Soil Depth Calculator
Whether filling a raised garden bed, top-dressing a lawn, or prepping flower borders, the depth of your soil determines root growth and health. Enter your bed dimensions and desired soil depth to get total cubic yards, cubic feet, and 40lb bags needed.
Results
Depth is what decides the order, not area
A bed's footprint is the easy half of the problem. The number that actually moves the order is depth, because volume scales with it directly: the same 8 × 4 ft bed takes 0.30 cubic yards at 3 inches and 0.59 at 6 inches. People measure the bed carefully, then guess the depth, and that is where the order goes wrong.
Formula: cubic feet = length × width × (depth ÷ 12), then cubic yards = cubic feet ÷ 27
Length and width are in feet, depth in inches. The division by 12 converts inches to feet; the division by 27 converts to cubic yards, which is the unit bulk soil is sold and delivered in.
How deep to go, by what you are growing
| Use | Depth | Why |
|---|---|---|
| Lawn top-dressing | 1/4 to 1/2 in | Any deeper smothers existing grass |
| Flower borders and annuals | 6 to 8 in | Shallow, fibrous root systems |
| Most vegetables | 8 to 12 in | Tomatoes, peppers, beans, squash |
| Root crops | 12 to 18 in | Carrots and parsnips fork if they hit hardpan |
| Shrubs and small trees | 18 to 24 in | Needs room below the root ball |
If the bed sits on open ground rather than on a hard surface, roots can travel below it, so the bed itself does not have to hold the full depth. On a patio, deck or driveway it does.
Bags or bulk
This calculator assumes a 0.75 cubic foot bag, which is what a 40 lb bag of screened topsoil usually holds. That works out to 36 bags per cubic yard. Bag volume is not standardized, though — the same 40 lb label covers 0.5 to 0.75 cubic feet depending on how wet and how dense the soil is, so the same cubic yard can take anywhere from 36 to 54 bags. Weight is what the bag is sold by; volume is what fills the bed.
The crossover is around 1 cubic yard. Below that, bags win on convenience and there is no delivery fee to spread. Above 2 cubic yards, bulk is usually less than half the price per yard and you are no longer stacking and slitting 50-plus bags. Most suppliers have a minimum delivery of 1 to 2 yards, and many charge the same flat fee for anything up to about 5, so ordering slightly more than you need often costs nothing extra.
Topsoil is not the same as a raised-bed mix
Straight topsoil compacts in a raised bed. It drains poorly, and once it has settled it is dense enough to hold water against roots. A standard raised-bed mix is roughly one third topsoil, one third finished compost and one third aeration material such as coarse sand, perlite or pine bark fines.
Budget for settling either way. Fresh soil drops 10 to 20% over the first season as organic matter breaks down and irrigation packs it, so filling to the rim in spring leaves a visible gap by fall. Fill proud, or plan to top up the following year.
Frequently asked questions
How deep should soil be for raised garden beds?
Most vegetables need 8 to 12 inches of soil depth. Root vegetables like carrots and parsnips need 12 to 18 inches of loose topsoil, and shrubs need 18 to 24 inches. If the bed sits on open ground, roots can grow below it, so the bed does not have to hold the full depth on its own — but on a patio or deck it does.
How many 40lb bags of topsoil are in a cubic yard?
At 0.75 cubic feet per bag, which is what this calculator assumes, one cubic yard takes 36 bags. Bag volume is not standardized: a 40 lb bag holds anywhere from 0.5 to 0.75 cubic feet depending on moisture and density, so the real answer ranges from 36 to 54 bags. Bags are sold by weight, but beds are filled by volume.
Should I buy soil in bags or in bulk?
The crossover is around 1 cubic yard. Below that, bags are more convenient and carry no delivery fee. Above 2 cubic yards, bulk is typically less than half the price per yard, and you avoid handling 50 or more bags. Check the supplier minimum first — many charge one flat delivery fee for anything up to about 5 yards.
Why does my new soil sink after a few months?
Fresh soil settles 10 to 20% in its first season as organic matter decomposes and watering compacts the mix. That is normal and not a sign of a bad product. Fill slightly above the rim when you build the bed, or plan to top up once the following spring.