How the mulch and soil calculator works
There are three tabs:
- Garden bed: enter the size and shape of the bed, choose the material (bark mulch, compost, topsoil, garden soil mix, pea gravel or decorative stone), and set the depth. You get the volume, the number of bags, and the weight.
- Raised bed: enter the size and height of the bed and how much space to leave at the top. You get the soil volume and a shopping list for the mix you choose.
- Tree rings: the mulch for a ring around each tree, with a gap left round the trunk.
Volumes include 5% extra for uneven ground and settling. You can use US units (cubic yards and cubic feet) or metric (cubic metres and litres).
The formula
Volume is the area multiplied by the depth. In US units, cubic yards equal the area in square feet multiplied by the depth in inches, divided by 324. For example, 80 sq ft at 3 in deep is 240 divided by 324, which is 0.74 cu yd. One cubic yard is 27 cubic feet, so it fills about 13.5 bags of 2 cu ft.
How deep should it be?
| Material | Typical depth |
|---|---|
| Bark or wood mulch | 2 to 3 in (5 to 8 cm) |
| Compost, as a top dressing | 1 to 2 in (2.5 to 5 cm) |
| Topsoil, to level or build a bed | 3 to 6 in (8 to 15 cm) |
| Garden soil mix for new beds | 6 in (15 cm) |
| Pea gravel or stone, over fabric | 2 to 3 in (5 to 8 cm) |
Too little mulch lets weeds through and dries out quickly. Too much, more than about 4 in, holds water against the roots and keeps out air.
Bags or bulk?
Bags are easy to carry and good for small jobs. They are usually 1 to 3 cu ft, and cost more per cubic foot. Bulk, measured in cubic yards, is cheaper for more than about 2 cu yd (roughly 50 bags), but it is delivered as a pile in your driveway, so you need a wheelbarrow and a tarp. One cubic yard of mulch weighs about 700 lb, and soil weighs about 2,000 lb, so check what your vehicle can carry.
Filling a raised bed
A popular mix is 60% topsoil, 30% compost and 10% aeration material like perlite. A simpler mix is half topsoil and half compost. Leave 1 to 2 in at the top so water does not run over the edge. Soil settles by 10 to 20% in the first year, so keep some compost to top it up each spring.
In a deep bed of more than 12 in, fill the bottom third with logs, branches and leaves. They take up space, rot slowly and feed the soil, which cuts the amount you need to buy.
Mulching around trees
Spread mulch 2 to 4 in deep in a ring at least 3 ft (1 m) across for a young tree, and wider as it grows. Keep it 3 to 6 in away from the trunk. A mound of mulch piled against the bark, sometimes called a mulch volcano, traps moisture and causes rot and pests.
Worked examples
A 10 × 8 ft bed with 3 in of bark mulch
The bed is 80 sq ft, and 3 in of mulch is 20 cu ft. With 5% extra that is about 0.78 cu yd, or 11 bags of 2 cu ft. It weighs about 0.27 tons.
An 8 × 4 ft raised bed, 12 in high
With 2 in left at the top, the soil is 10 in deep, which is 26.7 cu ft, or about 1.04 cu yd with 5% extra. For the standard mix you need about 12 bags of topsoil, 6 of compost and 2 of perlite, in 1.5 cu ft bags.
Three tree rings, 36 in wide
Each ring has a 6 in gap round the trunk and 3 in of mulch, which is about 1.8 cu ft. For three trees you need about 0.2 cu yd, or 3 bags of 2 cu ft.
Common mistakes
- Piling mulch against the trunk or stems. It causes rot.
- Applying it too thick. More is not better.
- Forgetting to weed first. Mulch smothers new weeds, but not established ones.
- Measuring the bed without the curves. Measure the length and width at their widest, and use the average for an uneven bed.
- Buying topsoil as a raised bed mix. Pure topsoil compacts. Mix in compost and aeration material.
Tips
- Lay landscape fabric or cardboard under gravel or stone, to stop weeds and stop it sinking.
- Use an edging to keep mulch in the bed and out of the lawn.
- Mulch in spring, after the soil has warmed, and top up in autumn.
- Check the product’s label, because bag sizes and weights vary.
