A 4 by 8 ft raised bed filled 12 inches deep needs 32 cubic feet of soil, which is 1.19 cubic yards or 22 bags of 1.5 cubic feet. To work out how much soil for a raised bed, measure the inside length and width in feet, divide the depth in inches by 12, then multiply length by width by depth in feet. That gives cubic feet, the unit printed on most bags.
Filling a raised bed is one of the bigger costs of starting a vegetable garden. Buy too little and you are back at the garden center. Buy too much and you have bags stacked behind the shed. The maths is simple once every measurement is in the same unit.
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How Much Soil for a Raised Bed: The Quick Formula
Measure the inside length and width of the bed in feet and the depth of soil you want in inches. Then use these three steps.
Cubic feet equals length times width times depth divided by 12. Dividing the depth by 12 converts inches to feet, so you end up with cubic feet.
Cubic yards equals cubic feet divided by 27. Use this figure when you order bulk soil by the cubic yard.
Bags equals cubic feet divided by bag size. Bag size is printed on the front of the bag, most often 1 cubic foot, 1.5 cubic feet or 2 cubic feet. Always round bags up.
Try your own numbers in the raised bed soil calculator on this site. It does the conversion for you and adds a settling allowance.
Worked Example: A 4 by 8 ft Bed
A 4 by 8 ft bed filled 12 inches deep needs 4 times 8 times 12 divided by 12, which is 32 cubic feet. Divide 32 by 27 and you get 1.19 cubic yards. In 1.5 cubic foot bags that is 32 divided by 1.5, which is 21.3, so you would buy 22 bags.
Fresh soil settles, often by 10 to 20 percent after the first few waterings. If you want the bed full to the top after it settles, add that allowance. At 10 percent, 32 times 1.1 equals 35.2 cubic feet. At 20 percent, 32 times 1.2 equals 38.4 cubic feet, which is 26 bags of 1.5 cubic feet. Buying the extra bags at the start is cheaper than a second trip.
Here is a second example of how much soil for a raised bed with a smaller bed. A 4 by 4 ft bed filled 10 inches deep needs 4 times 4 times 10 divided by 12, which is 13.3 cubic feet, or 0.49 cubic yards. In 1.5 cubic foot bags that is 8.9, so you would buy 9 bags.
Raised Bed Soil Chart
The chart below covers the most common bed sizes. Figures exclude the settling allowance, so add 10 to 20 percent if you want the bed full after settling.
| Bed size | Soil depth | Cubic feet | Cubic yards | 1.5 cu ft bags |
|---|---|---|---|---|
| 4 by 4 ft | 6 in | 8 | 0.30 | 6 |
| 4 by 4 ft | 10 in | 13.3 | 0.49 | 9 |
| 4 by 8 ft | 8 in | 21.3 | 0.79 | 15 |
| 4 by 8 ft | 12 in | 32 | 1.19 | 22 |
| 3 by 6 ft | 10 in | 15 | 0.56 | 10 |
| 3 by 6 ft | 12 in | 18 | 0.67 | 12 |
| 4 by 12 ft | 12 in | 48 | 1.78 | 32 |
| 2 by 8 ft | 10 in | 13.3 | 0.49 | 9 |
| 2 by 8 ft | 6 in | 8 | 0.30 | 6 |
If your bed is not on the chart, the formula for how much soil for a raised bed still works. Measure the inside length and width, pick your depth, and run the three steps. The soil calculator on this site handles other shapes and units as well.
Bags by Bag Size
Bag size changes the bag count a lot, so check the bag before you buy. The table below shows how many bags of each common size you need for four typical volumes. All counts are rounded up to the next whole bag.
| Soil needed | 1 cu ft bags | 1.5 cu ft bags | 2 cu ft bags |
|---|---|---|---|
| 13.3 cu ft | 14 | 9 | 7 |
| 15 cu ft | 15 | 10 | 8 |
| 32 cu ft | 32 | 22 | 16 |
| 48 cu ft | 48 | 32 | 24 |
A 2 cubic foot bag looks like better value on the shelf, but only if you can lift it. A 2 cubic foot bag of damp soil is heavy, so many gardeners prefer 1.5 cubic foot bags for a large fill.
How to Measure Your Bed
To work out how much soil for a raised bed, measure the inside of the bed, not the outside. With 2 inch thick boards, a bed that measures 4 by 8 ft outside is only about 3.67 by 7.67 ft inside, roughly 12 percent less soil. That difference is about 4 cubic feet on a 12 inch deep bed, or 3 bags of 1.5 cubic feet.
Depth is measured from the top of the soil surface you want down to the base of the bed. Most gardeners fill to about 1 inch below the top of the boards so water stays in the bed when it rains.
What Should You Fill a Raised Bed With?
There is no single right answer, but most successful mixes balance three things: structure, which keeps air spaces open, organic matter, which is compost that feeds soil life and holds moisture, and drainage, so water moves through instead of pooling.
A mix of 60 percent topsoil and 40 percent compost is economical for deep beds, holds water well and behaves like good garden soil.
