Tree Mulch Ring Calculator
Calculate how much mulch you need for a tree ring.
Enter tree trunk diameter, ring width, and depth to get volume in cubic feet or cubic meters.
Tree mulch ring volume is calculated for a circular ring (annulus) around the base of a tree — you need the area between the outer radius and the inner radius (the trunk), then multiply by depth.
Area of a Ring Formula:
Area = π × (Outer Radius² − Inner Radius²)
Volume Formula:
Volume (cu ft) = Area × Depth (ft)
Cubic Yards = Volume / 27
Worked example: Tree trunk diameter: 8 inches → trunk radius = 4 inches = 0.33 ft Desired mulch ring outer diameter: 6 feet → outer radius = 3 ft Depth: 3 inches = 0.25 ft
Area = 3.14159 × (3² − 0.33²) = 3.14159 × (9 − 0.11) = 3.14159 × 8.89 = 27.93 sq ft Volume = 27.93 × 0.25 = 6.98 cu ft → 0.26 cubic yards
One standard bag of mulch = 2 cubic feet, so you’d need ~3.5 bags.
Recommended ring dimensions:
- Outer radius: At least 3 feet from trunk — preferably as wide as the drip line of the tree
- Inner gap: Leave 3–6 inches clear around the actual trunk (mulch should never touch bark)
- Depth: 2–4 inches is ideal. Deeper than 4 inches suffocates roots and promotes rot.
Why proper mulching matters:
- Retains soil moisture (reduces watering by 25–50%)
- Suppresses weeds
- Moderates soil temperature in summer and winter
- Prevents lawn mower damage to trunk base
“Volcano mulching” warning: Piling mulch high against the trunk traps moisture, invites disease and pests, and can kill trees over 3–5 years — even mature ones. Always leave the trunk flare exposed.