Mulch Ring Volume Calculator
Work out the mulch for a ring around a tree from the outer radius, the inner gap at the trunk, and the depth — in cubic yards and 2 cu ft bags.
Calculator
A mulch ring from 0.5 ft to 3.0 ft radius (27.5 sq ft) at 3.0 in deep is about 0.255 cu yd (~4 bags). Keep mulch off the trunk — a doughnut, not a volcano.
A ring of mulch around a tree is one of the cheapest, highest-value things you can do for it: it holds moisture, evens out soil temperature, keeps string trimmers away from the bark and feeds the soil as it breaks down. The trick is buying the right amount. Because the ring is a donut — a wide circle with the trunk left open in the middle — you compute the ring area, not a full circle. Enter the outer radius, the inner trunk gap and the depth, and this tool returns the volume in cubic yards and standard 2 cu ft bags.
Formula
A mulch ring is a donut, so its area is the big circle minus the trunk-gap circle, then the volume converts to cubic yards with the standard 324 factor:
ring area (sq ft) = π × (outer radius² − inner radius²)cubic yards = ring area × depth (in) ÷ 324bags = ceil( cubic yards × 13.5 ) (2 cu ft bags)
The 324 comes from 12 in × 27 cu ft/cu yd, and a cubic yard is about 13.5 bags of 2 cu ft each.
Worked example
A ring out to 3 ft with a 0.5 ft trunk gap at 3 in deep:
π × (3² − 0.5²) = π × (9 − 0.25) = 27.49 sq ft27.49 × 3 ÷ 324 = 0.255 cu yd
That is about 0.255 cubic yards, or roughly 4 bags of 2 cu ft mulch. Widen the ring to the drip line and it climbs quickly.
Mulching a tree the right way
A proper mulch ring holds moisture, moderates soil temperature and keeps mowers and trimmers away from the bark. The shape matters as much as the volume:
- Donut, not volcano. Keep an inner gap so mulch never piles against the trunk — mulch on the bark traps moisture, invites rot and pests, and can kill the tree.
- 2 to 3 inches deep. Deeper is not better; a thick mat can suffocate roots and shed water. Refresh to depth rather than adding a new layer every year.
- Wider is better. Extending the ring toward the drip line does more good than piling it deep near the trunk. That is why the outer radius drives the volume.
- Buy a little extra. Coverage per bag varies with how fluffy or settled the mulch is, so add about 5–10% and confirm the coverage printed on the bag.
For beds and broad areas rather than a ring, use the bed mulch calculator instead, and see the material coverage table for bags per cubic yard.
Reference table
Mulch for a ring at 3 in deep with a 0.5 ft trunk gap, by outer radius (pure geometry — widen the ring and it grows fast):
| Outer radius | Ring area | Volume | 2 cu ft bags |
|---|---|---|---|
| 2 ft | 11.8 sq ft | 0.109 cu yd | 2 |
| 3 ft | 27.5 sq ft | 0.255 cu yd | 4 |
| 4 ft | 49.5 sq ft | 0.458 cu yd | 7 |
| 5 ft | 77.8 sq ft | 0.720 cu yd | 10 |