Why garden beds get under-ordered
Soil, mulch and gravel are all sold by volume, but a garden bed is measured by eye, and the two don't line up as neatly as people expect. A bed that looks like it needs "a few bags" often needs a lot more once the actual area and depth are worked out properly, which is how a Saturday morning job to top up the garden turns into three separate trips to the hardware store. Getting the volume right before you buy saves the second trip and, just as often, saves you from buying too much and having half a cubic metre of leftover mulch sitting in a tarp on the driveway for a month.
This calculator works out cubic metres and litres for a rectangular bed at whatever spread depth you choose, and compares buying in bulk against buying by the bag.
Measuring a bed properly
Length and width are straightforward with a tape measure, but a garden bed is rarely a perfect rectangle. Measure the longest length and the widest width and use those, rather than trying to average a curved or tapering edge in your head. That will slightly overstate the volume for a bed with rounded corners, which is the safer direction to be wrong in, since a little leftover soil or mulch is far less annoying than a bed that's two-thirds covered because you ran out.
If a garden bed wraps around a corner of the house, curves around a tree, or is really two separate beds joined by a narrow path, break it into rectangular sections and run each one through the calculator separately, then add the totals. It takes a few extra minutes and gives a much more honest number than trying to force an irregular shape into one length and width figure.
Spread depth is the number that actually varies
The length and width of a bed rarely surprise anyone. The depth is where estimates go wrong, because the right depth depends entirely on what you're spreading and what it's doing. A thin refresh layer on established mulch is a different job to laying mulch on bare soil for the first time. A gravel path that needs to stay put under foot traffic wants more depth than a decorative scatter over the top of soil. Read the guidance on the specific product you're buying, whether that's on the bag, on the supplier's website, or from the yard staff when you order, rather than assuming one depth suits every material and every job. That's exactly why this calculator asks for a spread depth rather than assuming one on your behalf.
Measure depth as the actual thickness of the layer you're adding, not the total depth of soil or mulch already in the bed. If you're topping up an existing bed, only the new layer counts toward the volume you need to buy.
Soil, mulch and gravel behave differently
The three materials this calculator covers aren't interchangeable in how they settle or spread, even at the same nominal depth. Loose mulch compacts noticeably in the weeks after it's laid, especially after rain, which is part of why a wastage allowance is worth building in rather than ordering the bare minimum. Soil compacts too, particularly if it's a lighter garden mix rather than a heavier loam. Gravel is the most stable of the three once it's down, but it's also the heaviest by volume, which matters more for how you move it around the yard than for how much you order.
If you're not sure which product is right for the job, whether that's a particular mulch for water retention under a hot northern aspect, a soil mix suited to natives versus a vegetable bed, or a gravel size that won't wash out on a slope, that's a question worth putting to the yard or nursery you're buying from. They deal with the same local conditions every day and will generally steer you toward whatever works in your soil type and climate, at no extra cost to you.
Why the wastage allowance matters here specifically
Ordering the exact calculated volume and nothing more is one of the most common ways a garden job falls short. Some material is lost to an uneven bed base, where low spots soak up more than the calculated depth and high spots get less than planned. Some is lost to edging, where mulch or gravel spills over a garden edge that isn't quite tall enough to contain it. A wastage allowance of around ten percent covers most of this for a straightforward, reasonably flat bed. Push it higher for a bed with a lot of edges and corners, a sloped bed where material tends to migrate downhill after rain, or if you're laying gravel over a surface that isn't compacted and level, since gravel settles into an uneven base more than mulch or soil does.
Bulk versus bags
The crossover point between buying bags and ordering bulk by the cubic metre depends on the specific prices at your local supplier, which this calculator deliberately doesn't guess at, since prices vary by region and by season and any figure quoted here would be wrong somewhere. As a rule of thumb, though, once a job needs more than a handful of bags, it's worth ringing a landscape supply yard and asking what a bulk delivery would cost for the same volume. Bulk delivery usually needs vehicle access to tip the load, either straight into the garden bed or onto a tarp on the driveway for barrowing in, so check that access is workable before you commit to it over bags.
Bags are the better call for a small top-up, a bed you can't get a delivery truck near, or when you want an exact, known quantity without a tip pile to manage. The bag size field in this calculator exists because bag sizes genuinely differ, 25 litres and 30 litres are both common, and the number printed on the bag you're actually buying is always more reliable than a general assumption.
What to ask a landscape supplier
Once you've got a volume figure, a supplier still needs a few more details to give you an accurate price and delivery slot: whether you want the load tipped on a driveway or wheelbarrowed directly into the bed, whether there's truck access to the site at all, and what day you need it. It's also worth asking whether the cubic metre price they quote is loose volume or compacted volume, since a cubic metre of loose mulch and a cubic metre of compacted soil aren't quite the same thing once they're on the truck versus once they've settled in the bed.
A worked example
Say you've got a garden bed running along a fence line, 6 metres long and 1.2 metres wide, and you want to lay 75mm of mulch over it. Plan area is 7.2 square metres. At 75mm depth, that's 0.54 cubic metres before any allowance. Add a 10% wastage allowance for the uneven bed base and the edging along the fence, and you're at roughly 0.59 cubic metres, which comes out to about 590 litres. At 25 litres a bag, that's 24 bags, which is well past the point where a bulk delivery is worth pricing against buying bags one at a time off a pallet.
Run the same bed at 50mm instead of 75mm, a common depth for a light refresh rather than a first application, and the volume drops to around 0.4 cubic metres with the same allowance, closer to 16 bags. The point of walking through the numbers isn't the exact figures, which will be different for your bed. It's that a fairly modest change in spread depth moves the total by enough to change whether bags or bulk is the sensible option, which is exactly why this calculator treats depth as something you set rather than something it assumes.
Common mistakes that leave a bed under-covered
The most frequent one is measuring the bed once, early in the planning, and then not rechecking it once the actual edging or garden layout is finished. A bed that grew a bit wider once the edging went in, or a second small bed added at the last minute near a letterbox or a side path, is easy to forget when you're working from an old measurement rather than the finished layout.
The second is treating every part of a bed as the same depth when it clearly isn't. A bed with a raised garden border holds mulch or soil deeper near the edge and shallower toward the middle if the surface slopes at all. Rather than guessing at one number, measure the shallow and deep points and use something close to the middle, the same approach that works for an uneven slab or garden edge elsewhere on this site.
The third is forgetting that mulch settles. A freshly spread 75mm layer can compact down noticeably within a few weeks, especially after decent rain, which is a good reason not to trim the wastage allowance to zero even on a straightforward, flat bed.
When it's more than a weekend job
Topping up an existing bed or mulching a new one is a job most people handle themselves with a wheelbarrow and a weekend. A full landscaping job, retaining a sloped bed before it can hold soil at all, or a large-scale garden redesign involving drainage and grading, is a different scale of work and usually needs a licensed landscaper, particularly if retaining walls or significant earthworks are involved. Victorian councils generally require a permit for retaining walls over a certain height, so it's worth checking with your council before any structural work goes in, not after.
If you run a landscaping or gardening business, our page on websites for landscapers covers what makes a trade site convert a search like this one into a real enquiry, and our cubic metre calculator handles the more general version of this same maths for anything that isn't a garden bed. Our full set of calculators covers a wide range of other figures a trade business runs on.