Raised Garden Bed Soil Calculator

How to use this calculator

Measure the inside length and width of your bed and the depth you want to fill. Enter those three numbers and you'll get the volume in cubic feet and cubic yards — how bulk soil is delivered — plus the number of bags. Filling several beds? Add a row for each and they're totaled together.

The bag size menu is optional and starts on bulk. Pick the size printed on your bag to get a bag count, or leave it on bulk if you’re ordering loose soil. Most bagged raised bed mix is 1.5 cubic feet, but 1, 2 and 3 cu ft bags are all sold, so check the bag before you rely on the count.

The formula, explained

Soil is sold by volume, so there's no weight conversion to worry about. Measure the inside of the bed — the frame's outer dimensions include the boards, which don't hold soil.

Length (ft) × Width (ft) × (Depth in ÷ 12) = cubic feet
Cubic feet ÷ 27 = cubic yards

For bags, divide the cubic feet by the bag size. At the common 1.5 cu ft bag, one cubic yard is 18 bags.

Worked example: a 4 ft × 8 ft bed, 10 inches deep

1. Area: 4 × 8 = 32 square feet
2. Convert the depth: 10 ÷ 12 = 0.833 ft
3. Volume: 32 × 0.833 = 26.7 cubic feet
4. Cubic yards: 26.7 ÷ 27 = 0.99 cubic yards
5. In bags: 26.7 ÷ 1.5 = 18 bags

Eighteen bags to fill one modest bed is why raised beds surprise people. A 4 × 8 bed looks small from above, but it's nearly a full cubic yard of soil — and that's before settling.

The part everyone forgets: settling

Raised bed soil is not like gravel. It's roughly half organic matter, and organic matter decomposes. Over the first season, a freshly filled bed typically drops 10–20% as the compost breaks down and the mix settles under watering.

That 4 × 8 bed filled to 12 inches (1.19 cubic yards) sits at roughly 9.6 inches by the following spring. To still have a genuine 12 inches after settling, you'd order about 1.48 cubic yards — roughly 25% more.

You have two sensible options. Either overfill slightly at the start, mounding the soil an inch or two above the frame, or fill level and plan to top up with compost next spring. Most gardeners end up doing both. What you shouldn't do is fill to exactly 12 inches, assume it stays there, and wonder why your carrots are stunted.

How deep should the soil be?

Depth is driven by what you're growing and by what's underneath the bed.

Growing	Minimum depth	Comfortable depth
Herbs, lettuce, spinach, greens	6 inches	8 inches
Most vegetables (beans, peppers, cucumbers)	10 inches	12 inches
Tomatoes, squash, brassicas	12 inches	18 inches
Carrots, parsnips, potatoes	18 inches	24 inches

There's an important caveat. If your bed sits on open soil, roots can grow down past the frame, so the frame depth is a starting point rather than a hard limit. If your bed sits on a patio, deck, or legs, the depth you fill is the only soil the plant will ever get — so use the comfortable column, not the minimum. Our guide on how deep a raised bed should be covers this in more detail.

How much soil for common bed sizes

Bed size	Depth	Cubic yards	1.5 cu ft bags
2 × 4 ft	6 in	0.15	3
2 × 8 ft	10 in	0.49	9
4 × 4 ft	12 in	0.59	11
3 × 6 ft	12 in	0.67	12
4 × 8 ft	10 in	0.99	18
4 × 8 ft	12 in	1.19	22
4 × 8 ft	18 in	1.78	32
4 × 12 ft	12 in	1.78	32
4 × 4 ft (root crops)	24 in	1.19	22

What depth costs you

Depth scales everything, and on a deep bed the numbers get serious fast. The same 4 × 8 ft bed needs:

Depth	Cubic yards	1.5 cu ft bags
6 inches	0.59	11
10 inches	0.99	18
12 inches	1.19	22
18 inches	1.78	32
24 inches	2.37	43

Step by step: how to measure your bed

1. Measure the inside dimensions, not the outside. A bed built from 2×10 boards on a 4 × 8 ft footprint has an interior closer to 3 ft 9 in × 7 ft 9 in. Using the outside numbers overestimates by roughly 5–8%.
2. Measure the fill depth, not the frame height. You'll leave an inch or two of freeboard at the top so soil and mulch don't wash over the edge.
3. Add a row for each bed. Beds of different sizes or depths should never be averaged.
4. For an L-shaped or tiered bed, split it into rectangles and enter each separately.
5. Decide your settling strategy before ordering — add 10–20% now, or plan to top up next spring.

