The basic formula: compost, topsoil, and drainage material in layers

The best soil for a raised bed is not pure garden soil from your yard. Instead, you build it from three parts: topsoil (the dark, nutrient-rich layer from the top 6 inches of ground), compost (decomposed organic matter), and drainage material (coarse material like wood chips or perlite that keeps water from pooling). A common working ratio is 40% topsoil, 40% compost, and 20% drainage material, though the exact split depends on what you're growing and what you have on hand.

The reason you don't use pure topsoil is that it compacts over time, especially in a raised bed where water can't drain sideways into surrounding ground. Compost keeps the soil loose and feeds plants as it breaks down further. Drainage material prevents the bed from becoming waterlogged, which kills roots and causes rot. If you're building a bed in spring and plan to plant when ready, you can use this mix straight away. If you're building in fall, let it settle for a few weeks so the layers integrate before planting.

Key Takeaways

  • Mix topsoil, compost, and drainage material in roughly equal parts (40-40-20) rather than using garden soil alone, which compacts and holds too much water.
  • Topsoil is the dark layer from the top 6 inches of existing ground; compost can be homemade, bagged, or bulk-delivered; drainage material is coarse wood chips, perlite, or aged bark.
  • A 4-foot by 8-foot bed 12 inches deep needs roughly 32 cubic feet of mixed soil, which you can calculate by multiplying length × width × depth in inches, then dividing by 1,728.
  • You can skip the drainage layer if your raised bed sits on bare ground with drainage holes in the bottom, but include it if the bed sits on concrete, compacted soil, or a landscape fabric barrier.
  • Refresh the top 2 to 3 inches of soil each spring with fresh compost rather than replacing the entire bed, which saves money and maintains soil structure.

Where to source each ingredient and what to avoid

Topsoil comes from three places: your own yard (if you have good soil and can dig it), a landscape supply yard (by the cubic yard, usually $30 to $60 per yard depending on region), or bagged at a garden center ($3 to $8 per 40-pound bag). If you buy bagged topsoil, check the label for the weight per bag so you can calculate how many you need. Avoid topsoil labeled "fill dirt" or "construction soil"—these are often clay-heavy and compacted. If you're digging from your own yard, take soil from an area that drains well and has no weeds or disease.

Compost is the easiest to make yourself if you have kitchen scraps and yard waste. Finished compost is dark, crumbly, and smells earthy—not like rotting food. If you don't have homemade compost, buy bagged compost at a garden center (usually $3 to $6 per 40-pound bag) or order bulk compost from a landscape supplier. Bulk compost is cheaper per cubic yard but requires delivery. Avoid compost made only from wood chips or bark, which ties up nitrogen as it breaks down further. Look for compost labeled as "finished" or "garden compost," not "mulch."

Drainage material can be coarse wood chips (not fine mulch), aged bark, perlite, or even clean gravel. Coarse wood chips are cheapest and often free from tree services. Perlite is lightweight and doesn't break down, but costs more. Avoid fresh wood chips, which can heat up as they decompose and damage roots. If you use wood chips, let them age for 6 months to a year before mixing into soil.

Calculating how much soil you need

Measure your raised bed in inches: length, width, and depth. Multiply all three numbers together, then divide by 1,728 (the number of cubic inches in a cubic foot). This gives you the volume in cubic feet. For example, a bed that is 48 inches long, 24 inches wide, and 12 inches deep is 48 × 24 × 12 = 13,824 cubic inches, divided by 1,728 = 8 cubic feet of soil.

Once you know the cubic footage, divide by 3 to find how much of each ingredient you need. For the 8-cubic-foot bed above, you'd need roughly 3.2 cubic feet each of topsoil and compost, and 1.6 cubic feet of drainage material. If you're buying bagged soil, one 40-pound bag of topsoil or compost is roughly 0.75 cubic feet, so you'd need about 4 bags of topsoil, 4 bags of compost, and 2 bags of drainage material (or 2 bags of perlite). Bulk delivery is usually cheaper for beds larger than 16 cubic feet.

