How the calculator works
It multiplies the area by the depth to get a volume, then turns that into bags and cubic yards:
- Area. Length × width for a rectangle, or π × radius² for a circular bed.
- Volume. Area × depth, with the depth changed from inches to feet (divide by 12).
- Cubic yards. Cubic feet ÷ 27, because a cubic yard is 3 × 3 × 3 feet.
- Bags. Cubic feet ÷ the bag size, rounded up to whole bags.
The circle option measures the whole circle, including any empty center.
Iowa State Extension’s worked example is a 265 square foot bed with 1½ inches of mulch: 265 × 0.125 = 33.1 cubic feet, which is 17 bags of 2 cubic feet or about 1.2 cubic yards in bulk.
How deep to spread it
Choosing a material fills in a starting depth you can change:
| Material | Starting depth | Guidance |
|---|---|---|
| Mulch | 3 in | Iowa State: 2 to 4 inches of hardwood mulch |
| Compost for a new bed | 3 in | Oregon State: 3 to 4 inches, mixed into the top 8 to 12 inches |
| Compost top-up | 1 in | Oregon State: ¼ to 1 inch a year in existing beds |
| Lawn topdressing | ¼ in | Oregon State: ¼ to ½ inch after core aerating |
| Raised bed soil | 12 in | The depth you’ll actually fill |
Deeper layers cover less ground. One cubic yard covers about 324 square feet at 1 inch, 162 at 2 inches, 108 at 3 inches and 81 at 4 inches.
Raised beds
Measure the inside of the bed and use the depth you plan to fill, which is often a little below the top of the walls. A 4 × 8 foot bed filled 12 inches deep holds 32 cubic feet: 16 bags of 2 cubic feet or about 1.2 cubic yards. Oregon State advises against filling raised beds with compost alone, so check the mix your soil supplier recommends.
Bags or bulk?
Bag sizes are printed on the bag in cubic feet or litres. For larger jobs, bulk delivery is sold by the cubic yard or cubic meter; suppliers set their own increments and minimum orders. The optional extra percentage is added before rounding, for spills or for material that settles after spreading.
Keep your plan
Print your current plan or choose Save as PDF in the browser’s print dialog. Your inputs stay in a device draft when browser storage is available. An optional free account lets you save and sync the project in My projects. Calculating and printing stay open to everyone.