Understanding your result
The headline figure is the total volume of mix to buy for all your beds, including the settling allowance. Below it you will see how many bags that is, and what to ask for if you are buying loose from a landscape supplier.
The breakdown table then splits the total into ingredients. Each ingredient is rounded up to whole bags on its own, which is why the bag total can be one or two higher than the volume alone would suggest. You cannot buy 12.9 bags of topsoil, and the leftover part-bag is useful for topping up later.
The drawing shows the bed from the side. The shaded area is soil, and the space above it is the gap you chose to leave. That gap matters more than it looks. A bed filled level with the rim sheds water and mulch over the edge every time you use the hose.
How we calculate this
The mix choices use these shares:
- 60 / 30 / 10: 60% topsoil, 30% compost and 10% aeration material such as perlite, pumice or coarse sand. A dependable all-round mix that holds its level well.
- 50 / 50: half topsoil, half compost. Simple to buy and rich, but it settles more because of the larger compost share.
- Equal thirds: one third each of compost, peat moss or coir, and coarse vermiculite. Light, soilless, and popular for shallow intensive beds. It is the most expensive option per cubic foot.
- Ready-made mix: a bagged or bulk product sold as raised bed soil, treated as a single ingredient.
In metric, the footprint in square meters times the soil depth in centimeters, divided by 100, gives cubic meters. One cubic meter is 1,000 liters.
The assumptions behind the numbers
| Assumption | Value used | Why |
|---|---|---|
| Measurements | Inside faces of the bed | The boards take up room. Outside measurements overstate the volume |
| Gap at the top | 1 in (2.5 cm) unless you change it | Keeps water and mulch inside the frame |
| Settling allowance | 10% unless you change it | Fresh mixes pack down with watering. Use 15 to 20% for mixes that are half compost or more |
| Bag sizes | 1, 1.5 and 2 cu ft; 40, 50 and 70 L | Common retail sizes for soil and compost |
| Bulk rounding | Up to the next 0.5 cu yd or 0.25 m³ | The smallest step most suppliers will load |
| Weight | 1,400 to 2,200 lb per cu yd (830 to 1,300 kg per m³) | Blended mixes are lighter than plain topsoil, and moisture changes everything. For hauling decisions only |
Assumptions last reviewed October 3, 2026.
There is no single correct recipe for raised bed soil. The shares above are starting points that many gardeners use, and local suppliers blend their own versions. What the calculator does reliably is the arithmetic: once you settle on a recipe, the volumes follow.
Two worked examples
Two standard 4 × 8 beds
Two beds are built from stacked 2 × 6 boards, which makes the sides 11 inches tall. Inside, each bed is 8 feet by 4 feet. The plan is to leave a 1 inch gap and use the 60 / 30 / 10 mix, bought in 1.5 cubic foot bags.
- Footprint: 8 × 4 = 32 sq ft
- Depth of soil: 11 − 1 = 10 in
- Volume per bed: 32 × 10 ÷ 12 = 26.7 cu ft
- Two beds: 53.3 cu ft
- With 10% for settling: 58.7 cu ft, which is 2.17 cu yd
- Topsoil (60%): 35.2 cu ft, 24 bags
- Compost (30%): 17.6 cu ft, 12 bags
- Aeration material (10%): 5.9 cu ft, 4 bags
That is 40 bags to lift, load and open. In bulk, the same job is a single order of 2.5 cubic yards of blended mix, dumped on the driveway. For two beds or more, bulk is nearly always the easier route.
One tall bed, in metric
A single bed is 2.4 m by 1.2 m inside and 40 cm tall. It will be filled to 3 cm below the rim with a 50 / 50 mix of topsoil and compost. Because half the mix is compost, the settling allowance is raised to 15%.
- Footprint: 2.4 × 1.2 = 2.88 m²
- Depth of soil: 40 − 3 = 37 cm
- Volume: 2.88 × 37 ÷ 100 = 1.07 m³
- With 15% for settling: 1.23 m³, or 1,225 liters
- Topsoil (50%): 613 L, 13 bags of 50 L
- Compost (50%): 613 L, 13 bags of 50 L
- In bulk: 1.25 m³
A bed this deep is a good candidate for filling the bottom 10 to 15 cm with rough material such as prunings and leaves. Entering 30 cm as the bed height instead of 40 cuts the order by about a quarter.
Common mistakes
- Using the outside dimensions. Measure between the inside faces of the boards. On a small bed the wall thickness changes the answer noticeably.
- Filling to the brim. Leave a gap. Soil level with the rim washes out with the first heavy watering.
- Ignoring settling. If you buy exactly the geometric volume, the bed will look half-empty a month later. Add the allowance, or plan to top up.
- Buying only compost. It is tempting because it is the richest-looking bag on the shelf. A bed of pure compost shrinks fast and stays wet.
- Forgetting the mix is by volume, not weight. A recipe that says “60% topsoil” means six buckets in ten, not 60% of the weight on the label.
- Treating potting mix as raised bed soil. Potting mix is made for containers. It works, but it costs far more per cubic foot and dries out faster in a large open bed.
Printable shopping list
After you calculate, Print shopping list gives you a one-page sheet with the number of bags of each ingredient, the bulk alternative, and the bed sizes you entered. If you are buying loose, read the supplier the bulk figure and ask what their raised bed blend contains, because the recipe varies from yard to yard.
Mix the ingredients before they go in, or layer them and turn the top 8 inches with a fork. Water the bed thoroughly, let it sit for a few days, and top up any low spots before you plant.
Questions people ask
- How much soil do I need for a 4 × 8 raised bed?
- A 4 × 8 foot bed that is 12 inches deep holds 32 cubic feet when filled to the brim, which is about 1.2 cubic yards. Filled to an inch below the rim with 10% added for settling, it comes to about 32 cubic feet again, or 22 bags of 1.5 cubic feet.
- How many bags of soil fill a raised bed?
- Divide the volume in cubic feet by the bag size and round up. A 4 × 4 foot bed 10 inches deep needs about 13.3 cubic feet, which is 9 bags of 1.5 cubic feet or 7 bags of 2 cubic feet, before any settling allowance.
- Can I fill a raised bed with only compost?
- It is not a good idea. Pure compost holds a lot of water, shrinks quickly as it breaks down, and can be too rich for some crops. Most mixes keep compost to between a third and a half of the volume and use topsoil or another mineral ingredient for the rest.
- Why add extra for settling?
- Bagged and freshly blended mixes are fluffed up with air. Watering and gravity pack them down, and the compost in the mix keeps shrinking as it decomposes. A bed filled level with the rim in spring is often 2 inches lower by midsummer.
- Should I measure the inside or the outside of the bed?
- The inside. Timber sides are usually 1.5 to 2 inches thick, so an "8 foot" bed built from 2-inch lumber can be 4 inches shorter inside than out. On a small bed that difference is a whole bag.
- Do I need to fill a tall bed all the way with soil?
- No. Vegetable roots mostly live in the top 12 inches. In a bed taller than about 18 inches you can fill the bottom with logs, branches, leaves or upturned turf and put the soil mix on top. Enter only the depth of soil you plan to add.
Built and reviewed by Bertha
Last updated · How we work out our numbers