Last updated: 2026-06-16

The amount of mulch or garden soil a bed needs is its area multiplied by the depth of the layer. Getting the depth consistent across the whole bed matters more than the exact area: a layer that is too thin lets weeds through and dries out, while one that is too deep can starve roots of air and trap moisture against stems.

Volume from Area and Depth

The core formula treats the layer as a flat slab. Area times depth gives a volume, with the depth converted to the same length unit as the area:

A bed of 30 m² covered to 7.5 cm uses a depth of 0.075 m, so the volume is $30 \times 0.075 = 2.25\ \text{m}^3$. Because mulch and soil are sold in different units around the world, that figure is more useful once converted: 2.25 m³ is about 2.94 cubic yards or 79 cubic feet.

Converting to Bags

Bagged mulch comes in fixed sizes — commonly 50 litres or 2 cubic feet. The number of bags is the total volume divided by one bag's volume, rounded up, since bags cannot be split:

For the 2.25 m³ bed above, 50 L bags hold 0.05 m³ each, so the job needs $\lceil 2.25 / 0.05 \rceil = 45$ bags.

Bulk versus Bagged

Once a project runs past ten to fifteen bags, bulk delivery by the cubic yard is usually cheaper per unit than bagged material, though it has to be moved by wheelbarrow rather than carried in. Bagged mulch is easier for small beds and stores cleanly between jobs.

Depth and Over-Mulching

A layer of 5–8 cm suits most beds and shrubs. Piling mulch high against a tree trunk — sometimes called a mulch volcano — holds moisture against the bark, invites rot and pests, and can kill the tree over time. Mulch should be kept level and pulled back a few centimetres from trunks and stems. Related estimators include Concrete Volume Calculator for edging and paths and Paint Coverage Calculator for fences and sheds.