What soil should you put in it?

This is where raised beds succeed or fail, and it isn't a quantity question. The widely used starting point is a 60/40 mix: 60% quality topsoil, 40% compost. The topsoil provides structure and mineral content; the compost provides nutrients, drainage, and biology.

Fill	Verdict	Why
60% topsoil + 40% compost	Recommended	Structure plus nutrition; holds shape and feeds plants
Bagged "raised bed mix"	Good, costly	Pre-blended and convenient; expensive at volume
100% compost	Avoid	Too rich, holds water, and shrinks dramatically as it breaks down
100% topsoil	Avoid	Compacts hard, drains poorly, little nutrition
Soil dug from your yard	Avoid	Compacts in a frame and brings weed seeds and pests with it

The pure-compost mistake is the expensive one. It looks like the richest possible option, and beds filled that way can drop several inches in a single season. Our guide on topsoil vs. compost explains what each one actually does.

Filling a deep bed without filling it with premium mix

For beds over about 12 inches, filling the whole depth with good soil is expensive and unnecessary — most vegetable roots live in the top 12 inches. Fill the bottom third with rough organic matter: logs, branches, leaves, grass clippings, cardboard.

On a 4 × 8 ft bed at 18 inches, filling only the top 12 inches with soil takes 1.19 cubic yards instead of 1.78 — a 33% saving, roughly 10 fewer bags. The woody material breaks down slowly, feeding the bed for years and improving drainage. The trade-off is more settling in the first two seasons, so plan to top up.

What will it cost?

• Bulk garden soil mix: roughly $30–$60 per cubic yard
• Bulk topsoil: roughly $15–$40 per cubic yard
• Bulk compost: roughly $30–$50 per cubic yard
• Delivery: $50–$100 locally
• Bagged raised bed mix: roughly $5–$10 per 1.5 cu ft bag

Worked cost: one 4 × 8 ft bed at 12 inches

That's 1.19 cubic yards, or 22 bags.

• Bags: 22 × $5–$10 = $110–$220
• Bulk: 1.19 × $30–$60 = $36–$71, plus $50–$100 delivery → roughly $86–$171

Where bags stop making sense

Volume	Bags	Bulk (incl. delivery)
0.5 cu yd (9 bags)	$45–$90	$65–$130
1 cu yd (18 bags)	$90–$180	$80–$160
2 cu yd (36 bags)	$180–$360	$110–$220
3 cu yd (54 bags)	$270–$540	$140–$280

The crossover is around 1 cubic yard — roughly a single 4 × 8 ft bed. Below that, bags win and you can buy as you go. Above it, bulk pulls away fast: at 3 cubic yards, bags cost roughly double. Bagged soil is one of the most marked-up materials in the garden center, so if you're filling two or more beds, order bulk and mix it yourself.

Common mistakes when filling a raised bed

• Filling with 100% compost. It's too rich for most crops, holds too much water, and shrinks so much that the bed can drop several inches in one season.
• Using soil dug from the yard. Native soil compacts into a brick inside a frame, drains badly, and brings weed seeds and pests along for free.
• Ignoring settling. Fill to exactly 12 inches and you'll have about 10 by next spring. Overfill by 10–20% or budget for a compost top-up.
• Measuring the outside of the frame. The boards don't hold soil. On a 4 × 8 bed, using outside dimensions overestimates by roughly 5–8%.
• Filling a deep bed entirely with premium mix. Roots live in the top foot. Fill the bottom third of a deep bed with rough organic matter and save a third of the cost.
• Lining the bottom with plastic or landscape fabric. It traps water and blocks roots and worms. If you need to stop burrowing pests, use hardware cloth, which blocks gophers but lets water and roots through.

