Raised Bed Soil Calculator — Bags, Yards and the Mix

How to work it out

1. Measure the inside of the frame

   Soil goes inside the bed, so measure the internal dimensions. A bed built from 2x lumber has walls about 1.5 inches thick, so an 8 ft board makes a bed that is 8 ft outside and roughly 7 ft 9 in. inside on the long side.

   On a small bed that difference is a few per cent. On a set of six beds it is most of a bag.

2. Decide the depth by what you are growing

   Six inches works for lettuce, herbs and other shallow-rooted crops. Twelve inches suits most vegetables. Root crops like carrots, parsnips and potatoes want 18 inches or more.

   If the bed sits on open soil rather than a hard surface, roots will grow down past the frame, so you can be less generous. On a patio, deck or gravel, whatever depth you build is all the root run there is.

3. Leave a gap at the top

   Fill to about 90% of the frame height. Watering a bed filled flush to the rim washes soil over the edge every time, and there is no room for mulch or a top-up of compost next spring.

   Expect the level to drop anyway. A new mix settles by 10 to 20% over the first season as the organic matter breaks down, which is normal and is why beds get a compost top-up each year rather than a full refill.

4. Buy the ingredients separately, or bulk

   Bagged raised bed mix is the easy answer for one small bed. For anything more, buying topsoil and compost separately and mixing them is usually much cheaper, and you can see exactly what you are getting.

   Above about a cubic yard, bulk delivery beats bags on both price and effort. A cubic yard is 27 cubic feet, which is eighteen 1.5 cu ft bags — a lot of plastic and a lot of lifting.

Soil volume by bed size

Cubic feet at 90% fill, and the 1.5 cu ft bags that takes.

Bed size	6 in. deep	12 in. deep	18 in. deep	24 in. deep
2 × 4 ft	3.6 cu ft / 3 bags	7.2 / 5	10.8 / 8	14.4 / 10
4 × 4 ft	7.2 cu ft / 5 bags	14.4 / 10	21.6 / 15	28.8 / 20
4 × 8 ft	14.4 cu ft / 10 bags	28.8 / 20	43.2 / 29	57.6 / 39
4 × 12 ft	21.6 cu ft / 15 bags	43.2 / 29	64.8 / 44	86.4 / 58
3 × 6 ft	8.1 cu ft / 6 bags	16.2 / 11	24.3 / 17	32.4 / 22

A cubic yard is 27 cu ft. Once you pass that, price bulk delivery instead of bags.

What goes in a raised bed mix

Three mixes that work, and what each ingredient is doing.

Mix	Proportions	Best for
General purpose	60% topsoil, 40% compost	Most vegetable beds, cheapest to make
Square foot garden	1/3 compost, 1/3 peat or coir, 1/3 vermiculite	Light, fast-draining, no native soil needed
Heavy soil areas	50% topsoil, 30% compost, 20% coarse sand or perlite	Where the topsoil available is clay-heavy
Bagged mix	As sold	One small bed, or topping up

Compost feeds the bed and holds water. Vermiculite, perlite or coarse sand keep it open. Topsoil gives it body and mineral content.

How much soil for a 4x8 raised bed?

A 4 by 8 ft bed at 12 inches deep, filled to 90%, needs 28.8 cubic feet of soil. That is twenty 1.5 cu ft bags, or just over a cubic yard.

At that size, bulk delivery is almost certainly the better buy — a cubic yard is 27 cu ft, and twenty bags is a lot of plastic, a lot of lifting, and usually more money. If you are mixing 60/40, that is 17 cu ft of topsoil and 11.5 cu ft of compost.

Two beds that size is 58 cu ft, or 2.1 cubic yards. Order it loose.

Why straight topsoil is the wrong answer

It is the obvious thing to fill a raised bed with, and it is the most common mistake. Topsoil sold in bulk is mineral soil — often subsoil with some organic matter mixed in — and it behaves very differently once it is in a box with no surrounding ground.

In a bed it compacts. Watering settles the particles together, air is squeezed out, drainage stops, and roots cannot get through it. By the second season you have a solid block that dries into a crust and cracks.

Compost is what prevents that. It holds the structure open, feeds the soil life that keeps it open, and retains water without waterlogging. Forty per cent by volume is the usual minimum in a raised bed, and more does no harm.

Filling deep beds without paying for it

A bed 24 inches deep on a patio holds nearly 58 cubic feet at 4 by 8 ft, and most plants will never send roots into the bottom six inches of it. Filling all of that with good mix is expensive and unnecessary.

The usual approach fills the bottom third with bulkier, cheaper organic material — branches, logs, turf laid upside down, leaves, straw — and puts the proper mix in the top 12 to 18 inches. The bottom layer rots down slowly, feeding the bed from below.

