard and top-up are the two that matter: one is the unit, the other is the number most people get wrong.