Multiply the bed’s area in square feet by the depth in inches and divide by 12 to get cubic feet. Divide by 27 for cubic yards, or by the bag size for bags. A 10 × 10 ft bed, 3 inches deep, takes 25 cu ft: 0.93 cubic yards, or 13 bags of 2 cu ft.
The math, with your numbers
These lines follow whatever you typed into the calculator above.
A circle’s area is π × (diameter ÷ 2)². Bags are rounded up, because a store won’t sell part of a bag; bulk yards you can order as the supplier allows.
How to measure your beds
- Beds and borders: measure the length and the width in feet. For an L-shape or a curve, split it into rectangles and run each one, or add the areas and use “I know the area”.
- Around a tree: mulch a ring to the drip line, the outer edge of the branches, if you can (Penn State). For a large, older tree, the Morton Arboretum mulches at least a 4 to 5 foot circle. Measure across the ring and choose “Circle or tree ring”.
- Several beds the same size: enter one and set “How many beds”.
How deep to spread mulch
- Most organic mulch, such as wood chips and bark: 2 to 4 inches, and no more (Penn State). The Morton Arboretum uses 3 to 4 inches of medium- to coarse-textured mulch around trees and shrubs.
- Fine or double-shredded mulch: 1 to 2 inches (Penn State). Fine mulch packs down and holds water (Morton Arboretum).
- Perennial beds: a couple of inches is enough (Morton Arboretum).
- Poorly drained soil: use less (Penn State).
- Winter mulch on garlic and bulbs is thicker straw or leaves; see when to plant garlic.
Don’t pile mulch against tree trunks: Penn State calls “volcano” mulching never recommended. Pull mulch 3 to 5 inches away from young trees and 8 to 10 inches from mature ones, so the flare at the base of the trunk shows.
How much a yard or a bag covers
A cubic yard is 27 cubic feet: 18 bags of 1.5 cu ft, 13½ bags of 2 cu ft, or 9 bags of 3 cu ft. Spread out, they cover:
| Depth | 1 cubic yard | 3 cu ft bag | 2 cu ft bag | 1.5 cu ft bag |
|---|---|---|---|---|
| 1 in | 324 sq ft | 36 sq ft | 24 sq ft | 18 sq ft |
| 2 in | 162 sq ft | 18 sq ft | 12 sq ft | 9 sq ft |
| 3 in | 108 sq ft | 12 sq ft | 8 sq ft | 6 sq ft |
| 4 in | 81 sq ft | 9 sq ft | 6 sq ft | 4½ sq ft |
Our arithmetic: volume ÷ (depth ÷ 12). Some bags print a slightly smaller coverage; a Scotts 1.5 cu ft bag lists 8 sq ft at 2 inches (RONA listing) against 9 by the math.
Bags or bulk? Bulk mulch is sold by the cubic yard. Two yards is 54 cubic feet, or 27 bags of 2 cu ft, so for a big job a delivery saves a lot of carrying.
Topping up an old bed
Organic mulch breaks down, finely shredded mulch fastest, so it needs topping up (Penn State). The Morton Arboretum checks the depth every year and adds mulch as needed. Push a trowel in, measure what’s left, and enter it as “Already there”: the calculator works out only what’s missing.
Questions people ask
How many bags of mulch are in a yard?
13½ bags of 2 cu ft, 9 bags of 3 cu ft, or 18 bags of 1.5 cu ft. A cubic yard is 27 cubic feet.
How many cubic feet are in a yard of mulch?
27. A yard is 3 ft × 3 ft × 3 ft.
How much area does a yard of mulch cover?
324 sq ft at 1 inch deep, 162 at 2 inches, 108 at 3 inches and 81 at 4 inches.
How many bags of mulch do I need for 100 square feet?
At 3 inches deep, 25 cu ft: 13 bags of 2 cu ft, or 9 bags of 3 cu ft. At 2 inches, 17 cu ft: 9 bags of 2 cu ft.
How much mulch for a tree ring?
A ring 6 feet across is about 28 sq ft. At 3 inches deep that’s 7 cu ft: 4 bags of 2 cu ft. Keep the mulch a few inches off the trunk.
Sources
- Penn State Extension, Mulching Landscape Trees (David R. Jackson, updated 2018): 2–4 in of organic mulch and no more; 1–2 in of fine or double-shredded mulch; less on poorly drained soil; to the drip line; 3–5 in from young trees and 8–10 in from mature ones; no volcano mulching; periodic reapplication.
- The Morton Arboretum, How to Mulch Trees and Shrubs: 3–4 in of medium- to coarse-textured mulch; fine mulch packs down; a couple of inches in perennial beds; at least a 4–5 ft circle around large trees; check the depth every year.
- Bag sizes: Scotts Nature Scapes mulch, for example, is sold in 2 cu ft bags (Ace Hardware listing) and 1.5 cu ft bags (RONA listing, 8 sq ft at 2 in).
- Volumes and coverage: our arithmetic (1 cubic yard = 27 cubic feet; 1 cubic foot = 28.32 liters).