🪵How much soil does your raised bed need?

✓ Tested & verified Updated: Aug 6, 2026 How we calculate this

Raised bed soil calculator

Bed length ft

Bed width ft

Fill depth (inches)

Number of beds

Settling allowance

Bag size you can buy

35.8 cubic feet of mix

= 1.3 cubic yards · about 24 bags (1.5 cu ft)

Topsoil, 50%: 17.9 cu ft
Compost, 30%: 10.8 cu ft
Potting mix or aeration, 20%: 7.2 cu ft
Before the settling allowance: 32 cu ft

Fill depth is the soil, not the frame. A 12-inch board holds about 11 inches of mix once you leave a watering gap.

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How much soil does a raised bed need?

A 4 × 8 ft bed filled 12 inches deep holds 32 cubic feet of mix — 1.2 cubic yards — and you should buy about 1.3 yards, because a blended mix settles roughly 12 percent after the first few waterings. Length × width × depth in feet is all there is to it: 8 × 4 × 1 = 32.

Split that into a mix rather than one product. Fifty percent screened topsoil, 30 percent finished compost and 20 percent potting mix or an aeration ingredient gives roughly 17.9, 10.8 and 7.2 cubic feet for this bed. Compost is an amendment, not a growing medium — past about 30 percent of the volume it holds too much water, settles hard and burns young roots on nitrogen it releases too fast.

Filling a deep bed entirely with bought mix is the expensive mistake. Beds over 12 inches can have their bottom third filled with coarse organic material — branches, turf sods laid upside down, leaves — which rots down into the profile and cuts the purchased volume by a third.

Raised beds are the one job on this site where people routinely order twice what they need, because the volume looks small on paper and enormous in bags. A single 4 × 8 bed at full depth is more than a cubic yard — 22 bags, or most of a pickup load.

The table gives the finished volume for the standard timber sizes at 12 inches, before the settling allowance. Halve the numbers for a 6-inch bed, add half again for 18.

Bed size	Cubic feet	Cubic yards	1.5 cu ft bags
2 × 4 ft	8	0.30	6
4 × 4 ft	16	0.59	11
3 × 6 ft	18	0.67	12
4 × 8 ft	32	1.19	22
4 × 12 ft	48	1.78	32

Two 4 × 8 beds put you past two cubic yards, which is where a bulk delivery of blended raised bed mix becomes both cheaper and far less work than stacking 44 bags into a car.

Frequently asked questions

How much soil does a 4x8 raised bed need?

32 cubic feet at 12 inches deep, which is 1.19 cubic yards, or about 22 bags of the 1.5 cu ft size. At 6 inches it halves to 16 cubic feet and 11 bags. Add roughly 12 percent on top of whichever figure applies, because a mix with compost in it settles noticeably in the first month.

What is Mel's Mix and does it work?

It is the Square Foot Gardening recipe: one third compost, one third peat moss or coir, one third coarse vermiculite by volume. It drains and roots beautifully in year one and is expensive to build, and because it holds almost no mineral soil it needs compost topped up every season. A 50/30/20 topsoil, compost and potting mix blend performs nearly as well for a fraction of the cost.

How deep should a raised bed be?

Six inches is enough over open ground that roots can reach into, twelve where the bed sits on a hard surface such as a patio or a weed membrane over gravel, and eighteen if you want carrots, parsnips or long-rooted brassicas. Depth past 18 inches adds cost without adding yield for almost every vegetable.

Can I fill a raised bed with just compost?

No. Pure compost drains too fast, then slumps into a dense airless layer as it finishes decomposing, and it can hold enough soluble salts to check seedlings. It also loses a third of its volume in the first season. Compost belongs at 25 to 30 percent of the finished mix, with mineral soil making up the bulk.

Why does raised bed soil keep sinking?

Because the organic fraction is still decomposing and because the mineral particles settle into the air gaps a few waterings after filling. Losing an inch or two in the first year is normal and healthy. Top up with compost each spring rather than refilling with fresh mix, and expect it to slow after the second season.

Is bagged or bulk soil better for raised beds?

Bulk, above about a cubic yard — which is one 4 × 8 bed at full depth. Two beds is 2.4 yards, which would be 44 bags to carry, and bulk raised bed mix costs $45 to $80 a yard against $100 or more for the same volume in bags. Below one yard the delivery fee wipes out the saving.

How we calculate this

Volume is length × width × fill depth, with the depth converted from inches to feet, multiplied by the number of beds; 27 cubic feet make a cubic yard. The settling allowance is applied as a percentage on top: 10 percent for a mostly mineral fill, 12 for the usual blend, 15 for a compost-heavy mix. The 50 / 30 / 20 split of topsoil, compost and potting mix keeps compost inside the 25–30 percent ceiling that extension guidance sets for amendment as a share of finished soil volume. Bag counts use the sizes in our bag table and round up. Nothing here is stored or sent anywhere.

Worked example

With Bed length ft 8, Bed width ft 4, Fill depth (inches) 12, Number of beds 1, Settling allowance 12% — the usual blended mix, Bag size you can buy 1.5 cu ft — Topsoil, potting mix, dyed and rubber mulch, this page works out 35.8 cubic feet of mix. = 1.3 cubic yards · about 24 bags ( 1.5 cu ft )

Topsoil, 50%17.9 cu ft
Compost, 30%10.8 cu ft
Potting mix or aeration, 20%7.2 cu ft
Before the settling allowance32 cu ft

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