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Short answer: Multiply the bed's floor area by its depth in feet. For a rectangle that's length × width; for an oval and a circle the shape costs you real volume, and most calculators get this wrong. An 8 × 4 ft bed needs 32 cubic feet of soil at 12 inches if it's rectangular, but only 28.6 if it's oval. Charts for every YoYoBloom size are below.
Soil is usually the biggest surprise in a raised bed budget, and it is often more than the bed itself. Work it out before you drive to the garden center, not after.
The formulas
Everything in feet. If your depth is in inches, divide by 12 first: 12" = 1 ft, 24" = 2 ft, 36" = 3 ft.
Rectangle: length × width × depth
Oval: [(length − width) × width + π × (width ÷ 2)²] × depth
Circle: π × (diameter ÷ 2)² × depth
cubic feet ÷ 27 = cubic yards
One adjustment worth making: don't fill to the rim. Leave an inch or two below the top edge so soil and mulch don't wash over the sides when you water. That takes roughly 5% off the figures below.
Exact soil volumes for every YoYoBloom bed
Bag counts are rounded up. Being two bags short on planting day is worse than having one left over.
Rectangular and square beds
| Bed size | Cubic feet | Cubic yards | Bags (1.5 cu ft) | Bags (2 cu ft) |
|---|---|---|---|---|
| 8 × 4 ft, 12" tall | 32.0 | 1.19 | 22 | 16 |
| 8 × 4 ft, 24" tall | 64.0 | 2.37 | 43 | 32 |
| 8 × 4 ft, 36" tall | 96.0 | 3.56 | 64 | 48 |
| 6 × 3 ft, 12" tall | 18.0 | 0.67 | 12 | 9 |
| 6 × 3 ft, 24" tall | 36.0 | 1.33 | 24 | 18 |
| 6 × 3 ft, 36" tall | 54.0 | 2.00 | 36 | 27 |
| 12 × 4 ft, 12" tall | 48.0 | 1.78 | 32 | 24 |
| 4 × 4 ft, 36" tall | 48.0 | 1.78 | 32 | 24 |
| 4 × 2 ft, 32.4" tall | 21.6 | 0.80 | 15 | 11 |
| 8 × 2 ft, 32.4" tall | 43.2 | 1.60 | 29 | 22 |
Oval beds
| Bed size | Cubic feet | Cubic yards | Bags (1.5 cu ft) | Bags (2 cu ft) |
|---|---|---|---|---|
| 8 × 4 ft, 12" tall | 28.6 | 1.06 | 20 | 15 |
| 8 × 4 ft, 24" tall | 57.1 | 2.12 | 39 | 29 |
| 8 × 4 ft, 36" tall | 85.7 | 3.17 | 58 | 43 |
| 6 × 3 ft, 12" tall | 16.1 | 0.60 | 11 | 9 |
| 6 × 3 ft, 18" tall | 24.1 | 0.89 | 17 | 13 |
| 6 × 3 ft, 24" tall | 32.1 | 1.19 | 22 | 17 |
| 6 × 3 ft, 36" tall | 48.2 | 1.79 | 33 | 25 |
| 8 × 2 ft, 12" tall | 15.1 | 0.56 | 11 | 8 |
| 8 × 2 ft, 24" tall | 30.3 | 1.12 | 21 | 16 |
| 4 × 2 ft, 12" tall | 7.1 | 0.26 | 5 | 4 |
| 4 × 2 ft, 24" tall | 14.3 | 0.53 | 10 | 8 |
Round beds
| Bed size | Cubic feet | Cubic yards | Bags (1.5 cu ft) | Bags (2 cu ft) |
|---|---|---|---|---|
| 2 ft diameter, 12" tall | 3.1 | 0.12 | 3 | 2 |
| 2 ft diameter, 24" tall | 6.3 | 0.23 | 5 | 4 |
| 3 ft diameter, 12" tall | 7.1 | 0.26 | 5 | 4 |
| 3 ft diameter, 24" tall | 14.1 | 0.52 | 10 | 8 |
| 3 ft diameter, 36" tall | 21.2 | 0.79 | 15 | 11 |
| 4 ft diameter, 12" tall | 12.6 | 0.47 | 9 | 7 |
| 4 ft diameter, 24" tall | 25.1 | 0.93 | 17 | 13 |
| 4 ft diameter, 36" tall | 37.7 | 1.40 | 26 | 19 |
For multi-packs, multiply by the number of beds. A 4-pack of 8 × 4 ft oval beds at 24 inches needs about 229 cubic feet, or 8.5 cubic yards. At that volume, one bulk delivery beats 153 bags by a wide margin.
