Multiply the length and width of your bed in metres to get the area, then multiply by the depth in metres. A bed of ten square metres at seventy-five millimetres deep needs about 0.75 cubic metres of mulch. Most garden beds suit a depth of roughly fifty to one hundred millimetres.
Standing in the garden with a tape measure, trying to work out whether that is one cubic metre or three, is where most orders begin. Guess low, and you are back for more on Sunday. Guess high, and a pile sits on the driveway for a fortnight. We run gardening and mulch services across Sydney, and this is the question we field most at this time of year.
What follows is a simple way to calculate quantity, the depth that actually does the job, and the mistakes that either waste material or harm plants. November is the month to get it down, before the summer heat arrives.
Work out the area of your beds
Measure the planted area of each bed in metres, work out its area, then add the beds together before you think about volume.
A rectangular bed is length times width. A bed four metres long and one and a half metres wide is six square metres. An irregular bed is easiest to handle by breaking it into rectangles and adding them, rather than trying to be clever about the shape.
For a curved or kidney-shaped bed, treat it as the rectangle that would contain it and take off roughly a quarter. A curved bed sitting inside a four by two metre footprint comes out at about six square metres. For a circular bed, multiply the radius by itself and then by 3.14. A round bed three metres across has a radius of one and a half metres, so it is about seven square metres.
Measure the planted area rather than the whole yard, and write the numbers down bed by bed as you go. A front bed of six square metres, a side strip of four and a back bed of ten give you twenty square metres in total, and twenty is the number you carry into the next step.
Convert area into cubic metres
Mulch is sold by volume, so the area has to be multiplied by depth, and the depth has to be converted from millimetres into metres first. That conversion is the step people skip, and it is the reason an order comes back wrong by a factor of a thousand.
Area in square metres multiplied by depth in metres equals the volume in cubic metres. Fifty millimetres is 0.05 of a metre, seventy-five millimetres is 0.075, and one hundred millimetres is 0.1.
A small courtyard bed of six square metres at fifty millimetres: 6 x 0.05 = 0.3 cubic metres. A larger garden with twenty-four square metres of beds at seventy-five millimetres: 24 x 0.075 = 1.8 cubic metres. That is the whole calculation.
Order a small margin over the number you land on. Mulch settles once it is spread and watered in, and running short on the last bed is a far bigger nuisance than having a barrow load left over for the pots.
| Bed area | Depth | Mulch needed |
| 5 m² | 50 mm | 0.25 m³ |
| 10 m² | 50 mm | 0.5 m³ |
| 10 m² | 75 mm | 0.75 m³ |
| 20 m² | 75 mm | 1.5 m³ |
| 30 m² | 100 mm | 3 m³ |
How deep should mulch be?
Most garden beds suit a layer of roughly fifty to one hundred millimetres, as general practice rather than a fixed specification.
Where you sit in that range depends on the material. Coarser mulches work at the deeper end, because the larger pieces still let light and air move through the layer. Finer material belongs at the shallower end, because it packs down tightly and, once it does, it can shed water across the surface instead of letting it soak in.
Both ends of the mistake cost you something. Too shallow and weed seeds come straight through, and the soil underneath dries out as fast as bare ground, which defeats the point of laying it. Too deep and water struggles to get past the layer to the soil, roots start growing up into the mulch where they dry out quickly, and damp material held against stems and trunks puts the plant at risk of collar rot.
The rule that matters more than the number: keep mulch clear of trunks, stems and the crowns of plants. Leave a gap of bare soil around the base of every tree and shrub, and never pile it against a trunk. A neat ring of mulch heaped up against a trunk is one of the most common ways a healthy tree gets into trouble in a suburban garden.
Treat the depth range as a guide that shifts with the mulch and the planting, not as a measurement to hit exactly.
Bags or bulk?
Bags suit small areas and top-ups, while bulk makes sense once carrying bags would take the whole afternoon.
