How to Fill a Raised Garden Bed
Filling a raised bed is a chance to build ideal soil from the start. This article covers layering options, soil blends, how much to buy for common bed sizes, and what to expect after settling.
By the CalcAndPlan Editorial Team · Published · 3 min read
In this article
Two ways to fill
Some gardeners fill the whole bed with a uniform soil blend. It is simple, predictable, and plants root evenly from day one. Others use a layered approach for tall beds, putting coarse organic material such as untreated logs, branches, and leaves in the bottom and good soil on top. Layering saves money on deep beds but settles more as the bottom layers decay.
A balanced blend
A mix of quality topsoil and compost is the usual foundation. Topsoil provides body and mineral content; compost adds organic matter and helps the bed hold moisture. Avoid filling with pure compost, which can shrink dramatically, or heavy native clay, which can drain poorly when confined in a box.
Run your numbersRaised Garden Bed CalculatorHow much the common sizes hold
Volume is the inside length times the inside width times the fill height. A few reference points help with budgeting:
- 3 × 6 ft at 12 in: 18 cubic feet
- 4 × 4 ft at 12 in: 16 cubic feet
- 4 × 8 ft at 12 in: 32 cubic feet
- 4 × 8 ft at 18 in: 48 cubic feet
After the first season
Expect the surface to drop a little as the soil settles and organic matter breaks down. A fresh inch or two of compost each year usually brings it back up and feeds the next crop.
Choosing a fill height
Most vegetables root well in 8 to 12 inches of good soil, and deeper beds help carrots, parsnips, and tomatoes. If your bed is taller than your plants need, either use a filler layer at the bottom or plan for deeper-rooted crops. Whatever you decide, use the soil depth — not the frame height — when calculating how much to buy.
Bagged mix or bulk blend?
Bagged raised-bed mixes are consistent, clean, and easy to carry, which makes them a good fit for one or two small beds. For several beds, bulk blends from a landscape yard are usually much cheaper per cubic foot, and many yards sell a ready-mixed garden or raised-bed blend.
If you buy bulk topsoil and compost separately, mix them on a tarp or in the bed as you fill. Aim for an even blend rather than distinct layers, so roots don’t hit a sudden change in texture.
Planting the first season
Let freshly filled beds settle with a good watering or two before planting, if you have time. Top up low spots, then plant. Mulching the surface helps the soil hold moisture and reduces crusting after rain.
At the end of the season, check the soil level. Adding compost to bring it back up each year keeps the bed productive and replaces organic matter used by the crops.
Worked example: two 4 × 8 ft beds and one 3 × 6 ft bed, all 12 in deep
- 4 × 8 × 1 = 32 cu ft each, so 64 cu ft for two
- 3 × 6 × 1 = 18 cu ft
- Total: 82 cu ft = 3.04 cu yd
- With 10% for settling: 90.2 cu ft = 3.34 cu yd
- In 1.5 cu ft bags: 82 ÷ 1.5 = 54.7, so 55 bags before the allowance
About 3.3 cubic yards — at this size, bulk delivery is usually worth pricing.
Helpful tips
Pro tips
- Measure inside the frame. Thick boards reduce the fill area more than you’d expect on small beds.
- Water each layer as you fill to settle it before planting.
- Leave an inch or two of rim so mulch and water stay in the bed.
Frequently asked questions
Can I fill the bottom with cardboard?
Cardboard at the base is often used to smother grass and weeds. It breaks down within a season, so it doesn’t replace soil volume.
Do raised beds need a bottom?
Most garden beds sit directly on the ground so roots and water can move into the soil below. Beds on patios or decks are the exception.
The bottom line
Calculate the inside volume of every bed, decide between a uniform blend and layering, and plan for settling. A topped-up inch of compost each year keeps the bed productive.
Further reading
General information for planning. Conditions vary by region, site, and product — check product labels and local guidance.