About the Mulch & Soil Calculator
A garden bed's mulch or soil needs are really a volume calculation dressed up as a depth recommendation, and it is easy to buy several bags too few when going only by bag size instead of true cubic footage.
How It Works
The calculator converts your bed's length, width, and depth into cubic feet, then divides by 27 to give cubic yards, the unit most nurseries and landscape suppliers sell mulch and soil in bulk.
Examples
A perennial border
A 20×10 ft bed at 3 inches of mulch needs about 1.85 cubic yards, or roughly 25 bags if buying 2 cu ft bagged mulch instead of bulk.
Deeper coverage for weed control
The same bed at 4 inches instead of 3 needs about 2.47 cubic yards, noticeably more material for just one extra inch of depth.
Advantages
- Compares four common depths (2, 3, 4, 6 inches) in one table
- Shows results in both cubic yards and cubic meters
- Works equally well for mulch, topsoil, or compost using the same volume math
Common Mistakes
- Estimating by bag count instead of calculating true volume first
- Not accounting for settling over the growing season
- Treating a curved or irregular bed as a simple rectangle without breaking it into sections
Edge Cases to Watch For
- Bagged mulch is typically sold in 2 cubic foot bags, so converting the cubic yard total back to bags means multiplying by roughly 13.5 bags per yard.
- Mulch settles and compresses over the season, so beds that need to stay at a consistent depth may need a light top-up later in the year.
- Irregularly shaped beds are harder to measure accurately than a simple rectangle, and breaking an odd-shaped bed into smaller rectangular sections gives a more accurate total.
Common Use Cases
- Ordering bulk mulch or soil delivery for a garden bed
- Estimating bagged mulch quantities for a small planting area
- Comparing material needs across different mulch depths