Bagged mulch is the sensible choice for pots, a single small bed or patching thin spots. It carries through a house, down a set of steps and around the side without a barrow, and whatever is left over stacks in the shed for next time.
Bulk delivery takes over once the volume climbs. The trade-offs are real ones: you need somewhere for the pile to sit, a barrow to move it, and the time to shift it bed by bed before it is in the way.
In Sydney, access usually decides it before volume does. A narrow side passage, stairs down to a back garden, no verge to tip onto or a shared strata driveway can all rule out a bulk drop regardless of how many square metres you have measured. Work out where a pile could physically go before you order one.
Laying it so it works
Mulch only does its job if the ground under it is prepared first.
- Clear the existing weeds, roots and all, rather than burying them.
- Water the bed thoroughly before you cover it.
- Spread the mulch evenly across the bed instead of tipping it in heaps and pushing it around.
- Rake it back from trunks, stems and the base of every shrub.
- Water again once the layer is down, to settle it.
- Check the depth with your hand in a few spots rather than judging it by eye.
Weeds come out first because a layer of mulch over established weed growth is a short-term fix at best. Anything with a running root system will push straight back through, and you will be pulling it out of fresh mulch within weeks, so it is worth taking the time to clear the weeds first and get them out properly.
Watering before and after matters more than it sounds. Dry soil under a fresh layer stays dry, because mulch is just as good at keeping water out as it is at holding it in.
Topping up, and when to do it
Organic mulch breaks down into the soil over time, which is a feature rather than a fault, so beds need topping up rather than stripping and replacing.
Judging it is straightforward. Push a hand down through the layer in a few spots and feel where the soil starts, look for bare patches showing through, and watch for weed seedlings germinating at the surface, which usually means the layer has thinned past the point of suppressing them.
Before new material goes on, rake the old layer over to break up any crust that has formed, so water can move through it again. If a bed has also collected prunings, matted old growth and weed material that is past being turned back in, clear that out and have someone take the old material away rather than burying it under the new layer.
The weeks before summer are the most valuable time to do this in Sydney, because that is when the layer is doing its hardest work holding moisture in the soil.
Frequently Asked Questions
How do I calculate how much mulch I need?
Multiply the bed’s length by its width for the area in square metres, then multiply that by the depth converted into metres. A bed of eight square metres at seventy-five millimetres works out at 8 x 0.075, or 0.6 cubic metres. Order a small margin extra, since mulch settles once it is watered in.
How many square metres does a cubic metre of mulch cover?
It depends entirely on the depth you lay it at. One cubic metre spread at fifty millimetres covers about twenty square metres. The same cubic metre at one hundred millimetres covers about ten. Depth is the variable you control, so decide the depth first and the coverage follows from it.
Can you put mulch on too thick?
Yes. An excessively deep layer stops water reaching the soil underneath, encourages roots to grow up into the mulch where they dry out, and holds damp material against trunks and stems where it can cause rot. Keep the layer within the usual range and always keep it clear of trunks.
Should I remove old mulch before adding new?
Normally you top up rather than remove it, because organic mulch breaks down into the soil and improves it as it goes. Rake the old layer first to break up any crust that has formed on the surface, then add enough new material to bring the depth back up to where you want it.
When is the best time to put mulch down in Sydney?
Late spring, before the summer heat arrives, is the most valuable time, because the layer then holds soil moisture through the hottest months. That said, it can go down in any season. Whenever you do it, water the soil thoroughly before covering it, or it will stay dry underneath.
The short version
The arithmetic is simple once you convert the depth into metres; the depth matters more than the exact quantity, and keeping the layer off trunks and stems matters more than either.
If you would rather the beds were measured, cleared and covered properly before summer, the team at A1 Gardening & Landscaping Sydney has been looking after Sydney gardens for more than 15 years and can work out the volume, bring it in and lay it at the right depth. Talk to us about what your beds need, or get in touch to arrange a time.