Two warnings. The level will drop noticeably in the first two years as that layer breaks down, so expect to top up. And keep fresh wood chips out of the root zone — they tie up nitrogen as they decompose. Below 12 inches they are fine.

What the bed sits on

On open soil, the simplest preparation is to loosen the ground underneath with a fork and leave it. Roots and worms move between the bed and the ground below, which is a genuine advantage.

If the ground is lawn or weeds, put down cardboard first. It smothers what is underneath, and it rots away within a season once the bed is in use. Plastic sheeting does the opposite — it stops drainage and cuts the bed off from the soil entirely.

On a hard surface, drainage is the whole problem. The bed needs drainage holes or open slats at the base, and a couple of inches of coarse gravel underneath is worth the effort. A bed sitting in a puddle on a patio will kill everything in it.

Bags or bulk: where the line is

The crossover is around one cubic yard, which is 27 cubic feet or eighteen 1.5 cu ft bags. Below that, bags are convenient and the price difference is not worth a delivery fee. Above it, bulk wins on both counts, usually by a wide margin.

Bulk comes with its own logistics. It arrives as a pile on your driveway and it has to be moved by wheelbarrow, which for a cubic yard is around fifteen loads. It cannot sit on the drive for a week in the rain. And most suppliers have a minimum order, often one or two yards.

A middle path that works well: buy the bulk of the volume as bulk topsoil, and the compost in bags. The topsoil is where the volume is and where the price difference bites; the compost is where quality matters most and bagged is more consistent.

Topping up, year after year

A raised bed is not filled once. The organic fraction breaks down and the level drops — typically an inch or two a year, more in the first season after building.

The annual routine is simple: an inch or two of compost spread on top each spring, which works out at roughly 3 to 6 cubic feet for a 4 by 8 ft bed. That is two to four bags. Do not dig it in; let the worms do it, which keeps the structure intact.

Budget for that when you plan. A set of beds is a small ongoing compost habit, not a one-off purchase, and it is the single thing that keeps them productive.

How this is worked out

No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.

1. volume = length × width × depth ÷ 12 Length and width in feet, depth in inches, giving cubic feet.
2. × number of beds × fill level 90% leaves a gap below the rim so watering does not wash soil out.
3. bags = volume ÷ bag size, rounded up Garden soil is commonly 1.5 cu ft; compost is often 1 cu ft.
4. cubic yards = volume ÷ 27 A cubic yard is 3 × 3 × 3 feet — the bulk delivery unit.
5. 60/40 mix = volume × 0.6 topsoil + volume × 0.4 compost The compost fraction is what stops the bed compacting.
6. thirds mix = volume ÷ 3 of each Compost, peat or coir, and vermiculite in equal parts by volume.

Common questions

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

About 28.8 cubic feet at 12 inches deep filled to 90% — twenty 1.5 cu ft bags, or just over a cubic yard. At that volume, bulk delivery is usually cheaper than bags.

Can I fill a raised bed with just topsoil?

You can, but it will compact within a season and drain badly. Topsoil in a box behaves differently from topsoil in the ground. Mix in at least 40% compost by volume — that is what keeps the structure open.

How deep should a raised bed be?

Six inches for salad and herbs, 12 inches for most vegetables, 18 or more for carrots, parsnips and potatoes. If the bed sits on open ground you can go shallower, because roots will grow down past the frame.

How many bags of soil for a raised bed?

Divide the volume in cubic feet by the bag size. A 4 × 4 ft bed at 12 inches is 14.4 cu ft, which is ten 1.5 cu ft bags. Check the bag — sizes range from 1 to 3 cubic feet and the prices do not scale with them.

What should I put in the bottom of a raised bed?

On open soil, nothing — just loosen the ground, or lay cardboard if there is grass. In a deep bed, the bottom third can be branches, logs, turf or leaves to save on mix. Never plastic sheeting, which stops drainage completely.

Is it cheaper to buy soil in bags or bulk?

Bulk, above about one cubic yard — that is 27 cubic feet or eighteen 1.5 cu ft bags. Below that, bags win once you account for delivery fees. A good compromise is bulk topsoil and bagged compost.

Why does the soil level drop every year?

The organic matter is decomposing, which is exactly what you want it to do. Expect an inch or two a year, more in the first season. Top up with an inch or two of compost each spring and leave it on the surface for the worms to take down.

Do I need to change the soil in a raised bed?

No. A raised bed does not need emptying and refilling. An annual compost top-up and a rotation of what you grow keeps it going indefinitely. Full replacement is only worth considering after a serious soil-borne disease.

Where these figures come from

Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.

• Cooperative Extension raised bed guidance — recommended bed depths and soil blend proportions
• Square Foot Gardening method — the equal-thirds compost, peat and vermiculite mix
• Bagged soil and compost manufacturer volumes — cubic feet per bag by product type

Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 12, 2026.