Mulch Calculator

Enter the area and mulch depth to see how much mulch to buy in cubic metres, bales, bags and trailer loads, and which way of buying it costs least.

Mulch is one of the cheapest ways to save water, suppress weeds and feed the soil, but it is bought in confusing units: bales, bags of various sizes, or cubic metres. Working out the area and depth first, then translating that into whatever the supplier sells, avoids both a second trip and a pile of leftovers.

Mulch Calculator

Choosing a mulch type loads a typical depth, bale size and example prices. They are examples only — bale sizes, bag sizes and prices vary widely, so replace them with the details from your supplier. A price of 0 means “not buying it that way”.

Full guide, more detail, and a printable version at https://hollyshomestead.com.au/tools/mulch-calculator.html

New to mulching? See our mulching guide, the no-dig gardening guide and building healthy soil.

How the Quantity Is Worked Out

Area multiplied by depth gives volume. Ten square metres at 7 cm (0.07 m) is 0.7 cubic metres, or 700 litres. The calculator adds an allowance for settling, uneven ground and the bits that end up where you did not intend, then converts the total into the units you can actually buy: a number of bales, a number of bags of your chosen size, or a bulk order rounded up to the next half cubic metre.

Bales are the awkward one. A bale is sold by weight or by the bale, but you spread it loose, and loose volume varies with how tightly it was compressed and how much you fluff it. The default figures are typical for a small garden bale; check the supplier’s stated coverage and edit the bale volume if it differs.

How Deep Should Mulch Be?

Most coarse mulches work best at 5 to 10 cm. Less than about 5 cm lets weeds through and dries out faster; much more than 10 cm can shed rain, go anaerobic and smother roots. Fine materials such as compost or leaf mulch are laid thinner, about 3 to 5 cm, while chunky woodchip is happy at 8 to 10 cm. Always pull mulch back a few centimetres from stems and trunks so the bark stays dry and does not rot.

Near buildings in bushfire-prone areas, choose and place mulch with your local fire authority’s guidance in mind, because dry organic material close to walls can burn. Settled straw mulches also need topping up each year or two as they break down into the soil, which is a feature as much as a cost; see our mulching glossary entry for the principles.

Bales, Bags or Bulk?

Bales suit straw-type mulches and small beds. Bags are convenient for a few square metres of bark or compost. Once you need more than about half a cubic metre of woodchip, bark or compost, bulk is nearly always far cheaper per litre, and arborists will sometimes drop chip for free if you ask. Compare using the same example units: the calculator shows each option and the cheapest priced one. For feeding the soil beneath, see our compost C:N calculator and soil volume calculator.