Tree Mulch Ring Calculator
Mulch for a ring around a tree, from ring radius, trunk diameter, and depth.
Keeps the mulch-free gap at the bark and gives litres, cubic yards, and bags.
Placing mulch around trees is one of the most beneficial things you can do for tree health. A proper mulch ring conserves soil moisture, moderates soil temperature, suppresses weeds, prevents compaction from foot traffic, and adds organic matter as it decomposes.
Ideal Mulch Ring Dimensions
- Ring radius: Ideally reaches the drip line, the point on the ground directly under the outermost branches. At a minimum, 90–120 cm (3–4 feet) out from the trunk.
- Mulch depth: 5–10 cm (2–4 inches) is ideal. Going deeper suffocates roots and holds excess moisture against the trunk.
- Keep away from trunk: Leave a gap of 5–10 cm (2–4 inches) around the bark itself. Piling mulch against the trunk, the practice known as volcano mulching, causes bark rot, disease, and pest problems.
The Formula
A mulch ring is a donut shape (annulus). The volume is: V = π × (R_outer² − R_inner²) × depth
Where:
- R_outer = the outer radius of the ring, measured from the centre of the trunk
- R_inner = where the mulch starts, which is half the trunk diameter plus the mulch-free gap
- depth = mulch depth
That second line is the one worth reading twice. The gap you leave is measured outward from the bark, but the annulus formula needs a radius from the centre of the tree, so the trunk’s own radius has to be added in. On a mature tree with a 40 cm trunk and a 10 cm gap, the mulch does not start until 30 cm from the centre, and treating the gap alone as the inner radius over-orders the mulch by a noticeable margin.
Volume to Bags or Cubic Yards
| Package | Volume |
|---|---|
| Standard bag (2 cu ft) | 0.056 cubic meters |
| Large bag (3 cu ft) | 0.085 cubic meters |
| Cubic yard | 0.765 cubic meters |
| Cubic yard | 27 cubic feet |
Mulch Types
| Type | Best For | Notes |
|---|---|---|
| Wood chips | Trees and shrubs | Long-lasting, excellent for trees |
| Shredded bark | Ornamentals | Attractive, holds in place on slopes |
| Pine needles | Acid-loving plants | Slightly acidifies soil over time |
| Straw | Vegetable gardens | Cheap, decomposes fast |
| Pea gravel | Paths, xeriscaping | No organic benefit, good drainage |
Metric and Imperial Reference
Ring radius is measured from the centre of the trunk, and the figures below assume a 20 cm (8 in) trunk with a 10 cm (4 in) mulch-free gap around it, at a 10 cm (4 in) depth.
| Ring Radius | Ring Diameter | Mulch Needed at 10 cm Deep |
|---|---|---|
| 60 cm / 2 ft | 120 cm / 4 ft | 101 L / 3.6 cu ft (2 bags) |
| 90 cm / 3 ft | 180 cm / 6 ft | 242 L / 8.5 cu ft (5 bags) |
| 120 cm / 4 ft | 240 cm / 8 ft | 440 L / 15.5 cu ft (8 bags) |
| 180 cm / 6 ft | 360 cm / 12 ft | 1,005 L / 35.5 cu ft (18 bags) |
Note how fast that column climbs. Doubling the radius does not double the mulch, it roughly quadruples it, because you are filling a ring whose area grows with the square of the radius. Going from a 3 ft radius to a 6 ft one takes you from five bags to eighteen. This is the single most common surprise on a mulching weekend, and it is why so many trees end up with a ring that stops well short of where the roots actually are.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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