Measure your raised bed, then multiply to find your soil volume
The amount of soil you need depends on three things: the length, width, and depth of your raised bed. Multiply those three numbers together, and you have your volume in cubic inches. Divide by 1,728 to convert to cubic feet — that's the unit garden soil is sold in. A 4-by-8-foot bed that is 12 inches deep, for example, needs 32 cubic feet of soil.
Most raised beds are 12 inches deep, but some are 18 or 24 inches. Deeper beds drain better and give roots more room, but they also need more soil and cost more to fill. If you haven't built your bed yet, 12 inches is the standard depth and works for most vegetables and herbs.
The math is straightforward, but it's straightforward to make a mistake. Use the calculator below, or work through it step by step with your actual measurements written down.
Key Takeaways
- Multiply length × width × depth (in inches), then divide by 1,728 to get cubic feet — the unit soil is sold in.
- A standard 4-by-8-foot bed at 12 inches deep needs 32 cubic feet of soil.
- Soil settles after watering, so buy 10 to 15 percent extra to top off the bed after the first week.
- Bagged soil is convenient for small beds but expensive for large ones; bulk delivery is cheaper for beds over 50 cubic feet.
- Fill your bed with a mix of topsoil and compost rather than topsoil alone, which compacts and drains poorly.
Common raised bed sizes and their soil needs
These are the most common dimensions you'll build or buy. Measure your actual bed before ordering — even a few inches difference changes the total.
| Bed Size (Length × Width) | Depth | Cubic Feet Needed |
|---|---|---|
| 2 × 2 feet | 12 inches | 4 cubic feet |
| 2 × 4 feet | 12 inches | 8 cubic feet |
| 3 × 6 feet | 12 inches | 18 cubic feet |
| 4 × 4 feet | 12 inches | 16 cubic feet |
| 4 × 8 feet | 12 inches | 32 cubic feet |
| 4 × 8 feet | 18 inches | 48 cubic feet |
If your bed is a different size, use the formula: (length in feet × width in feet × depth in feet) = cubic feet. For a 3-by-5-foot bed at 12 inches deep, that's 3 × 5 × 1 = 15 cubic feet.
Account for settling and buy a little extra
Soil compacts and settles after you water it, especially in the first week. Plan to buy 10 to 15 percent more soil than your calculation shows, so you can top off the bed after it settles. For a 32-cubic-foot bed, that means ordering 35 to 37 cubic feet instead.
This is particularly important if you're filling the bed all at once and watering it when ready. If you fill it gradually over a few days and water lightly, settling will be less dramatic. Either way, having extra on hand saves a second trip to the garden center.
Bagged soil versus bulk delivery
Garden soil is sold in bags (usually 2 or 3 cubic feet per bag) or delivered in bulk by the cubic yard (27 cubic feet). For small beds under 20 cubic feet, bags are convenient — you can carry them yourself and don't need a truck. For larger beds, bulk is almost always cheaper per cubic foot, even with a delivery fee.
A 32-cubic-foot bed needs about 11 bags of 3-cubic-foot soil, which costs $80 to $150 depending on your region and soil quality. The same bed filled with bulk soil costs $40 to $80 plus delivery, which is usually $50 to $100. If you're filling multiple beds or a very large one, bulk becomes the clear choice.
Check whether your local garden center or landscape supplier delivers. Many will drop a pile of soil in your driveway for a flat fee, and you spread it yourself. Others charge by the cubic yard. Get a quote before you decide between bags and bulk.
What type of soil to buy and how to mix it
Don't fill your raised bed with topsoil alone. Topsoil compacts over time, especially in a confined space, and it drains poorly. Instead, mix topsoil with compost — aim for about 60 percent topsoil and 40 percent compost by volume. This blend drains well, holds nutrients, and stays loose enough for roots to grow.
If you're buying in bulk, you can order a mix already blended, or you can buy topsoil and compost separately and mix them as you fill. Compost is more expensive per cubic foot than topsoil, so a 60/40 blend is a practical balance. Some gardeners use 50/50, which is richer but costs more.
For a 32-cubic-foot bed at 60/40, you'd order about 19 cubic feet of topsoil and 13 cubic feet of compost. If you're buying bags, look for bags labeled "garden soil" or "raised bed mix," which are already blended. These cost more per cubic foot than straight topsoil, but you save the work of mixing.
Circular and irregular-shaped beds
If your bed is round, measure the diameter (the distance across the center) and use this formula: (diameter ÷ 2) squared × 3.14 × depth in feet = cubic feet. A circular bed 4 feet across and 12 inches deep needs (2 × 2 × 3.14 × 1) = 12.56 cubic feet, so round up to 13.
For an irregular shape, divide it into rectangles, calculate each one separately, and add them together. A bed shaped like an L, for example, can be split into two rectangles. This method isn't perfect, but it gets you close enough that you won't be far off when you order.
Frequently Asked Questions
Do I need to fill the bed all the way to the top?
No. Leave 1 to 2 inches of space at the rim so water doesn't spill over the edge when you water. This means if your bed is 12 inches tall, you only fill it to about 10 or 11 inches. Account for this when you calculate — subtract 1 to 2 inches from your depth measurement.
Can I reuse soil from year to year?
Yes, but refresh it. Each year, mix in 1 to 2 inches of compost or aged manure to replace nutrients plants used up. This is cheaper than replacing all the soil, and the bed will stay healthy. If the soil becomes compacted or drains poorly, that's a sign to replace the top 6 inches or refresh the whole bed.
What if I want to use a soil-less mix instead of topsoil and compost?
Soil-less mixes (peat moss, coco coir, and perlite blends) work well in raised beds and drain better than topsoil. They cost more and dry out faster, so you'll water more often. Calculate volume the same way — you'll just be ordering a different product. These mixes are sold bagged, not in bulk.
How deep should a raised bed be for vegetables?
Twelve inches is standard and works for most vegetables, herbs, and shallow-rooted plants like lettuce and spinach. Root vegetables like carrots and parsnips need 18 inches. If you're growing only herbs or greens, 8 to 10 inches is enough. Deeper beds give you more flexibility and better drainage.
Should I put cardboard or landscape fabric on the bottom?
Cardboard kills weeds underneath and breaks down over a year or two, adding organic matter. Landscape fabric lasts longer but doesn't add anything to the soil. Either one works. If you're building on grass or weeds, cardboard is the simpler choice — just lay it down, overlap the seams, and fill on top. Your soil calculation doesn't change either way.