Building the bed layer by layer

Start with the drainage layer at the bottom. If your raised bed has a solid bottom or sits on concrete, spread 2 to 3 inches of coarse wood chips or perlite across the entire floor. If your bed sits directly on bare ground with drainage holes drilled in the bottom, you can skip this step—water will drain into the soil below. If you're unsure whether your bed drains, pour a bucket of water into an empty bed and watch where it goes. If it pools, add a drainage layer.

Next, add your topsoil. Spread it evenly across the bed until it reaches about 4 to 5 inches deep. Then add your compost layer, spreading it to another 4 to 5 inches. Finally, if you have any remaining drainage material, scatter it across the top and mix it in lightly with a garden fork or shovel. Water the whole bed gently so the layers settle and integrate. The soil will compact slightly over the first week, so you may need to add another inch or two of compost on top.

Adjusting the mix for what you're growing

The 40-40-20 formula works for most vegetables, herbs, and flowers. But some plants need tweaks. Root vegetables like carrots and parsnips prefer looser, sandier soil, so add an extra 10% coarse sand or perlite and reduce the topsoil slightly. Heavy feeders like tomatoes and squash benefit from extra compost—use 50% compost, 35% topsoil, and 15% drainage material. Acid-loving plants like blueberries need compost made from pine needles or sulfur-amended soil; ask your supplier if their compost is suitable.

If your tap water is very hard (high in minerals) or your region has alkaline soil, test your compost's pH before mixing. Most bagged compost is neutral to slightly acidic, but some are alkaline. A straightforward soil test kit from a garden center costs $10 to $20 and tells you the pH and nutrient levels. If you're growing in a very small bed (under 4 cubic feet), you might use a soilless mix like coco coir and perlite instead, which drains faster but requires more frequent feeding.

Maintaining and refreshing your soil year to year

After the first season, your soil will have settled and plants will have used up some nutrients. Each spring, top-dress the bed with 2 to 3 inches of fresh compost. Spread it evenly and work it into the top layer with a fork. This feeds the soil as the compost breaks down and keeps the bed from compacting. You don't need to replace the entire bed unless it's been in use for 5 or more years and has become dense and depleted.

If your bed drains poorly after a year or two, it usually means the compost has broken down and the soil has compacted. Add another 2 to 3 inches of compost and mix it in. If the problem persists, you may need to add more drainage material—carefully dig out the top 6 inches, mix in coarse material, and replace it. Avoid walking on the bed or compacting it with heavy tools, which speeds up settling.

Frequently Asked Questions

Can I use only compost without topsoil?

You can, but the bed will settle faster and may become too dense after a season. Topsoil adds structure and weight that keeps the bed stable. If you use only compost, plan to top-dress with 3 to 4 inches each spring instead of 2 to 3.

What if I can't find or afford bulk compost?

Make your own from kitchen scraps and yard waste in a bin or pile. It takes 2 to 6 months depending on temperature and how often you turn it. In the meantime, use bagged compost from a garden center, or increase the topsoil ratio to 50% and reduce compost to 30%, adding extra drainage material to compensate.

Do I need to test the soil before planting?

A soil test is helpful but not required. A basic test from your local extension office or a garden center kit tells you pH and nutrient levels, which helps you adjust the mix for specific plants. If you're growing vegetables and using quality compost and topsoil, the soil is usually ready to plant without testing.

Can I reuse soil from an old raised bed?

Yes. Remove the top 2 inches (which may have disease or weeds), then mix the remaining soil with fresh compost at a 1:1 ratio. This refreshes the nutrients and structure without buying entirely new soil.

What's the difference between compost and mulch?

Compost is fully decomposed organic matter that feeds soil and plants. Mulch is partially decomposed material (usually wood chips or bark) that sits on top of soil to retain moisture and suppress weeds. Use compost mixed into the bed, and mulch as a top layer around plants.