Tree mulch ring calculator
Enter how wide you want the mulch ring and the trunk thickness — we subtract the trunk so the numbers are what you actually spread.
Result
Ring mulch needed
How big should a mulch ring be?
Arborists' rule: mulch to the drip line (the reach of the branches) — for a young tree that's a 3–4 ft ring, for a mature oak it could be 20+ ft. Real-world minimum: as wide as you can tolerate mowing around, 8 ft across is a good target for a young tree. Volume is the ring area minus the trunk:
Formula
ring ft² = π × (ring Ø ÷ 2)² − π × (trunk Ø ÷ 2)²
ft³ = ring ft² × depth ÷ 12 · yd³ = ft³ ÷ 27
Example: an 8 ft ring around a 12 in trunk at 3 in deep: 50.3 − 0.8 = 49.5 ft² × 3/12 = 12.4 ft³ = 0.46 yd³ — seven 2 ft³ bags (plus the waste buffer).
The three tree-mulching rules
- No volcanoes. Mulch touching the bark holds moisture against the trunk → rot, insects, girdling roots. Leave a 3–6 in donut of bare soil.
- 2–4 inches deep, not 8. Roots need oxygen.
- Refresh, don't stack. Rake the old layer before adding new; depth above 4 in does harm.
For big beds of many trees, the plain mulch calculator handles rectangles and full circles.
Frequently asked questions
How much mulch do I need around my tree?
An 8 ft ring around a 1 ft trunk at 3 in needs ~0.5 yd³ (7 bags). Enter your ring size above — trunk excluded automatically.
How wide should mulch be around a tree?
Ideally out to the branch tips (drip line); realistically at least 3–4 ft out from the trunk for young trees. Wider rings = cooler roots and fewer mower injuries.
Why is volcano mulching bad?
Piled bark stays moist, rots the trunk's outer bark, harbors rodents that chew bark, and suffocates surface roots. Mulch should look like a donut, not a volcano.
Do I need to remove old mulch first?
If the total depth would exceed 4 inches, yes — rake off or thin the old layer before topping up.