Mulch Calculator
This mulch calculator works out how much a bed needs, in cubic yards for bulk delivery and in bags if you are picking it up yourself.
Rectangular Bed
Tree Ring or Circular Bed
Leave a gap at the trunk, mulch piled against bark rots it.
Bags vs Bulk
Bags win on small beds. Bulk wins once delivery is spread over enough yards. This finds the line.
How to use the Mulch Calculator
Bed size and depth are the only two inputs that matter. Three inches is the standard for weed suppression; two is safer around shallow-rooted shrubs and in beds you top up every year.
The result gives both cubic yards and bag count, so you can price it either way. Use the Bags vs Bulk tab once you have local prices.
Mulch formula
Volume is the same geometry as any spread material:
- Bed:
length × width × depth - Tree ring:
π × [(outer ÷ 2)² − (gap ÷ 2)²] × depth cubic yards = cubic feet ÷ 27bags = cubic feet ÷ bag size
Because a cubic yard is 27 cubic feet, coverage falls out cleanly:
| Depth | One cubic yard covers | 2 cu ft bags per 100 sq ft |
|---|---|---|
| 1 in | 324 sq ft | 4.2 |
| 2 in | 162 sq ft | 8.3 |
| 3 in | 108 sq ft | 12.5 |
| 4 in | 81 sq ft | 16.7 |
A cubic yard is 13.5 bags of 2 cu ft, or 9 bags of 3 cu ft.
Worked example
A 20 ft × 6 ft bed at 3 inches:
- Area: 120 sq ft. Depth: 0.25 ft. Volume: 30 ft³
- Cubic yards: 30 ÷ 27 = 1.11 yd³
- Bags: 30 ÷ 2 = 15 bags of 2 cu ft
At roughly $4.50 a bag that is about $68, against a yard and a bit of bulk plus a delivery fee, which is why small beds usually go in bags.
What the result means
Mulch is sold by volume, so the cubic yard and bag figures are the ones that matter. Weight is shown only because bulk suppliers occasionally quote by the ton, and mulch is light enough that a yard of bark weighs a fraction of a yard of gravel.
Bags are consistent; bulk is fluffier and settles. A “yard” delivered loose often looks like more than a yard once it is raked out, then less again after a season.
Common mistakes
- Volcano mulching. Piling mulch against a trunk traps moisture and rots the bark. Keep it three to six inches clear, that gap is what the tree ring tab subtracts.
- Going too deep. More than about four inches starves roots of air and can shed water rather than absorb it.
- Measuring for a full depth on a top-up. If last year’s mulch is still an inch deep, you only need to make up the difference.
- Forgetting bulk is delivered loose. You need somewhere to put a heap of it, and a way to move it.
Frequently asked questions
How many bags of mulch are in a cubic yard?
Thirteen and a half bags of 2 cubic feet, or nine bags of 3 cubic feet. A cubic yard is 27 cubic feet, so the arithmetic is exact.
How much does a cubic yard of mulch cover?
108 square feet at 3 inches deep, 162 at 2 inches, or 81 at 4 inches. The often-quoted “100 square feet” is that 3 inch figure rounded down.
How deep should mulch be?
Two to three inches for most beds. Three suppresses weeds well; two is kinder to shallow-rooted plants. Going past four starts to smother roots and shed water instead of letting it through.
Is it cheaper to buy mulch in bags or bulk?
Bulk is cheaper per yard but carries a delivery fee, so bags usually win under about two yards and bulk wins above it. The exact crossover depends on your local prices, the Bags vs Bulk tab works it out from figures you enter.
How much mulch do I need for a tree ring?
A 6 foot ring at 3 inches, with a 12 inch gap left at the trunk, takes about 6.9 cubic feet, roughly 4 bags of 2 cubic feet per tree.
Sources and assumptions
Volume and coverage are exact arithmetic from the 27 cubic feet in a cubic yard, no estimation involved.
Weights are approximate. Published figures put bark mulch near 500 lb per cubic yard and wood chips near 675, with moisture moving both considerably. Weight is a secondary figure here because mulch is sold by volume.
Bag sizes assume the 2 and 3 cubic foot bags common in the United States. Check the bag, some brands sell 1.5 cubic feet at a similar price.