A new raised bed looks small until you start buying soil for it. A 4 × 4 ft bed that is 10 inches tall holds about 13 cubic feet, which is roughly seven 2-cubic-foot bags or nine 1.5-cubic-foot bags. Here is how to work out your own number and fill the bed in layers so you are not paying for premium mix all the way to the ground.
1. Work out the volume
Multiply length × width × height in feet. For a 48 × 48 × 10 in. bed: 4 × 4 × 0.83 = 13.3 cubic feet. For a 96 × 28 × 10 in. bed: 8 × 2.33 × 0.83 = 15.5 cubic feet. Soil settles by 10–20% after watering, so add a little extra or plan to top up after the first few weeks.
2. Pick the spot first
Most vegetables want six or more hours of direct sun. Set the frame on level ground before filling it; once it is full of wet soil it is too heavy to shift. If you are placing it on grass, you do not need to dig the turf out.
3. Fill in layers
- Bottom: a layer of plain cardboard with the tape removed. It smothers grass and weeds and breaks down over the season.
- Middle (optional, for taller beds): coarse organic matter such as small branches, leaves or straw. It takes up space cheaply and feeds the soil as it breaks down.
- Top 6–8 inches: this is where most roots grow, so use your best material here: a mix of roughly equal parts topsoil and compost, or a bagged raised-bed mix.
In a 10-inch bed, keep the coarse middle layer thin or skip it; you want most of the depth to be good soil.
4. Water, settle, then plant
Water each layer as you go, then let the bed settle for a day before planting. Top up any low spots with compost.
5. Keep it going
Each spring, add an inch or two of compost to the surface. You will not need to replace the soil, only feed it.
Planning a new bed? Browse our raised beds and garden supplies; every listing shows its exact dimensions so you can do the math before you order.