How the mix calculator works
First it works out the total volume, exactly like the raised bed soil calculator: length × width × soil depth for each bed, times the number of beds, plus your extra for settling. Then it divides that volume by the parts in your recipe.
amount of one ingredient = total volume × (its parts ÷ total parts)
The parts don’t have to add up to 100. Type 2, 1 and 1 and the calculator treats it as 50%, 25% and 25%.
Worked example: one 4 × 8 ft bed, 12 inches deep, 60/30/10 recipe.
- Volume: 4 × 8 × 1 = 32 cubic feet; with 10% extra, 35.2 cubic feet (1.30 cubic yards)
- Topsoil, 60%: 35.2 × 0.6 = 21.1 cubic feet (0.78 cubic yards)
- Compost, 30%: 35.2 × 0.3 = 10.6 cubic feet (0.39 cubic yards)
- Other, 10%: 35.2 × 0.1 = 3.5 cubic feet (0.13 cubic yards)
The same bed with equal thirds needs 11.7 cubic feet of each ingredient.
The recipes here are examples, not rules. Your native soil, climate, the crops you grow and what is sold locally all matter. If you can, get a soil test on the topsoil you buy and adjust from there.
Common raised bed soil recipes
| Recipe | Parts by volume | Character | Good for |
|---|---|---|---|
| Topsoil-based blend | 60% topsoil, 30% compost, 10% coarse material | Holds water and nutrients, settles least | Most vegetable beds, deep beds |
| Half and half | 50% topsoil, 50% compost | Richer and lighter, settles more | Heavy feeders, beds topped up yearly |
| Equal thirds (Mel’s Mix style) | ⅓ compost, ⅓ peat moss or coco coir, ⅓ coarse vermiculite | Very light, drains fast, no topsoil | Shallow beds around 6 in, beds on patios |
| Bulk raised bed mix | supplier’s own blend | Ready to use, varies by supplier | Large or many beds |
“Coarse material” means something that keeps the mix open: aged bark fines, perlite, pumice or coarse sand are all used. Ask your supplier what they have; the calculator just calls it “Other”.
What makes good raised bed soil
The best raised bed soil for vegetables does four jobs at once:
- Drains. Water should soak in and not sit on top or pool at the bottom of the bed.
- Holds moisture. It should stay damp a few inches down between waterings. Compost and fine soil particles do this.
- Feeds plants. Organic matter from compost releases nutrients slowly and feeds soil life.
- Stays put. Mineral soil (topsoil) gives the mix body, so the bed doesn’t sink by a third in a year.
That is why most recipes combine a mineral part (topsoil), an organic part (compost) and something coarse. Native clay dug from the yard, on its own, usually drains too slowly in a box. Bagged potting mix works but is expensive at bed scale and very light. More on the differences in topsoil vs garden soil.
Mixing it yourself
- Mix on a tarp or in the bed itself in layers: a few inches of topsoil, an inch or two of compost, a sprinkle of the coarse part, then turn it with a fork. Repeat until full.
- Moisten as you go. Dry peat and coir are hard to wet once they are in the bed.
- Measure in buckets or wheelbarrows. A typical wheelbarrow holds about 6 cubic feet when level, and a 5-gallon bucket holds about 0.67 cubic feet. Count loads to keep the ratio right.
- Fill an inch above the final level. The mix will settle after the first few waterings.
Buying ready-mixed versus blending
Many landscape yards sell a bulk “raised bed mix” or “garden mix”. It saves the labour of blending, and one delivery fills several beds. Blending your own lets you control the recipe and use compost you already have. Price both: work out the cost per cubic foot of each ingredient and of the ready-made mix (price per yard ÷ 27). The compost calculator helps with the compost part.
Weight on the "Tons" line is for a typical bulk raised bed mix, about 0.7 to 1 ton per cubic yard. A thirds mix of peat, vermiculite and compost is much lighter.
Questions gardeners ask
- What is the best soil mix for raised beds?
- There is no single best recipe. A blend of good screened topsoil and compost, with a little coarse material for drainage, suits most vegetable beds. Roughly 60 percent topsoil, 30 percent compost and 10 percent bark fines or similar is a common starting point.
- What is Mel's Mix?
- The soilless mix from Mel Bartholomew's Square Foot Gardening method: equal parts by volume of compost (ideally blended from several sources), peat moss or coco coir, and coarse vermiculite. It is light and drains well but costs more per cubic foot than a topsoil-based blend.
- Should I mix by volume or by weight?
- By volume. Recipes are given in parts by volume, such as buckets, wheelbarrow loads or cubic feet, because the ingredients weigh very different amounts.
- Can I fill a raised bed with only compost?
- It is not usually recommended. Pure compost shrinks a lot as it breaks down, can be very rich, and dries out quickly. It works better as one part of a blend.
- Is bagged raised bed soil ready to use?
- Usually, yes, but products labelled raised bed soil vary a lot. Read the ingredient list. Some are mostly bark or peat, which is light and settles, and some are mostly topsoil.