A mix of 50 percent topsoil, 30 percent compost and 20 percent perlite or pumice gives better drainage where the topsoil is heavy.
Equal parts compost, peat moss or coco coir, and coarse vermiculite is a light, soilless blend. It is best in shallow beds because it is the most expensive per cubic foot.
Whatever you use, avoid filling a large bed with potting mix alone. It is designed for containers, costs more and can dry out quickly. The compost calculator will tell you how much compost you need for the compost share of your mix.
How Deep Does the Soil Need to Be?
Most vegetable roots do the bulk of their work in the top 12 inches. Lettuce, radishes and herbs grow happily in 6 to 8 inches. Tomatoes, squash and root crops like carrots and parsnips appreciate 12 inches or more.
If your bed sits on open ground, roots can also grow down into the soil below, so you can loosen the native soil with a fork before filling and get away with a shallower bed.
If your bed sits on concrete, paving or a roof, the soil depth is all the root room the plants get, so do not skimp. A bed on a hard surface also needs drainage holes or a gap at the base, or water will pool.
Cutting the Cost of Tall Beds
Beds that are 24 to 30 inches tall are wonderful for your back but can swallow over a cubic yard of soil each. A 4 by 8 ft bed that is 24 inches tall needs 64 cubic feet if you fill it all with soil. To save money, fill the bottom third with untreated logs, branches, leaves and old straw, then top with 12 to 18 inches of good soil.
Using the example above, filling only the top 16 inches with soil needs 4 times 8 times 16 divided by 12, which is 42.7 cubic feet. That saves 21.3 cubic feet against the full fill, or about 14 bags of 1.5 cubic feet. The woody material breaks down slowly, so plan to top it up with compost each year. When you work out how much soil for a raised bed, count only the depth of soil you are actually buying, not the full height of the boards. Only use untreated wood in the base layer.
Bagged or Bulk?
For one or two small beds, bags are convenient and easy to move. You can carry them one at a time and open them as you need them.
Once you need more than about a cubic yard, bulk delivery usually works out cheaper per cubic foot, but compare the delivery fee and check the quality before it is tipped. A 4 by 12 ft bed at 12 inches deep needs 1.78 cubic yards, so order the next whole yard up. The mulch calculator can help you plan the mulch layer that goes on top once the bed is filled.
Keeping Beds Topped Up
Soil level in raised beds drops a little every year as organic matter breaks down. Adding 1 to 2 inches of compost each autumn or spring keeps the level up. For a 4 by 8 ft bed, 1 inch of compost is 2.7 cubic feet and 2 inches is 5.3 cubic feet, which is 4 bags of 1.5 cubic feet.
Top up before you plant, not after, so the compost is mixed into the top few inches and roots can reach it. If the soil is more than 3 inches below the top of the boards, add soil as well. The square foot garden calculator can help you plan what to plant once the bed is ready.
Mistakes to Avoid
The most common mistake is measuring the outside of the bed instead of the inside. With thick boards the difference is about 12 percent, which is enough to leave a bed short by several bags. The second mistake is forgetting the settling allowance, so a bed that looks full on filling day can be several inches low a month later. The third mistake is filling a deep bed entirely with expensive mix, when logs and branches in the base would do. The fourth mistake is using potting mix alone in a large bed.
Frequently Asked Questions
Q: How much soil for a raised bed do I need for a 4 by 8 ft bed?
A: A 4 by 8 ft bed filled 12 inches deep needs 32 cubic feet of soil, which is 1.19 cubic yards or 22 bags of 1.5 cubic feet. Add 10 to 20 percent for settling.
Q: How many bags of soil do I need for a raised bed?
A: Divide the cubic feet your bed needs by the bag size. A 4 by 8 ft bed at 12 inches deep needs 32 cubic feet, so 32 bags of 1 cubic foot, 22 bags of 1.5 cubic feet, or 16 bags of 2 cubic feet.
Q: How many cubic yards of soil do I need for a raised bed?
A: Divide cubic feet by 27. A 4 by 8 ft bed at 12 inches deep needs 1.19 cubic yards. A 4 by 12 ft bed at 12 inches deep needs 1.78 cubic yards.
Q: How deep should soil be in a raised bed?
A: Most vegetables do the bulk of their root work in the top 12 inches. Lettuce, radishes and herbs grow well in 6 to 8 inches, while tomatoes, squash and root crops appreciate 12 inches or more.
Q: What is the best soil mix for a raised bed?
A: A mix of 60 percent topsoil and 40 percent compost suits most deep beds and keeps costs down. For heavy soil add drainage with 20 percent perlite or pumice.
Q: How much does soil settle in a raised bed?
A: Fresh soil often settles by 10 to 20 percent after the first few waterings. For a 4 by 8 ft bed at 12 inches deep that means adding 3.2 to 6.4 cubic feet. Top up with compost each year, about 1 to 2 inches per year.
Measure the inside, run the three steps for how much soil for a raised bed, and buy the settling allowance with the first load. A bed that starts full stays productive for years with a compost top up each spring.
