How Much Mulch Do I Need? Bark Coverage Made Simple
Quick answer: For general use, apply mulch about 50–75mm (2–3in) deep. At that depth, 1 cubic metre of bark mulch covers roughly 13–20 m², or about 1 bulk bag per 10–13 m². As a quick rule, you need around 50–75 litres of mulch per m² at a 50–75mm depth. Deeper mulch suppresses weeds better; too thin and weeds push straight through.
Mulch is one of the most cost-effective things you can add to a garden — it suppresses weeds, holds moisture in the soil, moderates temperature, and feeds the soil as it breaks down — but it only works if you lay it deep enough. Too thin a layer lets weeds through and dries out; too thick against plant stems can cause rot. This guide gives coverage figures, the right depth, what to use, and how to work out your quantity. Use the calculator above for your exact amount.
The right depth for mulch
Depth is what makes mulch work. Too thin and weeds grow straight through; too deep around plant stems and you risk rot. For borders and around shrubs, 50–75mm (2–3in) is the general sweet spot — enough to suppress weeds and hold moisture. For heavier weed suppression on open ground, go to 75–100mm (3–4in). Always keep mulch clear of stems and trunks, leaving a small gap so bark doesn’t sit against plant bases and cause rot.
For most gardens, aim for 50–75mm across the bed, tapering to nothing right at the base of each plant.
Coverage figures
At common depths, rough coverage:
| Depth | Coverage per m³ | Approx. per m² |
|---|---|---|
| 50mm (2in) | ~20 m² | ~50 litres/m² |
| 75mm (3in) | ~13 m² | ~75 litres/m² |
| 100mm (4in) | ~10 m² | ~100 litres/m² |
Bark mulch is usually sold in litre bags (large ones around 70–100 litres) or in bulk bags. One bulk bag typically covers 10–13 m² at a 75mm depth, making bulk the economical choice for larger borders.
Working out your quantity
It’s two steps: measure the area to be mulched, then multiply by your chosen depth to get the volume. For a border of 8 m² at 75mm deep: 8 m² × 0.075m = 0.6 m³ = 600 litres of mulch.
For irregular borders — which is most of them — break the shape into rough rectangles, work out each area, and add them together, the same approach that works for any awkward shape. Then multiply the total area by your depth. Round up so you have enough to finish the bed evenly.
Bark vs other mulches
“Mulch” covers several materials, all applied similarly. Bark and wood chip are long-lasting and attractive, great for borders and paths, and break down slowly over a couple of years. Composted mulch or well-rotted manure feeds the soil faster as it breaks down, but decomposes sooner and needs replacing more often. Gravel or stone acts as a permanent mulch for gravel gardens, calculated the same way by area × depth but sold by weight rather than volume.
Whichever you choose, the volume calculation is the same — area × depth — though stone mulches convert that volume to tonnes via density. For most ornamental borders, bark strikes the best balance of looks, longevity, and cost.
A membrane under bark?
Under bark on a border you plant into, a non-woven membrane can help suppress weeds while still letting water and air through to the plants — cut crosses to plant through it. On a purely decorative area with no planting, a membrane keeps the bark from mixing into the soil. For beds you actively garden and replant, many gardeners skip the membrane and simply refresh the bark, since a good depth of mulch suppresses most weeds on its own.
Top up, don’t over-order
Organic mulch breaks down and thins over a year or two, so borders need topping up periodically rather than a one-off deep layer that lasts forever. Adding around 5–10% to your calculated volume covers uneven spreading so you finish the bed without a gap. Plan to refresh bark every one to two years as it decomposes into the soil — which is a feature, not a fault, since it’s feeding the soil as it goes.
Common mistakes to avoid
The main mistakes are laying mulch too thin (so weeds push through), piling it against plant stems (which causes rot), and calculating for the area without accounting for depth. One more: expecting bark to last forever. Organic mulch breaks down and needs topping up every year or two — that’s normal, and it’s part of how it improves the soil.
Frequently asked questions
How deep should mulch be?
About 50–75mm (2–3in) for general use — enough to suppress weeds and hold moisture. For heavier weed control, go to 75–100mm. Always keep mulch clear of plant stems and trunks to avoid rot.
How much does a bulk bag of bark cover?
Roughly 10–13 m² at a 75mm depth. Coverage increases at shallower depths — around 20 m² at 50mm — and decreases at deeper ones. Bulk bags are the economical choice for larger borders.
How much mulch do I need per square metre?
Around 50 litres/m² at 50mm deep, or 75 litres/m² at 75mm. Multiply by your total area, breaking irregular borders into sections and adding them up, then round up so you have enough to finish evenly.
How often do I need to replace mulch?
Organic mulches like bark break down over one to two years and need topping up. Gravel and stone mulches are effectively permanent. The breakdown of organic mulch is beneficial — it feeds the soil — so a periodic top-up is expected rather than a problem.
Can I put mulch straight onto soil?
Yes — bark and organic mulch go straight onto the soil surface around plants. On purely decorative, unplanted areas a membrane underneath keeps the mulch from mixing into the soil. On planted borders, a good depth of mulch suppresses most weeds on its own.
What’s the best mulch for a garden border?
Bark or wood chip strikes the best balance of appearance, longevity, and cost for ornamental borders. Composted mulch and well-rotted manure feed the soil faster but need replacing sooner. Choose bark for looks and low maintenance, composted material for feeding.