With mulch, the depth is the decision and the area is arithmetic. Published guidance gives a minimum and an ideal thickness rather than one number, so both are offered here as cited starting points you can change.
A 100 m² border at 50 mm depth, with 70 L bags.
5,000 litres (L)
| Depth | 50 mm |
|---|---|
| Overage | not entered — no allowance added |
| Bag size | 70 L |
| Density | not entered — no weight shown |
This is an example, not your result. It matches golden vector B1 and B3 in the calculation methodology. Enter your own measurements and select Calculate.
With mulch, the depth is the decision and the area is arithmetic. 100 m² at 5 cm needs 5 m³, which is 5,000 litres, or 72 bags at 70 litres each. The same area at 7.5 cm needs 7.5 m³. Half the job is choosing between those two numbers.
The published guidance gives a minimum and an ideal rather than one figure, which is why both are offered above as editable presets rather than baked in as a default.
Do not pile mulch against the stems of woody plants. A collar of damp mulch held against bark keeps the stem wet, which invites rot and gives pests somewhere to work unseen. Leave a clear gap around trunks and woody stems, and mulch trees out to the canopy radius rather than in a neat ring at the base.
Low, mat-forming Mediterranean plants such as thyme are usually better left unmulched altogether: they hold too much moisture around the stems and foliage.
The material selector above changes the labels on your result and nothing else. That is a deliberate design decision: bark, wood chip and other biodegradable mulches occupy volume the same way, and pretending otherwise would mean asserting densities and breakdown rates this site has no source for.
If you would rather use a compost or soil-improver mulch, the compost calculator carries the cited figures for that, in both litres and kilograms.
Every step is arithmetic you can repeat by hand. Full precision is kept through the calculation and rounded once for display.
| Step | Formula |
|---|---|
| Depth to metres | depth m = mm ÷ 1000, or cm ÷ 100, or m × 1 |
| Cubic metres to litres | 1 m³ = 1000 L |
| Net volume | net volume m³ = net area m² × depth m |
| Gross volume after overage | gross volume = net volume × (1 + overage % ÷ 100) |
| Bag count | bags = ceiling(gross litres ÷ bag litres) |
| Mass from a density you entered | kg = gross m³ × density kg/m³, and tonnes = kg ÷ 1000 |
The calculation methodology sets out every formula, bound and rounding rule this site uses.
SourceRHS — Mulches and Mulching
https://www.rhs.org.uk/soil-composts-mulches/mulch
SourceRHS — Organic Matter: How to Use in the Garden
https://www.rhs.org.uk/soil-composts-mulches/organic-matter-how-to-use-in-garden