What This Calculator Figures Out
Filling a raised bed is a volume problem: length times width times depth. The hard part is that soil is priced three different ways — by the cubic foot, the cubic yard, and the bag — and the numbers rarely match. This calculator takes your bed's measurements and gives you all three at once, so you can walk into a garden center or call a bulk yard already knowing exactly how much to ask for.
A Worked Example
Take the most common bed, 4 ft × 8 ft, filled 12 inches deep. The math is 4 × 8 × 1 = 32 cubic feet. Divide by 27 and that is about 1.2 cubic yards, or 21 of the 1.5 cu ft bags most stores stock. At roughly 40 pounds per cubic foot, that bed holds around 1,280 pounds of moist mix, which is why a bulk delivery beats hauling two dozen bags from the car.
Bags or Bulk?
The cubic-yard number is the one that decides bags versus bulk. Below about a cubic yard, bags are convenient and the price gap is small. Once you are filling two or more 4×8 beds — roughly 2.4 cubic yards — a bulk delivery at $30 to $60 per yard undercuts bagged soil that runs $67 to $108 per yard equivalent. Run your real dimensions above and let the yard figure make the call.
Raised Bed Soil Calculator
Soil Volume for Common Raised Bed Sizes
Rectangular beds at 12" depth, 2 cu ft bags
| Bed Size | Cubic Feet | Cubic Yards | 2-cu-ft Bags |
|---|---|---|---|
| 4×4 ft | 16.0 | 0.59 | 8 |
| 4×8 ft | 32.0 | 1.19 | 16 |
| 4×12 ft | 48.0 | 1.78 | 24 |
| 3×6 ft | 18.0 | 0.67 | 9 |
| 2×8 ft | 16.0 | 0.59 | 8 |
| 6×6 ft | 36.0 | 1.33 | 18 |
How to Use This Calculator
- Pick Your Bed Shape: Choose rectangular for a standard framed bed, or circular for a round planter or ring. The inputs change to match.
- Enter Dimensions and Depth: Type the length and width (or diameter) in feet and the depth in inches. A classic bed is 4 × 8 ft at 12 inches; set the number of beds if you are filling more than one.
- Choose a Bag Size: Match the bag size to what your store sells — 1.5 and 2 cu ft are the most common. This drives the bag count in the results.
- Read the Results: Check the cubic-feet figure for the mix, the cubic-yard figure for a bulk order, and the bag count for a store run. Add about 15 percent if you want a settling buffer.
How It Works
Enter your bed's length, width, and depth and the calculator returns the fill volume in cubic feet, then converts it to cubic yards for bulk orders and to the number of bags at your chosen bag size.
The basic rule:
- Rectangular: volume = length × width × depth (in feet)
- Circular: volume = π × (diameter/2)² × depth
- Convert inches to feet: divide depth by 12
- 1 cubic yard = 27 cubic feet
- Soil mix weighs approximately 40 lbs per cubic foot
For a bed 18 inches or deeper, fill the bottom third with logs, branches, or straw (the hugelkultur method). A 4×8 bed at 18 inches needs 48 cu ft; filling the lowest 6 inches with wood drops the soil you buy to about 32 cu ft.
Tips & Considerations
- Measure the inside of the frame, not the outside boards, or you will overbuy by the width of the lumber on every side.
- Order about 15 percent extra to cover settling — compost and peat mixes drop 10 to 20 percent after the first few waterings.
- Split the total into thirds for a balanced fill: topsoil, compost, and an aeration material like perlite or coarse sand.
- For beds over 18 inches deep, line the bottom third with logs or branches (hugelkultur) to cut the soil you buy nearly in half.
- Once your total passes one cubic yard, price a bulk delivery — it usually beats bagged soil by 50 to 70 percent.
Frequently Asked Questions
How much soil does a 4×8 raised bed need?
At a 12-inch depth, a 4 ft × 8 ft bed holds 4 × 8 × 1 = 32 cubic feet of fill. That is 1.19 cubic yards (32 ÷ 27), which works out to 16 of the 2 cu ft bags or 21 of the smaller 1.5 cu ft bags. Double the depth to 24 inches and you need 64 cubic feet.
How do I convert cubic feet of soil to cubic yards?
Divide cubic feet by 27, since one cubic yard is a 3 ft × 3 ft × 3 ft cube. So 32 cu ft ÷ 27 = 1.19 cu yd, and 54 cu ft ÷ 27 = 2 cu yd. Bulk soil yards are almost always sold by the cubic yard, so this is the number to give a supplier.
How many bags of soil equal a cubic yard?
It depends on the bag. A cubic yard is 27 cubic feet, so you need 27 of the 1 cu ft bags, 18 of the 1.5 cu ft bags, or about 14 of the 2 cu ft bags. Bag sizes are printed on the front — always check, because store brands mix 1, 1.5, 2, and 3 cu ft sizes on the same shelf.
What soil mix should I put in a raised bed?
A common blend is 1/3 topsoil, 1/3 compost, and 1/3 aeration such as perlite, vermiculite, or coarse sand. For a 32 cu ft bed that is roughly 11 cu ft of each. Mel's Mix from Square Foot Gardening swaps in 1/3 peat moss, 1/3 vermiculite, and 1/3 blended compost from several sources.
Should I account for settling when buying soil?
Yes. Fresh mix, especially anything heavy in compost or peat, settles 10 to 20 percent over the first few weeks of watering. Buy about 15 percent extra so the bed stays full, or plan to top it off with an inch or two of compost after the first season.
Is bulk delivery cheaper than bagged soil?
Usually by a wide margin. Bulk soil runs $30 to $60 per cubic yard delivered, while bagged soil at $5 to $8 per 2 cu ft bag comes to $67 to $108 per cubic yard equivalent. Bulk starts making sense once you pass roughly one cubic yard, or about two 4×8 beds.
How deep should a raised bed be?
Six inches is enough for lettuce and herbs, 12 inches suits most vegetables, and 18 to 24 inches is best for carrots, potatoes, and other root crops. If the bed sits on concrete or hardpan where roots cannot reach the ground below, go deeper than you otherwise would.