Pro tips for a bed that performs

• Order bulk and mix on a tarp. For anything over a cubic yard, buy topsoil and compost separately and blend 60/40 yourself. It's cheaper and you control the mix.
• Water it in before planting. Fill, soak thoroughly, wait a day, then top up. You'll find the settling immediately instead of in June.
• Leave 1–2 inches of freeboard. Soil filled flush with the top washes over the edge every time you water.
• Top up with an inch of compost each spring rather than rebuilding. It replaces what decomposed and feeds the bed for the season.
• Use hardware cloth, not plastic, at the base if you have gophers or voles — it stops them without stopping drainage.
• Buy topsoil by sight if you can. Ask to see it before delivery; "topsoil" is an unregulated term and quality varies enormously between yards.

A note on accuracy

This calculator gives you a clean volume figure from your measurements. Bagged soil is fluffier than settled soil, bulk soil compresses in the pile, and every mix settles once watered — so treat the result as a close planning number, round up, and expect to top up after the first season. Bulk suppliers often have a minimum delivery, so it's worth a quick call before you book.

Where these figures come from

The bed depth and soil-mix guidance built into this calculator comes from the published sources below. The arithmetic itself — converting measurements to volume, and volume to bags or tons — is straightforward unit conversion, and you can check it in the full working the calculator shows. Material densities and bag yields vary by product and supplier, so confirm the figure printed on the bag or quoted by your yard before you order.

• Growing Vegetables in Raised Beds Source: University of Maryland Extension. Bed depth, filling, and what to do when a bed sits on a hard surface.
• Raised Bed Gardening (FS 270) Source: Oregon State University Extension. Bed depth, soil mix, and loosening the native soil beneath the bed.
• Raised Bed Gardening Source: Utah State University Extension. Why beds should be at least 6 to 12 inches deep.
• Soil to Fill Raised Beds Source: University of Maryland Extension. Raised-bed soil mixes and how much compost to blend in.
• How to Use Compost in Gardens and Landscapes (EM 9308) Source: Oregon State University Extension. Why a bed should not be filled with compost alone.

Frequently asked questions

How much soil do I need for a 4x8 raised bed?

A 4 ft by 8 ft bed at 10 inches deep needs about 26.7 cubic feet — roughly 0.99 cubic yards, or 18 bags at 1.5 cu ft each. At 12 inches it's 1.19 cubic yards (22 bags), and at 18 inches it's 1.78 cubic yards (32 bags).

How many bags of soil are in a cubic yard?

18 bags at the common 1.5 cubic foot size, since a cubic yard is 27 cubic feet. At 1 cu ft per bag it's 27 bags, at 2 cu ft it's 14, and at 3 cu ft it's 9. Bag sizes vary a lot for soil, so check yours.

What's the best soil mix for a raised bed?

Roughly 60% quality topsoil to 40% compost. The topsoil gives structure and mineral content; the compost provides nutrients, drainage, and biology. Avoid filling with 100% compost, which is too rich and shrinks dramatically, and avoid soil dug from your yard, which compacts and brings weed seeds.

How deep should a raised bed be?

Six to eight inches works for herbs and salad greens, 10–12 inches suits most vegetables, and root crops like carrots and potatoes want 18–24 inches. If the bed sits on open soil, roots can grow deeper than the frame. If it sits on a patio, deck, or legs, the fill depth is all the plant will ever get.

Why does my raised bed soil keep sinking?

Because it's roughly half organic matter, and organic matter decomposes. Most beds drop 10–20% in the first season as compost breaks down and the mix settles under watering. It's normal. Either overfill by 10–20% at the start or top up with an inch or two of compost each spring.

Should I buy soil in bags or in bulk?

The crossover is around one cubic yard — about a single 4 × 8 ft bed. Below that, bags win once delivery is counted. Above it, bulk pulls away quickly: at 3 cubic yards, bags cost roughly double. Bagged soil is heavily marked up, so for two or more beds, order bulk and blend it yourself.

Can I fill the bottom of a raised bed with something cheaper?

Yes, and for deep beds you should. Most vegetable roots live in the top 12 inches, so fill the bottom third with logs, branches, leaves, or cardboard. On a 4 × 8 ft bed at 18 inches, that cuts the soil needed from 1.78 to 1.19 cubic yards — a 33% saving. Expect a bit more settling in the first two seasons.

Should I line the bottom of my raised bed?

Not with plastic or landscape fabric — both trap water and block roots and worms from reaching the soil below. If burrowing pests are a problem, staple hardware cloth across the base instead. It stops gophers and voles while letting water, roots, and worms through.