Mulch Coverage Calculator
Find how much mulch covers an area at a chosen depth.
Depth is the whole calculation
Mulch volume is simply area multiplied by depth, and the depth is where the decision lies. Below about 5cm, mulch fails at its main job — light still reaches weed seeds and they germinate through it. Above about 10cm it starts to impede water and air reaching the soil, and on heavy ground can keep the root zone waterlogged. Roughly 5–8cm suits most beds; the shallow end of that range is right around shallow-rooted plants, the deeper end for weed suppression on open ground.
A 10m² bed at 7cm needs 700 litres — 0.7 cubic metres, or about ten 70-litre bags. As with soil, bulk delivery becomes markedly cheaper well before you reach that quantity.
Keep it off the stems
Piling mulch against trunks and stems traps moisture against bark, which invites rot, fungal infection, and rodent damage, and can encourage roots to grow into the mulch layer instead of the soil. Leave a clear gap of several centimetres around every stem and pull mulch back from the crowns of perennials. The volcano of bark heaped around a young tree is one of the most common and most damaging garden habits.
Choosing a material
Bark and wood chip last one to three seasons and suit paths, shrubs, and permanent plantings. Straw breaks down within a season and is well suited to vegetable beds, though it often carries weed seed. Compost as mulch feeds the soil while it suppresses weeds but decomposes fastest of all. Gravel and slate last indefinitely and suit drought-tolerant plantings, but they are effectively permanent — removing them later is unpleasant work.
One persistent worry deserves addressing: woody mulch does tie up nitrogen, but only at the thin interface where it meets the soil. Spread on the surface, it does not deplete the root zone. Digging fresh wood chip into soil is what causes a real nitrogen deficit.
How to use it
- Enter the area to be covered and your chosen depth.
- Read the volume in litres, cubic metres, and bag equivalents.
- Keep a clear gap around every stem and trunk.
- Compare bulk delivery against bags — the crossover comes sooner than expected.
Before you plant
How deep should mulch be?
Roughly 5–8cm for most beds. Thinner than 5cm lets light through and weeds germinate; thicker than about 10cm starts to block water and air reaching the soil, which is a problem on heavy ground.
Does wood mulch steal nitrogen from my plants?
Only at the thin layer where it meets the soil surface, which does not affect the root zone. The problem arises when fresh wood chip is dug into the soil rather than spread on top.
Why should mulch not touch stems?
Mulch held against bark traps moisture and invites rot, fungal disease, and rodent damage, and can encourage roots to grow into the mulch instead of the soil. Leave a few centimetres clear around every stem.
Which mulch should I choose?
Bark and wood chip last one to three seasons and suit shrubs and paths; straw breaks down in a season and suits vegetable beds; compost feeds the soil but decomposes fastest; gravel lasts indefinitely but is difficult to remove later.
Disclaimer: These are general planning estimates. Seed packets, product labels, and regional frost, water, and pesticide guidance always take priority over any figure produced here. Local soil, climate, and variety differences can change the result substantially.