Why oval beds need about 11% less soil than you'd think
Nearly every raised bed soil calculator online assumes a rectangle. If your bed is oval and you use one, you will overbuy.
An oval bed — technically a stadium shape, a rectangle with a half-circle at each end — has less floor area than the rectangle it fits inside. For an 8 × 4 ft footprint:
- Rectangle: 8 × 4 = 32 sq ft
- Oval: 28.6 sq ft — about 11% less
At 36 inches deep that gap is over 10 cubic feet, or roughly seven bags.
The reason it works out this way is neat: the two rounded ends together form one complete circle whose diameter equals the bed's width. So an oval bed is just a square middle section plus one circle. Worked for 8 × 4: (8 − 4) × 4 = 16, plus π × 2² = 12.6, giving 28.6 sq ft.
The gap narrows in practice, though. Oval beds have no interior corners — and corners are exactly where soil stays coldest, drains slowest, and gets planted last. You lose area on paper and gain usable growing space.
Bagged or bulk?
The break-even point is around one cubic yard.
Under a cubic yard — bagged is fine. You can carry it yourself, store the leftovers, and you don't need somewhere to dump a delivery.
Over a cubic yard — bulk is dramatically cheaper, often less than half the price per cubic foot. A single 8 × 4 ft bed at 24 inches already needs over two cubic yards. That's 40-plus bags, several car trips, and a significant premium over one delivery.
Two conversions worth remembering:
- 1 cubic yard = 27 cubic feet = about 18 bags at 1.5 cu ft each
- A cubic yard of moist soil weighs roughly 1,200–1,400 lbs — more than most car trunks should carry
When ordering bulk, ask what you're actually buying. "Garden soil," "raised bed mix," and "topsoil" mean different things at different yards. Ask what's in it before you order three yards of it.
What to fill it with
This trips up more first-time gardeners than the volume math does. The three things sold at garden centers are not interchangeable:
- Topsoil — mineral soil. Cheap, heavy, compacts badly on its own. Good as a base component, poor as the only ingredient.
- Potting mix — designed for containers. Light, fast-draining, expensive. Filling a whole raised bed with it costs a fortune and dries out constantly.
- Compost — organic matter. Excellent nutrition and structure, but pure compost shrinks dramatically as it breaks down and can hold too much water.
A reliable starting mix:
| Component | Proportion | What it does |
|---|---|---|
| Topsoil or bulk garden soil | 60% | Body, minerals, water retention |
| Compost | 30% | Nutrition, biology, structure |
| Aeration material | 10% | Drainage and air pockets |
For the aeration portion use perlite, coarse sand, pine bark fines or vermiculite. Avoid fine sand mixed into clay-heavy topsoil — that combination sets up hard.
On using soil from your own yard: it's free, which is tempting. But native soil brings weed seeds, and heavy clay will compact into a brick inside the bed. If you use it, keep it to a third of the total at most and mix it thoroughly with compost.
How to fill a deep bed for less
A 36-inch bed needs three times the soil of a 12-inch bed, but plants only use the top 12 to 18 inches. You do not have to buy three feet of soil.
