🌱 Mulch & Soil Calculator

Area and depth to yards and bags

The depth is converted to feet and multiplied by the bed area for cubic feet. Dividing by 27 gives cubic yards, the unit bulk material is sold in, and dividing by 2 gives the count of standard 2 cu ft bags.

cubic feet  = area(sq ft) x (depth(in) / 12)
cubic yards = cubic feet / 27
bags        = ceiling(cubic feet / 2)

A 200 sq ft bed at 3 in deep is 50 cubic feet: 1.85 cubic yards, or 25 bags. The handy rule behind that is one cubic yard covers 108 sq ft at 3 in deep, or 162 sq ft at 2 in.

Bags or bulk, and how much to add

A cubic yard is 13.5 of the 2 cu ft bags, so the 25 bags above are not quite two yards. Bagged material wins for one small bed and for anything you have to carry through a house; past about two yards, bulk delivery is normally much cheaper per yard, and the saving grows from there.

• Beds are not rectangles. Split a curved bed into rectangles and triangles and add them, rather than measuring the widest point and multiplying.
• Bulk is measured loose in the bucket. A scoop is not a certified yard, and material fluffs up when it is loaded and settles again once it is spread and rained on.
• Add 5 to 10% for what stays in the barrow, spills on the path and gets raked off the edges.

Depth is the input worth thinking about. 2 to 3 in of mulch suppresses weeds and holds moisture; much past 4 in starves roots of air and turns sour underneath. Bark and wood mulch also break down, losing something like half their depth over a year, so a topped-up bed usually needs 1 to 2 in rather than a fresh 3. For a new planting bed, topsoil or compost wants 4 to 6 in of workable depth, which is double the volume of a mulch layer over the same area.

Frequently asked questions