About the Raised Bed Soil Calculator
This raised bed soil calculator works out how much growing mix you need to fill one or more raised garden beds. Enter the inside length, width and fill depth of each bed and the number of beds, choose a soil recipe, and it returns the total volume in cubic feet and cubic yards, the number of bags and the quantity of each ingredient.
It is aimed at gardeners building new beds, topping up beds that have settled and anyone comparing bagged soil with a bulk delivery. Popular mixes are included — a 60/30/10 blend of topsoil, compost and perlite, a simple 50/50 topsoil and compost mix, and the square-foot-gardening "Mel's Mix" of equal parts compost, peat or coir, and vermiculite.
Fresh soil settles by roughly 10–20% in the first season, so a settling allowance is added by default. Measure the inside of the frame, and if the bed sits on good native soil you can fill only the top 8–12 inches with premium mix.
With the default inputs, the soil needed is 35.2 cu ft. Change any value above to recalculate instantly.
How to use the raised bed soil calculator
- 1Measure the inside length and width of the bed in feet.
- 2Enter the depth you want to fill in inches.
- 3Set the number of identical beds.
- 4Pick a soil recipe and settling allowance.
- 5Buy the total volume, or each ingredient separately, and mix before filling.
Formula and method
Each bed is a rectangular box, so its volume is length × width × depth, with depth converted from inches to feet by dividing by 12. That volume is multiplied by the number of beds and increased by the settling allowance, because loose, freshly mixed soil compacts once watered.
Cubic yards are cubic feet ÷ 27. Bags are rounded up to whole bags. Each ingredient is the total volume times its share of the chosen recipe; volumes of mixed components are approximately additive for these coarse materials.
- Depth
- Fill depth in inches
- Settling %
- Extra volume to allow for compaction
- 27
- Cubic feet per cubic yard
Worked examples
One 8 × 4 ft bed, 12 inches deep
An 8 × 4 ft bed filled 1 ft deep holds 32 cubic feet; adding 10% for settling gives 35.2 cu ft (1.3 cubic yards), or 18 bags of 2 cu ft. A 60/30/10 mix is 21.1 cu ft of topsoil, 10.6 of compost and 3.5 of perlite.
Three 6 × 3 ft beds of Mel's Mix, 10 inches deep
Each bed holds 6 × 3 × 10 ÷ 12 = 15 cu ft, so three beds need 45 cu ft: 15 cu ft each of compost, peat or coir, and vermiculite — or 30 bags of 1.5 cu ft.
4 × 4 ft bed, 18 inches, 50/50 mix
A 4 × 4 ft bed at 18 inches holds 24 cu ft; with 10% settling that is 26.4 cu ft, split into 13.2 cu ft of topsoil and 13.2 of compost, or 14 bags of 2 cu ft.
Frequently asked questions
How much soil do I need for a 4 × 8 raised bed?+
A 4 × 8 ft bed holds 32 cubic feet (about 1.2 cubic yards) at 12 inches deep, 21.3 cu ft at 8 inches and 48 cu ft at 18 inches. Add 10–20% for settling.
What is the best soil mix for raised beds?+
A common all-round mix is about 60% screened topsoil, 30% compost and 10% perlite or other aeration. Mel's Mix (equal parts compost, peat or coir and vermiculite) is lighter and popular for shallow square-foot beds.
How deep should a raised bed be?+
Most vegetables grow well in 8–12 inches of loose soil over native ground. Root crops and beds over concrete or poor soil benefit from 12–18 inches or more.
Is it cheaper to buy raised bed soil in bags or bulk?+
For more than about 1 cubic yard (27 cu ft), bulk soil or a delivered blend is almost always cheaper per cubic foot than bags, even after a delivery fee.
Why does my raised bed soil sink?+
Compost and organic matter settle and decompose, so beds commonly drop 10–20% in the first season. Top up each year with an inch or two of compost.