To fill a raised bed, multiply length by width by depth in feet to get cubic feet, then divide by 27 for cubic yards. A 4 by 8 foot bed 12 inches deep needs 32 cubic feet, about 1.2 cubic yards. Enter your bed size below to get the soil volume, bags needed, and a tomato-ready mix breakdown.
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What Your Result Means: the volume is how much soil your bed holds; the bag counts tell you how many standard bags to buy; and the mix breakdown splits that volume into a balanced blend that grows healthy tomatoes.
How Do You Use the Soil Volume Calculator?
Enter your bed shape and dimensions, set the soil depth, and press calculate. For round beds, the length field becomes the diameter.
- Choose the bed shape and how many identical beds you are filling.
- Enter length and width in feet (or the diameter for a round bed).
- Set the soil depth in inches, then calculate to get volume, bags, and a mix breakdown.
What Each Result Means
| Result | What it tells you |
|---|---|
| Cubic feet | Total soil volume, the base figure for buying |
| Cubic yards | Bulk soil is sold by the yard; 1 yard is 27 cubic feet |
| Bags needed | How many 1.5 or 2 cubic-foot bags to buy |
| Mix breakdown | How much compost, topsoil, and aeration to blend |
Common Raised-Bed Soil Volumes
Typical bed sizes at a 12-inch depth, so you can sanity-check your result.
| Bed size | Depth | Cubic feet | Cubic yards |
|---|---|---|---|
| 4 x 4 ft | 12 in | 16 | 0.6 |
| 4 x 8 ft | 12 in | 32 | 1.2 |
| 3 x 6 ft | 12 in | 18 | 0.7 |
| 4 x 8 ft | 18 in | 48 | 1.8 |
How Does the Volume Calculation Work?
The tool converts depth to feet, finds the bed area, multiplies for volume, then scales by the number of beds and splits the result into a soil mix.
Round: cubic feet = 3.1416 x radius squared x (depth divided by 12)
Cubic yards = cubic feet divided by 27
Mix = compost 30 percent, topsoil 50 percent, aeration 20 percent
Bag counts round up, because you cannot buy a partial bag, and the mix ratio follows a balanced blend that drains well and feeds tomatoes.
Worked Example
Say you fill one 4 by 8 foot bed to 12 inches. Depth is 1 foot, so volume is 8 times 4 times 1, or 32 cubic feet, about 1.2 cubic yards. That is roughly 22 bags of 1.5 cubic-foot soil, or a mix of about 10 cubic feet of compost, 16 of topsoil, and 6 of aeration such as perlite.
Factors That Change Your Volume
- Depth: tomatoes root deep, so going from 10 to 18 inches nearly doubles the soil you need.
- Settling: fresh soil and compost settle 10 to 20 percent, so buy a little extra to top up.
- Filler at the base: logs or branches in a deep bed reduce the soil required.
- Bed shape: round and tapered beds hold less than a full rectangle of the same footprint.
- Number of beds: volume scales directly with how many identical beds you fill.
Understanding Your Outputs
| Output | Meaning | Best used for |
|---|---|---|
| Cubic feet | Exact volume | Comparing bagged options |
| Cubic yards | Bulk-delivery unit | Ordering a soil drop for big beds |
| Bags needed | Store-bag count | Shopping for a single bed |
| Mix breakdown | Compost, topsoil, aeration split | Blending your own tomato mix |
When to Use It
Use the calculator when building a new raised bed and buying soil, when topping up settled beds each spring, and when deciding between bagged and bulk soil. Bulk by the cubic yard is usually cheaper once you pass about half a yard, which is roughly a single 4 by 8 foot bed.
Common Mistakes to Avoid
- Forgetting to convert depth: depth is in inches but volume needs feet, so divide by 12.
- Buying exact volume: soil settles, so order about 10 percent extra to top up.
- Using pure compost: straight compost slumps and stays soggy; blend with topsoil and aeration.
- Shallow beds for tomatoes: under 10 inches limits roots; aim for 12 inches or more.
- Ignoring bulk pricing: for several beds, a cubic-yard delivery beats dozens of bags.
Accuracy and Limitations
The calculator gives an accurate geometric volume, but real beds vary.
What it calculates accurately: the volume from your dimensions plus the bag counts and mix splits.
What it does not account for: soil settling, irregular or tapered bed shapes, base fillers such as logs, or how compacted the soil is. Order about ten percent extra to be safe.
Methodology
Method: geometric volume with a tomato soil-mix split. Inputs used: shape, length and width (or diameter), depth in inches, number of beds. Assumptions: mix of 30 percent compost, 50 percent topsoil or garden soil, 20 percent aeration (perlite or coarse sand); bags of 1.5 and 2 cubic feet. Rounding: volumes to one decimal, bag counts rounded up. Edge cases: zero or empty dimensions prompt for valid numbers.
Sources: Iowa State University Extension – Growing Tomatoes, Royal Horticultural Society – Tomatoes, The Old Farmer’s Almanac – Tomatoes. Last reviewed: September 24, 2026 · Reviewed by Aiza Anwar.
Frequently Asked Questions
How much soil do I need for a 4×8 raised bed?
A 4 by 8 foot bed filled 12 inches deep needs 32 cubic feet, about 1.2 cubic yards or roughly 22 bags of 1.5 cubic-foot soil.
How deep should a raised bed be for tomatoes?
Make raised beds at least 12 inches deep for tomatoes; 18 inches is better, since tomato roots grow deep and reach for water.
How many bags of soil fill a raised bed?
Divide your bed volume in cubic feet by the bag size. A 32 cubic-foot bed needs about 22 bags of 1.5 cubic feet or 16 bags of 2 cubic feet.
What is the best soil mix for raised-bed tomatoes?
A balanced mix is about 30 percent compost, 50 percent topsoil or garden soil, and 20 percent aeration such as perlite or coarse sand.
Is bagged or bulk soil cheaper for raised beds?
Bulk soil by the cubic yard is usually cheaper once you need more than about half a yard, roughly one 4 by 8 foot bed.
How much compost should I add to a raised bed?
Aim for about a third of the volume as compost. Too much straight compost holds water and slumps, so balance it with topsoil.
Can I fill the bottom of a deep bed to use less soil?
Yes. Logs, branches, or coarse organic matter at the base of a deep bed cut the soil needed and slowly enrich it as they break down.
How much does raised-bed soil settle?
Fresh soil and compost settle about 10 to 20 percent in the first season, so buy a little extra and top up in spring.
How many cubic feet are in a cubic yard of soil?
One cubic yard equals 27 cubic feet, which is how bulk soil is measured and delivered.
Related Tomato Tools
Visit the Growing Tomatoes hub
This calculator provides general planning estimates. Soil settles and bed shapes vary, so order about ten percent extra and adjust the mix to your local soil. Spotted an error? Tell us here.