What this calculator works out
This calculator converts an area and a mulch depth into a volume in litres, with an allowance added for uneven spreading.
Mulch depth is the decision that matters. Too thin and it suppresses nothing; too thick against stems and it causes rot.
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Before you spread
- 5 to 7 cm is the usual depth for weed suppression and moisture retention on beds and borders. Below about 5 cm, light reaches the soil and weeds germinate.
- Mulch onto moist soil. Applying it over dry ground keeps the rain out as effectively as it keeps moisture in.
- Keep mulch clear of stems, trunks and crowns — a few centimetres of bare soil around each plant prevents collar rot.
- The extra allowance covers uneven spreading and the awkward corners of a real border.
Annual weed seeds need light to germinate, and a 5 cm layer denies it. Established perennial weeds with deep roots push straight through any depth of mulch, which is why they need removing first. Mulch prevents new weeds; it does not clear existing ones.
How this calculator works
Area times depth, converted to litres:
Litres = area × depth × 10 × (1 + allowance ÷ 100)
One square metre at one centimetre deep is ten litresThe factor of ten comes from that relationship: a square metre one centimetre deep is 0.01 m³, which is 10 litres. Any bulk garden material follows the same conversion.
Worked example: a 20 m² border
Using the default figures — 20 m² at 7 cm deep with a 10% allowance:
- Base volume: 20 × 7 × 10 = 1,400 litres
- With allowance: 1,540 litres
1,540 litres is around two bulk bags, or roughly 26 bags at 60 litres each. Dropping the depth to 5 cm brings it to 1,100 litres — a saving of nearly a bulk bag from two centimetres. Since 5 cm is generally sufficient for suppression, the deeper option costs meaningfully more for little additional benefit.
Common mistakes
- Mulching over dry soil. It keeps rain out as well as moisture in.
- Piling mulch against stems and trunks. It causes rot at the collar.
- Mulching over perennial weeds. They grow through; remove them first.
- Going thinner than about 5 cm. Light gets through and weeds germinate.
- Digging fresh wood chip into the soil. It locks up nitrogen as it breaks down; use it on the surface.
Frequently asked questions
When should I mulch?
Spring is the usual time, once the soil has warmed and while it is still moist from winter. Autumn mulching also works and protects roots over winter. Avoid mulching frozen soil, which locks the cold in, or bone-dry soil, which keeps the rain off.
What material makes the best mulch?
It depends on the purpose. Composted bark and wood chip are long-lasting and good for weed suppression. Well-rotted compost or manure feeds the soil as it breaks down but suppresses weeds less effectively. Leaf mould is excellent for soil structure. Gravel and slate suit dry-garden planting and last indefinitely.
Does mulch really save water?
Yes, substantially. A mulch layer reduces evaporation from the soil surface, which is where most water is lost between rainfall and plant uptake. It also moderates soil temperature and reduces the crusting that makes water run off rather than soak in.
Will wood chip rob nitrogen from my plants?
Only if dug in. Fresh woody material decomposing within the soil draws nitrogen from it. As a surface mulch, the interaction is limited to the top few millimetres and does not affect established plants. Composted bark has largely broken down already and is not a concern either way.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Royal Horticultural Society — mulching guidance, materials and depths
- GOV.UK — peat-free growing media and garden waste guidance
Sources are checked at publication and can change — how I choose and check references.