How Much Soil Do You Need to Fill a Raised Bed?
To find out how much soil a raised bed needs, multiply its length by its width by the depth you will fill, with all three in feet. The answer is in cubic feet. Divide by the volume printed on the bag to get the number of bags, and round up. Divide the cubic feet by 27 if the supplier sells by the cubic yard. A 4 by 8 foot bed filled 12 inches deep needs 32 cubic feet, which is about 1.2 cubic yards. This guide gives the formulas, worked examples for common sizes and the points that make people order too little.
The Formula, Step by Step
- Measure the inside of the bed, not the outside. The wall thickness of the boards takes up room, and the inside length and width are what the soil fills.
- Choose your fill depth. Measure from the bottom of the bed to the level you want the soil to reach, and convert inches to feet by dividing by 12. A 10-inch fill is 10 divided by 12, or about 0.83 feet.
- Multiply: length in feet times width in feet times depth in feet. The result is cubic feet.
- Convert if needed. One cubic yard is 27 cubic feet (3 feet by 3 feet by 3 feet), so divide cubic feet by 27 for yards.
- Divide by the bag size. The label gives the volume of each bag. Divide your cubic feet by that number, then round up to a whole bag.
These steps are arithmetic, not a recommendation of any product, and the formula works the same for any rectangular bed, whether it is wood, metal or composite. For help choosing a bed itself, see our raised bed buying guide.
Worked Examples for Common Sizes
The table shows the volume for several rectangular sizes. The bag columns use example bag volumes of 1.5, 2 and 3 cubic feet so you can see how the count changes. Bags come in many sizes, so use the volume on the label of the bag you are actually buying.
| Bed (feet) and fill depth | Cubic feet | Cubic yards | 1.5 cu ft bags | 2 cu ft bags | 3 cu ft bags |
|---|---|---|---|---|---|
| 4 x 4, 12 in. deep | 16.0 | 0.59 | 11 | 8 | 6 |
| 3 x 6, 12 in. deep | 18.0 | 0.67 | 12 | 9 | 6 |
| 4 x 8, 6 in. deep | 16.0 | 0.59 | 11 | 8 | 6 |
| 4 x 8, 10 in. deep | 26.7 | 0.99 | 18 | 14 | 9 |
| 4 x 8, 12 in. deep | 32.0 | 1.19 | 22 | 16 | 11 |
| 4 x 8, 18 in. deep | 48.0 | 1.78 | 32 | 24 | 16 |
| 4 x 12, 12 in. deep | 48.0 | 1.78 | 32 | 24 | 16 |
Check a row yourself with one example: a 4 by 8 foot bed filled 12 inches deep is 4 times 8 times 1, which is 32 cubic feet. Divide by 1.5 and you get 21.3, so you round up to 22 bags of that size. At 27 cubic feet to the yard, 32 cubic feet is 1.19 cubic yards.
Round and Odd-Shaped Beds
For a round bed or large planter, use the area of a circle: pi (3.14) times the radius squared times the depth. The radius is half the diameter. A round bed 6 feet across and 12 inches deep has a radius of 3 feet, so the volume is 3.14 times 3 times 3 times 1, or about 28.3 cubic feet. A 4-foot round bed at the same depth is about 12.6 cubic feet. The same method works for round planters and stock tanks used as beds, and you can read about their winter care in our frost-proof planters guide.
For an L-shaped or other odd bed, split it into two or more rectangles, work out the volume of each and add them. For a bed with sloping sides, measure the average width and use that. When in doubt, round the measurements up slightly instead of down.
What Changes the Number
What goes in the bottom
Some gardeners fill the bottom of a tall bed with other material, such as wood, branches, cardboard or coarse compost, to cut down on the amount of soil. If you do, subtract the depth of that layer from your fill depth before you multiply. Our how to build a raised garden bed guide covers the bed itself, and raised bed gardening guide covers how to plan the growing side.
Settling
Fresh soil and compost are light and loose, and they settle after watering and rain. The surface usually sinks, so a bed that looks full on the day you fill it may be low a few weeks later. Plan for a top-up. Buy a modest amount more than your calculation, or leave room in your budget and schedule for a second delivery or a few extra bags. We do not quote a settling percentage because it depends on the material and how you pack it.
Leave room at the top
Do not fill right to the rim. Leaving some space below the top of the boards lets you water without runoff and keeps soil and mulch in place. Decide on the final level before you calculate, and use the distance from the bed floor to that level as your depth.
Bag size and units
Bag volumes are printed in cubic feet on many labels, but some show quarts or liters. Work in one unit, and check the label carefully, because a bag that looks large can hold less than you think. Bulk deliveries are sold by the cubic yard, and suppliers may have minimum orders and delivery rules, so ask before you commit.
Bags or Bulk?
Bags are easy to carry, store and buy as you need them, and they are the practical choice for one or two small beds. They cost more per cubic foot than bulk, and a large bed can take dozens of bags to fill. Bulk soil, usually sold by the cubic yard, comes in a truck load, and it suits several beds or a deep one. You need a place for the delivery to be dropped, and a plan to move it, which is where a wheelbarrow earns its keep. Ask the supplier what is in the mix, how it is sold and whether you can see a sample first.
What the soil is made of matters too. A raised bed is only as good as its fill, and a soil mix is a choice in its own right. Our garden soil guide explains soil basics, and improving clay or sandy soil covers fixing a heavy or sandy base. A soil test kit is worth considering once the bed is filled and the plants are chosen.
Bottom line. Length times width times fill depth in feet gives cubic feet. Divide by the bag volume and round up, or divide by 27 for cubic yards. Measure the inside of the bed, subtract any bottom layer, allow for settling and leave room below the rim.
Sources and verification
- The formulas and conversions are arithmetic: volume equals length times width times depth, 12 inches to the foot, 27 cubic feet to the cubic yard, and area of a circle equals pi times radius squared. The bag counts in the table use example bag volumes, not a specific product.
- Bag volumes, bulk delivery minimums and soil mix contents are set by the seller. Read the label or ask the supplier, and measure the inside of your own bed. No settling percentages or prices are quoted.
Frequently Asked Questions
How many cubic feet of soil do I need for a 4x8 raised bed?
A 4 by 8 foot bed filled 12 inches deep needs 32 cubic feet, because 4 times 8 times 1 foot is 32. That is about 1.2 cubic yards.
How do I convert cubic feet to cubic yards?
Divide by 27. One cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet.
How many bags of soil do I need?
Divide your cubic feet by the volume printed on the bag, then round up to a whole bag. For example, 32 cubic feet divided by a 1.5 cubic foot bag is 21.3, so you would buy 22.
Should I buy extra soil for settling?
Yes, plan for a top-up. Fresh soil and compost settle after watering, so the surface sinks. The amount depends on the material, so we do not quote a percentage.
How do I calculate soil for a round raised bed?
Multiply pi (3.14) by the radius squared by the depth in feet. The radius is half the diameter. A 6-foot round bed filled 12 inches deep is about 28.3 cubic feet.