Raised Bed Soil Calculator
Find out exactly how much soil your raised bed needs, in cubic feet, cubic yards, liters, and bags.
Filling a raised bed is the step most new gardeners get wrong, and it is an expensive mistake in both directions. Order too little and you are back at the garden center on a Sunday afternoon. Order too much and you have a pile of soil in the driveway. This raised bed soil calculator does the math for you: enter your bed's length, width, and depth, and it returns the volume you need along with how many bags that works out to and how to split it between topsoil, compost, and aeration.
Bags you will need
| Bag size | Bags needed |
|---|---|
| 1 cu ft bags | 36 |
| 1.5 cu ft bags | 24 |
| 2 cu ft bags | 18 |
| 3 cu ft bags | 12 |
| 50 quart bags (1.94 cu ft) | 19 |
Soil mix breakdown
Tip: measure the inside of your bed, not the outside. On a bed built from 2x lumber the boards eat about 1.5 inches on each side, which is enough to throw the total off on a long bed.
How to calculate soil for a raised bed
The formula is simple once every measurement is in the same unit. Convert your depth from inches to feet by dividing by 12, then multiply the three dimensions together:
Length (ft) × Width (ft) × Depth (ft) = Cubic feet of soil
A 4 ft by 8 ft bed filled 12 inches deep works out to 4 × 8 × 1 = 32 cubic feet. To convert that into cubic yards, which is how bulk soil is sold and delivered, divide by 27. So 32 ÷ 27 = about 1.2 cubic yards.
For a round or half-barrel bed, use the area of a circle instead: multiply pi (3.14) by the radius squared, then by the depth. The calculator above handles this for you when you switch the shape to round.
One last adjustment matters more than people expect: add about 10% to whatever number you land on. Fresh soil is full of air pockets, and after the first few deep waterings a bed will visibly drop. Buying that extra 10% up front saves a second trip.
How much soil for common raised bed sizes
These are the raw volumes for the most common bed footprints, before the 10% settling allowance. Cubic yards are shown in parentheses.
| Bed size | 6 in deep | 12 in deep | 18 in deep | Bags (1.5 cu ft, 12 in) |
|---|---|---|---|---|
| 2 ft x 4 ft | 4.0 cu ft (0.15 cu yd) | 8.0 cu ft (0.30 cu yd) | 12.0 cu ft (0.44 cu yd) | 6 bags |
| 3 ft x 6 ft | 9.0 cu ft (0.33 cu yd) | 18.0 cu ft (0.67 cu yd) | 27.0 cu ft (1.00 cu yd) | 12 bags |
| 4 ft x 4 ft | 8.0 cu ft (0.30 cu yd) | 16.0 cu ft (0.59 cu yd) | 24.0 cu ft (0.89 cu yd) | 11 bags |
| 4 ft x 8 ft | 16.0 cu ft (0.59 cu yd) | 32.0 cu ft (1.19 cu yd) | 48.0 cu ft (1.78 cu yd) | 22 bags |
| 4 ft x 12 ft | 24.0 cu ft (0.89 cu yd) | 48.0 cu ft (1.78 cu yd) | 72.0 cu ft (2.67 cu yd) | 32 bags |
The best soil mix for a raised bed
Knowing the volume is only half the job. What you put in the bed determines how it performs for the next several years. Do not fill a raised bed with bagged "potting mix" alone, which is designed for containers and dries out fast, and do not fill it with straight topsoil, which compacts. A blend works far better:
- 60% topsoil or garden soil — provides structure, mineral content, and the ability to hold water between waterings.
- 30% compost — supplies nutrients and the biology that makes them available to your plants. Worm castings are an excellent portion of this share.
- 10% aeration material — perlite, coarse sand, or fine pine bark to keep the mix from compacting into a solid block.
The calculator splits your total by these percentages automatically, and you can change the ratios if you are working from a different recipe. If you are building a deep bed and want to reduce how much soil you buy, hugelkultur is worth reading about — filling the bottom third with logs and branches cuts your soil volume meaningfully and improves moisture retention as the wood breaks down.
Bagged soil or bulk delivery?
The break-even point usually sits around 2 cubic yards, or roughly 54 cubic feet. Below that, bags are easier to handle and you avoid delivery fees. Above it, bulk soil by the yard is substantially cheaper per cubic foot, and hauling forty-plus bags from the car stops being fun quickly.
If you are filling two or more 4 ft by 8 ft beds, you are already past the break-even point. Call a local landscape supply yard and ask for a raised bed or garden mix rather than plain screened topsoil.
Frequently asked questions
How much soil do I need for a raised bed?
Multiply length x width x depth, with every measurement in feet, to get the volume in cubic feet. A 4 ft x 8 ft bed filled 12 inches deep needs 32 cubic feet, which is about 1.2 cubic yards. Divide cubic feet by 27 to convert to cubic yards, and add roughly 10% because soil settles after the first few waterings.
How many bags of soil do I need for a 4x8 raised bed?
A 4 ft x 8 ft bed filled 12 inches deep needs 32 cubic feet of soil. That works out to 22 bags of 1.5 cubic feet, 16 bags of 2 cubic feet, or 11 bags of 3 cubic feet. At that volume bulk soil is usually cheaper than bagged.
What is the ideal soil ratio for a raised bed?
A reliable raised bed mix is roughly 60% topsoil or garden soil, 30% compost, and 10% aeration material such as perlite, coarse sand, or fine pine bark. The topsoil gives the bed structure and water-holding ability, the compost supplies nutrients and biology, and the aeration keeps the mix from compacting into a brick.
How many cubic feet are in a 50 quart bag of potting soil?
One cubic foot equals about 25.7 dry quarts, so a 50 quart bag holds roughly 1.94 cubic feet. Two 50 quart bags give you just under 4 cubic feet.
Is 25 quarts the same as 1 cubic foot?
Almost, but not exactly. One cubic foot is 25.7 dry quarts, so a 25 quart bag is about 0.97 cubic feet. It is close enough to estimate with, but on a large bed the shortfall adds up, which is why buying a little extra is smart.
How many cubic feet is a 40 lb bag of potting soil?
Soil is sold by volume, not weight, and the weight of a given volume changes with moisture content. As a rough guide a 40 lb bag of topsoil holds about 0.75 cubic feet, while a 40 lb bag of lighter potting mix can hold closer to 1.5 cubic feet. Always check the cubic feet printed on the bag rather than working from the weight.
How deep should a raised bed be?
Six inches is the practical minimum, and it works for lettuce, herbs, and other shallow-rooted crops. Twelve inches suits most vegetables. Go 18 inches or deeper for carrots, parsnips, and potatoes, or if your bed sits on concrete or hard compacted ground where roots cannot reach into the native soil below.
Do I have to fill a deep raised bed entirely with soil?
No. In a bed deeper than about 18 inches you can fill the bottom third with logs, branches, and leaves, a technique called hugelkultur. The wood breaks down slowly, holds moisture, and cuts the amount of purchased soil you need. Just make sure the top 12 inches is real growing mix, since that is where nearly all root activity happens.
New to raised beds?
Getting the soil right is the single biggest factor in whether a raised bed thrives or struggles. The ultimate guide to raised bed gardening walks through siting, building, filling, and planting your first bed, and the benefits of raised garden beds covers why they outperform in-ground rows for most home gardeners.
Explore the Masterclass