The short answer: a 200 sq ft bed at 3 inches deep needs about 1.85 cubic yards of mulch, which is roughly 25 bags of the 2 cubic foot size. Here is the math for any bed.
The formula
cubic feet = length × width × depth in feet cubic yards = cubic feet ÷ 27 bags = cubic feet ÷ bag volume, rounded up
A 20 by 10 ft bed is 200 sq ft. At 3 inches, that is 0.25 ft deep, so 200 × 0.25 = 50 cubic feet. Divide by 27 for 1.85 cubic yards, or by 2 for 25 bags of 2 cubic foot mulch.
Bags or bulk
Bagged mulch is tidy for a small bed and easy to carry. Bulk by the cubic yard costs less once you pass roughly ten bags, and a single delivery beats hauling bag after bag.
| Bed | Depth | Cubic yards | 2 cu ft bags |
|---|---|---|---|
| 100 sq ft | 2 in | 0.62 | 9 |
| 200 sq ft | 3 in | 1.85 | 25 |
| 400 sq ft | 3 in | 3.70 | 50 |
How deep, and a caution
Two to three inches refreshes an established bed; three to four blocks weeds and holds moisture. Mulch settles and breaks down over the season, so a fresh layer thins out. Top it up rather than piling it deeper, and keep it pulled back from trunks and plant stems so they do not rot.
Get your exact number
Enter the bed size, depth and your bag size in the mulch calculator and it returns the cubic yards for bulk and the number of bags. For paths and driveway base instead of beds, see the gravel guide.