How Much Soil Do I Need for a Raised Bed?

A raised bed is a box

Which makes this the easiest calculation in the garden: length × width × depth. The only thing that catches people is that the bed is measured in feet and the depth in inches.

The arithmetic

A standard 8 by 4 foot bed, filled 12 inches deep.

Convert the depth to feet:

12 ÷ 12 = 1 foot

Multiply:

8 × 4 × 1 = 32 cubic feet

In cubic yards, which is how bulk soil is sold:

32 ÷ 27 = 1.19 cubic yards

In bags, at 1.5 cubic feet each:

32 ÷ 1.5 = 21.3, so 22 bags

Twenty-two bags. For one bed. That number surprises people every time, and it's the main reason to check before setting off to the garden centre with the car.

Fill depth isn't bed height

Worth separating, because the calculator asks for one and people enter the other.

If you build a 12-inch-high box you don't put 12 inches of soil in it. Fill to an inch or two below the rim, or every watering washes soil over the sides and onto the path.

And you're filling to the depth roots need, not to the top of whatever you built. A waist-high bed is a fine thing for your back, but the bottom foot of it is doing nothing except costing you soil — which is exactly what the "fill the bottom with logs" trick is for.

How deep does it actually need to be?

• 6–8 inches — salad, herbs, most leafy crops. Plenty.
• 10–12 inches — root vegetables, tomatoes, anything that wants to go down.
• Deeper — mostly for your back rather than the plants.

Depth is where the cost lives. Going from 6 inches to 12 doesn't add a bit of soil, it doubles the order. That's the single biggest lever on what this costs, so it's worth deciding deliberately rather than defaulting to "fill the box".

The other number: growing area

The calculator also reports your growing area — length × width, across however many beds you're filling. An 8 × 4 bed is 32 square feet of planting surface.

It's worth looking at alongside the volume, because the two answer different questions. The volume tells you what to order; the growing area tells you what you can actually plant. Seed packets and spacing guides are written in plants per square foot, so this is the figure you take to the plant spacing calculator — and it's the one that tells you whether a second bed is worth building before you've paid to fill the first.

Note that growing area doesn't change with depth. Going deeper costs more soil and buys you root room, not more planting space.

Bags or bulk?

Compare on volume, not on the sticker.

A cubic yard is 27 cubic feet — 18 bags at 1.5 cubic feet each. So our one bed is most of a bulk yard already.

Past roughly fifteen or twenty bags, bulk is usually cheaper per unit and always less carrying. The catch is that it arrives as a pile on your drive and doesn't move itself, so have a barrow and somewhere for it to sit.

Bulking out the bottom

You can fill the lower third with logs, branches, twiggy prunings or other coarse material. It's often called hügelkultur, and it does three useful things: cuts what you buy, holds moisture as it breaks down, and slowly feeds the bed.

The trade-off is settling. Wood breaking down means the bed drops noticeably in its first year or two, more than soil alone would. That's fine if you expect it and annoying if you don't.

It will look lower next spring

Not a mistake, and not something to prevent.

Fresh soil and compost settle as they're watered, and organic matter decomposes — that's the point of it. Most beds drop an inch or two over their first season.

The answer is to top up each spring, budgeting a bag or two per bed, rather than overfilling now and watching it wash over the sides all summer.

Beds that aren't rectangles

Split it into boxes, calculate each, add them together. An L-shape is two rectangles; a tiered bed is one box per tier.

Slight overestimates are the safer error here. The bed is going to settle anyway.

Where people get it wrong

• Forgetting to convert inches to feet. 8 × 4 × 12 gives 384 "cubic feet" — twelve times too much.
• Filling to the top of the box. Leave an inch or two below the rim.
• Buying deeper than the crop needs. Depth is the cost.
• Underestimating what a bed swallows. Twenty-two bags for one 8 × 4 bed.
• Using garden soil straight from the ground. It compacts in a bed and drains badly. A raised bed wants a mix with compost through it.
• Filling the bed before working out watering. Much easier to run a hose or a leaky pipe before there's 32 cubic feet on top of it.

The short version

Length × width × depth-in-feet gives cubic feet. Divide by 27 for yards, or by your bag size for bags. Fill to an inch below the rim, 6–8 inches for salad and 10–12 for roots, and expect to top it up every spring.