How to calculate mulch
Use net area and target depth to choose bag quantity
Back to calculator ↑Define covered area
Measure the bed and subtract paving, paths and other uncovered areas. Keep mulch away from woody stems as gardening guidance advises.
Sketch the actual mulched bed and subtract paths, rocks, paving and any other uncovered patches. Split curved beds into smaller shapes to improve the area estimate. Do not use the whole bounding rectangle when large trunks or planting clusters occupy part of it; mulch should stay clear of woody stems. Measure lengths in metres and enter exclusions in square metres.
Choose depth
RHS discusses at least about 5 cm for organic mulch in garden beds, but material and plants matter. Enter the layer appropriate to your project.
Enter the layer thickness after spreading, not the height of the delivery pile. RHS describes organic mulch around 5 cm or more for garden beds, but the material and plants should guide the choice. Excess depth against stems can retain unwanted moisture. Keep decorative gravel separate from organic mulch in planning because weight, settling and maintenance differ.
Default example
At defaults, 6 × 4 m gives 24 m². Convert 5 cm to 0.05 m, so 24 × 0.05 = 1.2 m³. A 10% allowance makes 1.32 m³ or 1,320 L, exactly 33 bags of 40 L. If 4 m² is excluded, use 20 m² at the start of the volume calculation; do not subtract bags after a rounded purchase count.
Check the site
Organic materials can settle, and bag volume may be a loose measure. Check the label and the space around plant bases before ordering.
Compare actual litres per bag, not only shelf price; smaller bags can cost more per litre. For a loose delivery, use the allowed cubic-metre volume rather than the bag count and ask whether the quoted volume is loose or placed. Organic mulch settles and decomposes over time, so plan for later topping up. Optional entered price is bag material only, excluding delivery and spreading.
Common questions
How many metres is 5 cm?
Divide 5 by 100 to obtain 0.05 m, then multiply square metres to get cubic metres. Treating 5 cm as 5 m would overstate volume by 100 times.
Can mulch touch trunks?
Leave a clear ring appropriate to the plant and garden guidance; do not pile mulch against bark where moisture can accumulate. Enter that clear area as an exclusion.
Does the estimate include delivery?
The cost multiplies whole bags by the entered bag price only; delivery and labour are separate. Leaving price blank preserves the quantity estimate without inventing a cost.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.