Materials

Cinder Block Raised Garden Beds

A cinder block raised bed is cheap, quick and nearly permanent: lay standard 8x8x16-in blocks on level ground, holes up, and fill. A 4x8 bed one block high takes about 16 blocks and can last for decades.

Written by the Raised Gardening Beds editors · Updated October 22, 2026

Leafy plant growing in the open core of a gray concrete block

Block counts for common bed sizes (outer dimensions)
Bed size	Blocks per course	1 course (about 8 in tall)	2 courses (about 16 in tall)
4x4 ft	10	10 blocks	20 blocks
4x8 ft	16	16 blocks	32 blocks
4x12 ft	22	22 blocks	44 blocks
5 ft 4 in x 8 ft	18	18 blocks	36 blocks

Cinder block vs concrete block

Most people say "cinder block," but what you buy today is almost always a concrete block: cement, sand and gravel. True cinder blocks, made with coal cinders or fly ash in the mix, were common decades ago and are now mostly found as old salvage.

For a garden bed, the practical differences are small. Both come in the standard 8x8x16-in size (the real measurement is about 7⅝ x 7⅝ x 15⅝ in, leaving room for mortar joints). Concrete blocks are heavier, at roughly 35 to 40 lb each, and stronger. Lightweight blocks made with expanded shale or similar aggregates run closer to 25 to 30 lb and are easier on the back.

Blocks cost a few dollars each, which makes this one of the cheapest long-lasting bed materials. For more low-cost options, see our cheap raised garden bed ideas.

The fly ash question

Fly ash is a byproduct of burning coal, and it can contain trace amounts of heavy metals. Some older cinder blocks, and some modern concrete mixes, include it. That raises a fair question about growing food next to them.

The evidence of actual harm to garden crops is limited, and metals bound in cured concrete don't move readily into soil. Still, if you want to be cautious:

Buy new concrete blocks rather than unknown salvaged ones.
Plant edibles in the main bed, a few inches back from the walls, and use the block holes for flowers.
Line the inside face of the blocks with heavy plastic sheeting if you want a full barrier. Leave the bottom open for drainage.

One more effect worth knowing: fresh concrete can raise soil pH slightly right next to the blocks. Most vegetables don't mind, but acid lovers like blueberries won't thrive in a block bed.

Planning size and block count

Keep the inside width no more than about 4 ft so you can reach the middle from both sides. Remember that blocks take up 8 in on each side, so a bed with outer dimensions of 4x8 ft gives you a growing area of about 2 ft 8 in by 6 ft 8 in, plus the holes. If you want a full 4-ft-wide planting area, make the outer width 5 ft 4 in.

To count blocks for any rectangle:

Measure the outer length and width in inches.
Subtract 8 from each.
Add the two numbers, double the total, and divide by 16.

That gives blocks per course. Sizes work out cleanly when both outer measurements are multiples of 16 in (4 ft, 5 ft 4 in, 8 ft, 12 ft); other sizes mean cutting a block in half with a masonry blade. For a 4x8 bed: (88 + 40) x 2 ÷ 16 = 16 blocks. Multiply by the number of courses, and buy one or two extra for breakage.

For depth, one course (about 8 in) suits lettuce, herbs and other shallow crops over loosened soil. Two courses (about 16 in) suit most vegetables. Fill with a good raised bed soil mix, not plain native soil.

Laying the blocks

Mark the outline with string and stakes, and check that it's square by measuring both diagonals.
Clear the strip where the blocks will sit. Remove sod and dig down 2 to 3 in.
Add a base. Spread 2 in of paver base or crushed gravel, then tamp it firm. This keeps blocks from rocking or sinking.
Set the first course with the holes facing up. Use a level along every block and across corners; tap high spots down with a rubber mallet.
Stagger the second course so each block straddles the joint below, like brickwork. Run a bead of masonry adhesive between courses to stop shifting.
Pin the corners, if you like, by driving a length of rebar through the holes into the ground.

Without mortar, keep to one or two courses. Taller block walls need proper footings and mortar, or they can lean outward as wet soil pushes on them. Our guide to building a raised garden bed covers site choice and filling in more detail.

Planting the block holes

Each hole, or core, holds a bit less than a gallon of soil. That's small and dries out fast, so pick tough, compact plants:

Herbs: thyme, oregano, chives, sage.
Flowers: marigolds, nasturtiums, alyssum, calendula.
Fruit: strawberries trail nicely over the edge.
Succulents: sedum and hens and chicks need almost no water.

Stuff a wad of newspaper or a small stone in the bottom of each hole so soil doesn't wash out the base, then fill with the same mix as the bed. Water the holes every day or two in hot weather.

How to choose

Go with blocks when

You want a durable bed for little money and don't mind a utilitarian look or the weight of moving them.

Pick new over salvaged

Fresh concrete blocks remove most of the fly ash worry and come in uniform sizes that stack level.

Stick to two courses

Unmortared walls stay stable at about 16 in. For a taller bed, choose wood or metal instead.

Related guides

Do cinder blocks leach chemicals into soil?

Modern concrete blocks are mostly cement, sand and gravel, and evidence of harm from them is limited. Older blocks with fly ash may hold trace metals, so cautious gardeners grow food in the bed and keep flowers in the holes.

Do you need mortar for a cinder block bed?

Not for one or two courses on a level, compacted base. Masonry adhesive between layers and rebar at the corners keep things steady. Go higher only with footings and mortar.

How many blocks does a 4x8 raised bed need?

About 16 blocks for each layer, measured as 4x8 ft on the outside. Two layers, roughly 16 in tall, take about 32 blocks.