Raised bed soil: getting the mix right
Last reviewed September 2026 · written from public-domain USDA/NOAA data and Cooperative Extension references — see how we source data
A raised bed is only as good as what goes in it. The goal is a mix that holds moisture and nutrients but still drains, and that you top up with organic matter every season.
A reliable fill
For most gardeners: roughly half a bulk "garden soil" or "raised bed" blend and half compost, mixed. The square-foot method's equal thirds of compost, coir or peat, and vermiculite also works well and is lighter.
Avoid filling with bagged potting mix alone (expensive, dries fast) or with native soil alone if it is heavy clay (compacts). Never use pure topsoil.
How deep the soil needs to be
Six inches of good soil grows lettuce, radishes and most herbs. The majority of vegetables do much better with 10-12 inches, and deep-rooted crops like tomatoes, squash and carrots appreciate 12-18.
If the bed sits on loosened native soil rather than paving, roots carry on down into the ground beneath, so a 10-12 inch bed is enough for almost everything. On concrete or a patio, go for 18 inches or more.
How much you need
Volume in cubic feet = length (ft) × width (ft) × depth (inches) ÷ 12. Divide by 27 for cubic yards, which is how bulk material is sold and delivered. Anything over about one cubic yard is usually cheaper delivered in bulk than in bags. The raised bed soil calculator does this and estimates bag counts.
How much soil a raised bed takes
| Bed | 6″ deep | 11″ deep | 12″ deep | 18″ deep |
|---|---|---|---|---|
| 4 × 4 ft | 8 cu ft0.30 yd³ · 6 bags | 15 cu ft0.54 yd³ · 10 bags | 16 cu ft0.59 yd³ · 11 bags | 24 cu ft0.89 yd³ · 16 bags |
| 6 × 3 ft | 9 cu ft0.33 yd³ · 6 bags | 17 cu ft0.61 yd³ · 11 bags | 18 cu ft0.67 yd³ · 12 bags | 27 cu ft1.00 yd³ · 18 bags |
| 8 × 2 ft | 8 cu ft0.30 yd³ · 6 bags | 15 cu ft0.54 yd³ · 10 bags | 16 cu ft0.59 yd³ · 11 bags | 24 cu ft0.89 yd³ · 16 bags |
| 8 × 4 ft | 16 cu ft0.59 yd³ · 11 bags | 29 cu ft1.09 yd³ · 20 bags | 32 cu ft1.19 yd³ · 22 bags | 48 cu ft1.78 yd³ · 32 bags |
| 12 × 4 ft | 24 cu ft0.89 yd³ · 16 bags | 44 cu ft1.63 yd³ · 30 bags | 48 cu ft1.78 yd³ · 32 bags | 72 cu ft2.67 yd³ · 48 bags |
Filling deep beds cheaply
For beds deeper than about 12 inches, the bottom third can be coarse organic matter — small logs, branches, leaves, straw — with the growing mix on top. It breaks down over a few years, feeding the bed, and you top up as it settles.
What goes under the bed
Over lawn, lay overlapping plain brown cardboard (tape and labels removed) directly on the mown grass and soak it before filling. It smothers the turf and rots away within a season.
Where gophers or voles are a problem, staple galvanized hardware cloth across the bottom of the frame before filling.
Skip landscape fabric and gravel "drainage layers". Fabric blocks roots and worms, and gravel under soil actually slows drainage: water will not move from fine soil into coarse gravel until the soil above it is saturated.
Keeping it good
Soil in a bed loses volume and fertility as organic matter breaks down and crops remove nutrients. Add an inch or two of compost each season, keep the surface mulched or planted, and avoid compacting it by never standing on the bed.
Signs the soil needs attention
The level has dropped by two inches or more. That is normal as organic matter breaks down - top up with compost.
Water pools on the surface or runs off. Add organic matter and fork it lightly into the top few inches.
Crops grow slowly and lower leaves turn pale yellow. The bed needs feeding - see nitrogen deficiency.
After a hot spell the surface is dry, crusted and sheds water. Mulch it, and water slowly in several passes until it wets through again.
Common questions
What is the best soil for a raised bed?
A blend of roughly half screened topsoil or a bulk "raised bed" mix and half compost suits most vegetables: it holds water and nutrients but still drains. Bagged potting mix on its own dries out fast and is expensive at raised-bed volumes.
How much soil do I need for a 4×8 raised bed?
A 4×8 bed 12 inches deep holds 32 cubic feet - about 1.2 cubic yards, or 22 bags of 1.5 cubic feet. At 11 inches (two stacked 2×6 boards) it is about 29 cubic feet. The raised bed soil calculator works out any size.
Should I put rocks or gravel in the bottom of a raised bed?
No. A gravel layer under soil does not improve drainage and can make the bed wetter, because water will not move from fine soil into coarse gravel until the soil above is saturated. Fill the full depth with growing mix, or coarse organic matter in the bottom of very deep beds.
Do I need to replace raised bed soil every year?
No. Top it up with an inch or two of compost each year as it settles. Replacing it is only worth considering if it carries a serious soil-borne disease, which is rare.