The bottom third can be filled with logs and branches (a method called hugelkultur), fallen leaves, straw, grass clippings, untreated wood chips, or cardboard and small twigs. This routinely cuts soil cost by 30 to 50 percent, and the material breaks down into rich soil over the following seasons.
What not to put in the bottom:
- Pressure-treated or painted wood
- Plastic sheeting or bags
- Gravel or rocks. This is the most persistent myth in raised bed gardening. A layer of coarse material under finer soil does not improve drainage — it creates a perched water table that makes drainage worse.
- Anything that will compact into a solid layer
Settling: budget for it
Soil settles. Expect to lose one to three inches in the first year as air pockets close and organic matter breaks down. Beds filled with a layering method settle more, sometimes six inches or more in the first season.
This is normal, not a sign anything went wrong. Plan to top up each spring with an inch or two of compost, which also replaces the nutrition last season's crops used.
Practical implication: buy about 10% more than your calculation says.
Weight and placement
Moist soil runs roughly 1,300 lbs per cubic yard. An 8 × 4 ft bed weighs about 1,400 lbs at 12 inches, 2,750 lbs at 24 inches, and 4,100 lbs at 36 inches once it's watered.
- Level the ground first. A bed this heavy will not settle evenly on a slope, and once it's full you are not moving it.
- Check before putting a tall bed on a deck or balcony. Most residential decks are not built for this. A 12-inch bed is a far safer bet on an elevated structure.
- Decide the location before you fill. This sounds obvious. It is the single most common regret.
Worked example
Two 8 × 4 ft oval beds at 24 inches:
- Volume per bed: 57.1 cubic feet
- Two beds: 114.3 cubic feet
- In cubic yards: 114.3 ÷ 27 = 4.2 cubic yards
- Add 10% for settling: about 4.7 cubic yards
- Fill the bottom third with logs and leaves instead: purchase about 3.1 cubic yards
- Split into the mix: roughly 1.9 yd topsoil, 0.9 yd compost, 0.3 yd aeration material
Had you used a rectangular calculator for those same oval beds, you'd have ordered close to a full extra cubic yard you didn't need.
Frequently asked questions
How much soil do I need for an 8x4 raised bed?
For a rectangular 8 × 4 ft bed: 32 cubic feet at 12 inches deep, 64 at 24 inches, and 96 at 36 inches. For an oval bed with the same footprint: 28.6, 57.1 and 85.7 cubic feet respectively.
How many bags of soil for a 4x8 raised bed?
Using 1.5 cubic foot bags: about 22 bags at 12 inches deep, 43 at 24 inches, and 64 at 36 inches for a rectangular bed. Check the bag size before you buy — 1 cu ft and 2 cu ft bags are also common and change the count substantially.
How do I calculate soil for an oval or round raised bed?
For an oval, area = (length − width) × width + π × (width ÷ 2)². For a circle, area = π × (diameter ÷ 2)². Multiply either by the depth in feet. Rectangular calculators will overestimate an oval bed by roughly 11%.
Is bagged or bulk soil cheaper?
Bulk is cheaper past about one cubic yard, often by half or more per cubic foot. Below that, bagged is usually more convenient and the price difference is small.
Can I fill a raised bed with just topsoil?
You can, but it will compact and drain poorly. Mixing in about 30% compost and 10% aeration material makes a substantial difference to plant performance.
Do I need to fill the entire depth with soil?
No. Plants use the top 12 to 18 inches. In beds 24 inches and deeper, the bottom third can be filled with logs, branches, leaves or untreated wood chips.
How much does raised bed soil settle?
One to three inches in the first year for a bed filled entirely with soil, and more for beds using a layering method. Buy about 10% extra, and top up with compost each spring.
Should I put gravel or rocks in the bottom for drainage?
No. This is a common myth. A layer of coarse material beneath finer soil creates a perched water table that makes drainage worse, not better.
Not sure which depth you need in the first place? Read our guide to choosing between 12", 24" and 36" beds.