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Enter your bed size below to see exactly how much soil you need — in cubic feet, cubic yards and bags — and how much you save by filling the bottom with free material. Soil is usually the most expensive part of a raised bed, so getting the number right before you shop is the fastest way to save money.
How to measure your bed
Measure the inside of the frame, not the outside — block and timber walls can eat 8–16 inches of width on a small bed. For depth, measure from the ground (or the top of your filler layer) to about an inch below the rim, so soil and mulch don’t wash over the edge. For round or odd-shaped beds, measure the longest and widest points and take about 80% of the result as a quick estimate.
The formula behind the calculator
Volume in cubic feet = length (ft) × width (ft) × depth (in) ÷ 12. Divide by 27 to get cubic yards. Divide cubic feet by the bag size (usually 1.5 or 2 cu ft) and round up to get the number of bags.
Common bed sizes
| Bed size | Depth | Cu ft | Cu yd | 1.5 cu ft bags |
|---|---|---|---|---|
| 2×4 ft | 12 in | 8 | 0.30 | 6 |
| 3×6 ft | 12 in | 18 | 0.67 | 12 |
| 4×4 ft | 12 in | 16 | 0.59 | 11 |
| 4×8 ft | 8 in | 21.3 | 0.79 | 15 |
| 4×8 ft | 12 in | 32 | 1.19 | 22 |
| 4×8 ft | 18 in | 48 | 1.78 | 32 |
Bag counts are rounded up. Soil settles 10–20% in the first season, so leave room to top up.
Bags or bulk? The break-even point
Bagged soil is convenient and has no delivery fee, but it costs more per cubic foot. Bulk soil sold by the cubic yard is usually much cheaper per foot, but delivery often adds a flat fee. As a rule of thumb, once you need more than about one cubic yard (27 cu ft) — one deep 4×8 bed or two shallow ones — get a bulk quote. Below that, bags almost always win.
The 60/30/10 mix
The calculator splits your total into a simple, cheap blend:
- 60% topsoil — the bulk of the volume and the cheapest component.
- 30% compost — feeds plants and holds water. Many towns give away leaf compost free.
- 10% aeration — perlite, pine bark fines or coarse sand to keep it from compacting.
Pre-mixed “raised bed soil” is fine if you only need a few bags. For bigger beds, mixing your own from these three parts usually costs less.
Use free filler to cut soil by half
Vegetable roots mostly live in the top 8–12 inches. In a deep bed, the bottom layer can be logs, branches, leaves, grass clippings and cardboard — all free. As it breaks down it feeds the soil above (the idea behind hügelkultur). Tick the filler box in the calculator to see the reduced soil number. In our 4×8 cinder block bed receipt, free filler is what keeps the soil bill to about $77.
Two cautions: skip walnut wood and anything treated or painted, and expect a wood-filled bed to sink a little more in year one as the logs break down. Top it up with compost rather than new soil.
Topping up an existing bed
Older beds only need a refresh, not a refill. Measure how far the soil has dropped below the rim, enter that as the depth, and use the result for compost alone. One or two inches of compost each spring is usually enough to keep a bed productive.
FAQ
How many bags of soil for a 4×8 raised bed?
At 12 inches deep, 32 cubic feet: 22 bags of 1.5 cu ft or 16 bags of 2 cu ft. With free filler in the bottom half, about half that.
How many cubic yards for a 4×8×1 bed?
About 1.19 cubic yards (32 ÷ 27).
Can I use only topsoil?
You can, but pure topsoil compacts in a bed. Adding compost and an aeration ingredient makes plants grow better and drain properly.
Should I add extra for settling?
Plan for 10–20% settling in the first season. Top up with compost rather than buying extra soil upfront.
