Raised Bed Soil Calculator
Work out how much soil fills your raised garden beds, in cubic feet, cubic yards and bags, and how to split it between topsoil and compost. The example is two 4 × 8 ft beds, 12 in deep, with a little extra for settling.
How to use the raised bed soil calculator
- Enter the inside length and width of your first bed. Click Add another area for each extra bed (the example has two).
- Set the depth of soil you want: at least 8 in for salad greens and 12–24 in for tomatoes, peppers and squash (University of Maryland Extension).
- Keep an allowance of about 10% under Compaction for settling; fresh soil sinks after the first waterings.
- Read the cubic feet and yards. Split them about 70% topsoil and 30% compost (Penn State Extension), or use a ready-made raised-bed mix.
- Check your bag size: the default is 0.75 ft³ (a 40 lb bag of topsoil). Change it to match the bags you'll buy.
How much soil for a raised bed?
A raised bed is a box, so the soil volume is length × width × depth:
- Volume (ft³) = length (ft) × width (ft) × depth (in) ÷ 12 × (1 + settling allowance)
- Cubic yards = ft³ ÷ 27
- Bags = ft³ ÷ bag size, rounded up
Measure the inside of the frame: with 2 in thick boards, a bed sold as 4 × 8 ft can be closer to 3.7 × 7.7 ft inside. Add the beds together and work out the mix from the total.
Worked example: two 4 × 8 ft beds, 12 in deep
This is the setup loaded above.
- Area: 2 × (4 × 8) = 64 ft².
- Volume: 64 × 12 ÷ 12 = 64 ft³. Add 10% for settling: 70.4 ft³.
- Cubic yards: 70.4 ÷ 27 = 2.61 yd³.
- Mix: 70% topsoil = 49.3 ft³, 30% compost = 21.1 ft³.
- Bags: 70.4 ÷ 0.75 = 93.9, so 94 bags of 0.75 ft³ if you buy it all bagged.
Ninety-odd bags is a lot of plastic and carrying. At over 2½ yards, a bulk delivery of a garden blend or of topsoil and compost is usually cheaper, and many landscape yards sell a ready-made raised-bed mix by the yard.
What soil mix to use
Extension services agree that raised beds need a blend of mineral soil and organic matter rather than plain topsoil or pure compost:
- Penn State Extension recommends about 70% soil and 30% compost.
- University of Maryland Extension suggests compost and purchased topsoil in a 1:2 or 1:1 ratio, depending on the site, and, for beds on hard surfaces, compost and a soilless growing mix 1:1.
Buy both from reputable suppliers and ask whether the topsoil is screened. On good native soil, you can loosen the ground under the bed and fill less depth, since roots will grow down into it.
Soil for common raised bed sizes
| Bed size | 8 in deep | 12 in deep | 18 in deep | 12 in: yd³ | 12 in: topsoil / compost (70/30) | 12 in: 0.75 ft³ bags |
|---|---|---|---|---|---|---|
| 2 × 4 ft | 5.3 ft³ | 8 ft³ | 12 ft³ | 0.3 | 5.6 / 2.4 ft³ | 11 |
| 2 × 8 ft | 10.7 ft³ | 16 ft³ | 24 ft³ | 0.59 | 11.2 / 4.8 ft³ | 22 |
| 3 × 6 ft | 12 ft³ | 18 ft³ | 27 ft³ | 0.67 | 12.6 / 5.4 ft³ | 24 |
| 4 × 4 ft | 10.7 ft³ | 16 ft³ | 24 ft³ | 0.59 | 11.2 / 4.8 ft³ | 22 |
| 4 × 8 ft | 21.3 ft³ | 32 ft³ | 48 ft³ | 1.19 | 22.4 / 9.6 ft³ | 43 |
| 4 × 10 ft | 26.7 ft³ | 40 ft³ | 60 ft³ | 1.48 | 28 / 12 ft³ | 54 |
| 4 × 12 ft | 32 ft³ | 48 ft³ | 72 ft³ | 1.78 | 33.6 / 14.4 ft³ | 64 |
No settling allowance: fresh soil settles, so add 10–20% or plan to top up after the first few waterings. The 70/30 split is Penn State Extension's recommended soil-to-compost ratio.
Raised bed tips
- Fill in layers and water as you go. Water each layer lightly to settle it, so the bed doesn't sink by inches after planting.
- Leave an inch or two below the rim. Room for mulch and for watering without washing soil over the edge.
- Mix before filling. Blend topsoil and compost on a tarp or in a wheelbarrow so the bed isn't layered.
- Top up every year. Organic matter breaks down; plan on adding an inch or two of compost each season.
- Mulch the paths. Wood chips or gravel between beds keep weeds down and boots clean. See the mulch calculator or the pea gravel calculator.
Frequently asked questions
How much soil do I need for a 4×8 raised bed?
At 12 in deep, a 4 × 8 ft bed holds 32 ft³ (1.19 yd³), or 43 bags of 0.75 ft³. At 8 in it is 21.3 ft³; at 18 in, 48 ft³. Add about 10% for settling.
What is the best soil mix for raised beds?
A blend of topsoil and compost. Penn State Extension recommends about 70% soil to 30% compost; University of Maryland Extension suggests compost to topsoil at 1:2 or 1:1.
How deep should a raised garden bed be?
University of Maryland Extension gives at least 8 in for leafy greens, beans and cucumbers, and 12–24 in for peppers, tomatoes and squash, for beds on hard surfaces. On good soil you can loosen the ground below and build shallower.
How many bags of soil for a raised bed?
Divide the bed's volume in cubic feet by the bag size. A 4 × 8 × 1 ft bed (32 ft³) needs 43 bags of 0.75 ft³ or 22 bags of 1.5 ft³.
Should I buy raised bed soil in bags or bulk?
Bags suit one or two small beds. From about a cubic yard (36 bags of 0.75 ft³) upward, compare a bulk delivery of a garden blend; it is often cheaper and avoids the plastic.
More topsoil calculators
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- Pea Gravel CalculatorPea gravel for paths, patios, fire pits and play areas, in yards, tons and bags.