How to Calculate Soil Volume
Soil volume comes down to one thing: how much space needs to be filled. Multiply the area of your bed by the depth you want to fill, and you have cubic feet. Divide by 27 for cubic yards, or divide by the volume printed on the bag.
Note: convert inches to feet first — divide depth in inches by 12
A standard 1.5 cubic foot bag of garden soil covers about 18 square feet at 1 inch deep, or about 9 square feet at 2 inches deep. For a typical 4 × 8 raised bed filled to 12 inches, you need about 32 cubic feet of soil — that's 22 standard 1.5 cu ft bags or about 1.2 cubic yards of bulk soil.
Soil Depth by Project Type
- Raised bed (vegetables) — 10 to 12 inches. Most vegetable roots need 8 to 12 inches. Deeper is better for root vegetables like carrots and parsnips; 12 inches is the practical standard that handles almost everything.
- Raised bed (flowers / herbs) — 6 to 8 inches. Shallower roots. Six inches is the minimum for most annual flowers and herbs.
- Topdressing a lawn — 0.25 to 0.5 inches. Just enough to fill low spots and improve surface drainage. More than half an inch risks smothering grass.
- New garden bed (ground level) — 6 to 8 inches of amendment worked into existing soil. You are not filling the bed; you are improving the top layer.
- Container / planter — Fill to within 1 to 2 inches of the rim to allow for watering without overflow. Measure the actual interior depth of the container.
The Raised Bed Soil Mix (60/30/10)
The standard recommendation for raised beds is a three-part mix: 60% topsoil, 30% compost, and 10% perlite or peat moss. This ratio comes from the square foot gardening method developed by Mel Bartholomew and is widely endorsed by university extension services. It gives you drainage, nutrient retention, and aeration in one mix.
If you are buying a pre-blended "raised bed mix" from a garden centre, it already approximates this ratio. Check the bag — most quality blends contain compost, topsoil, and either perlite or vermiculite. The calculator's mix breakdown assumes you are blending your own from separate bags.
Topsoil vs Garden Soil vs Potting Mix
- Topsoil — the base layer. Dense, mineral-rich. Good as the bulk ingredient in a raised bed mix. Too heavy on its own for containers. Inexpensive — often sold by the cubic yard in bulk.
- Garden soil — topsoil with added compost and sometimes fertilizer. Designed for in-ground beds. Not recommended for raised beds or containers because it compacts in confined spaces.
- Raised bed mix — lighter blend formulated specifically for raised beds and large containers. Contains compost, bark, perlite or coconut coir. The right choice when filling a raised bed from scratch.
- Potting mix — the lightest option. Designed for pots and containers. Mostly bark, peat or coir, and perlite. No real topsoil. Wrong for in-ground beds but right for any container.
- Compost — organic matter, not a standalone growing medium. Used as an amendment — added to topsoil, not replacing it. Breaks down and improves soil structure over time.
Bags vs Bulk: When to Order Delivery
Once you need more than 1 cubic yard of soil — about 18 standard 1.5 cu ft bags — call a landscape supplier for a bulk quote. Bulk topsoil typically runs $30 to $60 per cubic yard delivered, depending on quality and distance. Bagged raised bed mix at the hardware store often costs $8 to $12 per 1.5 cu ft bag, which works out to $144 to $216 per cubic yard. The math tips decisively toward bulk for any project beyond a single raised bed.