How Do You Calculate How Much Soil a Raised Bed Needs?
Raised bed soil volume equals length times width times fill height. Multiply length by width in feet, multiply by the height in inches, divide by 12 for cubic feet, then multiply by the number of beds. Divide by 27 for cubic yards.
The formula is length (ft) × width (ft) × height (in) ÷ 12 × number of beds = cubic feet. Three 6 × 3 ft beds filled 10 inches deep hold 18 × 10 ÷ 12 × 3 = 45 cubic feet, which is 1.67 cubic yards or 30 bags of 1.5 cubic feet. Round beds and stock-tank planters use π × radius² for the area: a 4 ft round bed 12 inches deep holds 12.6 cubic feet. In metric, multiply length × width in meters by the height in centimeters and multiply by 10 for liters.
Measure the inside of the frame, not the outside, because 2-inch lumber or block walls take up part of the footprint. The raised bed calculator is one of several free garden calculators for planning beds and materials. Open ground, lawns and low spots without a frame are sized with the topsoil calculator for cubic yards, bags and tons, which also estimates the weight of bulk soil.
What Is the Best Soil Mix for Raised Beds?
The most common raised bed soil mix is 60% topsoil, 30% compost and 10% aeration material such as perlite or pumice. A simpler 50/50 blend of topsoil and compost also works well for vegetables, herbs and flowers.
Each ingredient has a job. Topsoil provides mineral structure and holds water, compost supplies nutrients and soil life, and perlite or pumice keeps the mix from compacting. The 50/50 blend is richer and suits heavy feeders such as tomatoes and squash. The square foot gardening mix — equal thirds of compost, peat moss or coir, and vermiculite — contains no topsoil at all, so it is light and weed-free but costs more to fill a deep bed.
Compost is usually the largest purchase after topsoil: a 4 × 8 ft bed 12 inches deep takes 9.6 cubic feet of compost in a 60/30/10 mix and 16 cubic feet in a 50/50 mix. The compost calculator for garden beds and top-dressing sizes the yearly compost layer that keeps raised bed soil fertile.
How Much Soil Does a 4×8 Raised Bed Need?
A 4 × 8 ft raised bed needs 16 cubic feet of soil at 6 inches deep, 32 cubic feet at 12 inches and 48 cubic feet at 18 inches. At 12 inches, that equals 1.19 cubic yards, 22 bags of 1.5 cubic feet or about 906 liters.
| Bed size | Cubic feet | Cubic yards | 1.5 cu ft bags | Liters |
|---|---|---|---|---|
| 4 × 4 ft, 6 in deep | 8 ft³ | 0.30 yd³ | 6 | 227 L |
| 4 × 4 ft, 12 in deep | 16 ft³ | 0.59 yd³ | 11 | 453 L |
| 3 × 6 ft, 12 in deep | 18 ft³ | 0.67 yd³ | 12 | 510 L |
| 4 × 8 ft, 6 in deep | 16 ft³ | 0.59 yd³ | 11 | 453 L |
| 4 × 8 ft, 12 in deep | 32 ft³ | 1.19 yd³ | 22 | 906 L |
| 4 × 8 ft, 18 in deep | 48 ft³ | 1.78 yd³ | 32 | 1359 L |
| 4 × 12 ft, 12 in deep | 48 ft³ | 1.78 yd³ | 32 | 1359 L |
| 4 ft round, 12 in deep | 12.6 ft³ | 0.47 yd³ | 9 | 356 L |
Volume = length × width × height in inches ÷ 12. Bag counts are rounded up; add about 10% for settling.
How Can You Fill a Deep Raised Bed With Less Soil?
Fill the bottom third of a deep raised bed with logs, branches, leaves or straw, then add at least 12 inches of soil mix on top. This hügelkultur-style base cuts the soil you need to buy and holds moisture as the wood breaks down.
An 18-inch bed filled with 6 inches of logs and branches needs 12 inches of soil — a third less than filling the full depth. Use untreated hardwood or softwood; avoid pressure-treated lumber and black walnut, whose wood contains juglone, a compound that inhibits the growth of many vegetables. Enter the depth of the filler layer in the calculator and it subtracts that height from the soil total.
Decomposing wood draws nitrogen from the soil around it during the first year or two, so a generous compost layer above the logs keeps vegetables supplied. The bed surface also sinks as the wood rots, and each year's top-up restores the level.
How Deep Should a Raised Garden Bed Be?
Most raised garden beds should be 10 to 12 inches (25–30 cm) deep. Beds on loose native soil can be as shallow as 6 inches, while beds on pavement, compacted clay or hardpan need 18 to 24 inches of soil.
Shallow-rooted crops such as lettuce, radishes, spinach and most herbs grow well in 6 to 8 inches of soil. Tomatoes, peppers, squash and carrots do best with 12 inches or more, especially when the bed sits on ground their roots cannot penetrate. Beds 24 to 30 inches tall bring the soil to a comfortable working height for gardeners who prefer not to kneel, at the cost of four to five times the soil of a 6-inch bed.
Soil in a raised bed warms earlier in spring and drains faster than the ground, but the exposed sides also let it freeze harder in winter. Perennials, shrubs and strawberries that stay in a raised bed year-round survive more reliably when they are rated about one zone colder than the local climate, so finding your USDA hardiness zone by ZIP code is a useful first step before planting them.
How Much Does Raised Bed Soil Settle?
Raised bed soil typically settles 10 to 20% in the first season as air pockets collapse and organic matter decomposes. Buy about 10% more than the calculated volume, or fill the bed to the rim and top it up with compost the next year.
A 4 × 8 ft bed filled with 32 cubic feet of fresh mix can drop 1 to 2 inches after a few months of watering. Mixes high in compost, peat or coir settle more than mixes built on topsoil. Watering the fill thoroughly as each 4-inch layer goes in reduces later settling and shows early whether more soil is needed. Watering needs also differ by crop and species, and the care guides for more than 800 plant species describe how much water, light and soil drainage each plant prefers.
Frequently Asked Questions
- How many bags of soil do I need for a 4×8 raised bed?
- A 4 × 8 ft raised bed 12 inches deep holds 32 cubic feet of soil: 22 bags of 1.5 cubic feet or 16 bags of 2 cubic feet. At 6 inches deep it holds 16 cubic feet, or 11 bags of 1.5 cubic feet.
- Can I fill a raised bed with only topsoil?
- Topsoil alone compacts in a raised bed, drains slowly and holds few nutrients. Blending it with 30 to 50% compost keeps the fill loose and fertile, which is why most raised bed mixes combine topsoil and compost.
- Can I use potting soil in a raised bed?
- Potting soil works in a small raised bed but costs more per cubic foot, dries out faster and settles more than a topsoil and compost blend. It is best kept for containers and shallow tabletop planters.
- Should I put gravel in the bottom of a raised bed?
- Gravel in the bottom of a raised bed does not improve drainage; water collects in the soil above the gravel until that layer saturates. Cardboard over existing grass and wire mesh against burrowing animals are more useful bottom layers.
- How often should raised bed soil be topped up?
- Raised bed soil needs a top-up once a year. Adding 1 to 2 inches of compost each spring or fall replaces the organic matter that breaks down and restores the level lost to settling.