Start with one-third compost, two-thirds existing soil or base material
The most reliable ratio for a new raised bed is to fill it one-third with compost and two-thirds with other material—either garden soil from your yard, topsoil you've bought, or a mix of both. This ratio gives you enough organic matter to feed plants and improve water retention without making the bed so rich that it drains poorly or costs too much to fill.
If you're refilling an existing bed that's been in use for a year or more, you can use less compost. Add a 2- to 3-inch layer of compost on top each spring, work it into the top few inches, and leave the rest of the bed undisturbed. This is cheaper and faster than emptying and refilling the whole thing.
The exact amount depends on three things: the size of your bed, what you're growing, and what kind of compost you're using. A bed that's 4 feet by 8 feet by 12 inches deep holds about 32 cubic feet of material. One-third of that is roughly 11 cubic feet of compost, which is a little over one cubic yard.
Key Takeaways
- A new raised bed should be filled one-third compost and two-thirds soil or topsoil to balance cost, drainage, and plant nutrition.
- For an existing bed, add a 2- to 3-inch layer of compost each spring instead of refilling the whole bed.
- A 4-by-8-foot bed that is 12 inches deep needs about 11 cubic feet (roughly one cubic yard) of compost for the one-third ratio.
- Finished compost (dark, crumbly, earthy-smelling) works better than partially decomposed material, which can tie up nitrogen as it breaks down.
- Vegetables and herbs need richer soil than ornamental plants, so you may add slightly more compost for a vegetable bed.
Calculate the volume of your raised bed
Measure the length, width, and depth of your bed in feet, then multiply all three numbers together. That gives you the total cubic feet the bed holds. Divide by three to find how much compost you need.
Common bed sizes and their one-third compost needs:
| Bed Size (L × W × D) | Total Volume | Compost Needed (1/3) |
|---|---|---|
| 4 ft × 4 ft × 12 in | 16 cubic feet | 5.3 cubic feet |
| 4 ft × 8 ft × 12 in | 32 cubic feet | 10.7 cubic feet |
| 3 ft × 6 ft × 10 in | 15 cubic feet | 5 cubic feet |
| 2 ft × 4 ft × 12 in | 8 cubic feet | 2.7 cubic feet |
Most garden centers and landscape suppliers sell compost by the cubic yard (27 cubic feet). For a 4-by-8-foot bed 12 inches deep, you'll need a little less than half a cubic yard of compost. For a 4-by-4-foot bed, you'll need about one-fifth of a cubic yard.
Use finished compost, not partially decomposed material
Finished compost is dark brown or black, crumbly, and smells earthy—like forest floor. It breaks apart easily in your hand and contains no recognizable food scraps or leaves. This is what you want in a raised bed.
Partially decomposed compost (sometimes called "compost in progress") is lighter in color, still has visible chunks, and may smell sour or ammonia-like. If you use it, you're adding material that will continue to break down in your bed. As it decomposes, it uses nitrogen from the soil, which can starve your plants in the first few weeks. It also shrinks as it finishes decomposing, so your bed level will drop noticeably over the first month or two.
If you're making your own compost and it's not fully finished, you can still use it—just add it to the bottom layer of the bed and cover it with finished compost or soil. This keeps it away from plant roots while it completes its breakdown.
Adjust the ratio for what you're growing
Vegetables and herbs are heavy feeders and benefit from richer soil. If you're planting tomatoes, peppers, lettuce, or herbs, you can increase compost to 40 or even 50 percent of the bed's volume. This gives roots more organic matter to draw nutrients from and improves water retention in sandy soil.
Ornamental plants, shrubs, and perennials are less demanding. They often do well with 25 to 30 percent compost. Too much compost can actually make the soil too rich and cause excessive leafy growth at the expense of flowers.
If you're planting a mixed bed with both vegetables and ornamentals, stick with the one-third ratio as a middle ground. You can always add more compost as a top layer next season if plants seem to be struggling.
Account for settling and decomposition over time
A newly filled raised bed will settle over the first few weeks as the materials compact and water moves through. You may see the level drop by an inch or two. This is normal and doesn't mean you added too little—it just means the materials have shifted and air pockets have closed.
Over the course of a growing season, compost and organic matter continue to break down, which lowers the bed level further. By the end of the year, a bed that started 12 inches full may be down to 10 or 11 inches. This is why adding a fresh layer of compost each spring keeps the bed at a workable depth without requiring a complete refill.
Where to source compost and what to expect to pay
Finished compost is sold by garden centers, landscape suppliers, and some municipalities. Prices vary widely depending on where you live and whether you pick it up or have it delivered. Bagged compost (typically 2 cubic feet per bag) costs more per cubic foot than bulk compost delivered by the yard, but requires no equipment to transport.
Some cities offer free or low-cost compost made from yard waste collected during leaf and branch pickup. Check your local waste management or parks department website to see if a program exists in your area. Quality varies, but municipal compost is usually finished and ready to use.
If you're filling a large bed or multiple beds, buying by the cubic yard is almost always cheaper than bagged compost. A landscape supplier can deliver it to your driveway, though delivery fees explore in some areas.
Frequently Asked Questions
Can I use 100 percent compost to fill a raised bed?
You can, but it's expensive and unnecessary. Pure compost drains very quickly and can dry out in hot weather, and it costs two to three times more than a compost-and-soil mix. A one-third compost ratio gives you the same benefits at a fraction of the cost.
What if I don't have enough compost for the one-third ratio?
Use what you have and fill the rest with topsoil or garden soil. Add a thicker layer of compost (3 to 4 inches) on top in the first season, and add more compost each spring. Your bed will gradually become richer over time, and plants will still grow well in the meantime.
Should I add compost to the bottom or mix it throughout?
Either works. Mixing it throughout gives even nutrition and water retention. Layering it (compost on the bottom, soil on top) is faster and works just as well for most plants. Roots will reach down to the compost layer as they grow.
Do I need to replace all the compost every year?
No. Add a 2- to 3-inch layer of fresh compost each spring and work it into the top few inches. This replenishes nutrients and organic matter without the cost and labor of emptying the whole bed. Most gardeners refill completely only every three to five years.
What's the difference between compost and potting soil for a raised bed?
Potting soil is lightweight and designed for containers; it drains very quickly and doesn't hold nutrients well over a full season. Compost is heavier, holds water and nutrients longer, and improves soil structure. Use compost for raised beds and potting soil only for pots and seed trays.