Cinder Block vs Concrete Block: What the Names Mean

Cinder block vs concrete block is mostly a naming question today. Nearly all gray blocks sold at home centers and masonry yards are concrete masonry units, or CMUs, made from portland cement, sand, and gravel or lightweight aggregate. True cinder blocks used coal cinders or fly ash as aggregate and were common in the early to mid 1900s. People still call any hollow gray block a “cinder block,” but the products you buy now are concrete block.

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Side-by-side

Feature	Cinder block (historic)	Concrete block (CMU)
Aggregate	Coal cinders or ash	Sand and gravel, or expanded shale or slag
Weight (8x8x16)	Lighter, often about 26 to 33 lb	About 35 to 40 lb normal weight; lighter versions exist
Strength	Lower	Higher, made to standard specifications
Availability	Rare new	Standard product
Common uses	Older foundations and walls	Foundations, retaining walls, buildings

Lightweight vs normal-weight block

Modern block comes in normal-weight and lightweight versions. Lightweight block uses expanded shale, clay, slate, or slag and can weigh 25 to 30 percent less, which helps on large walls. Both meet strength standards when made to spec. See how much does a concrete block weigh.

Is “cinder block” in an old house a problem?

Older cinder block walls are usually still serviceable if they are dry, straight, and not cracked. They can be weaker and more porous than modern CMU, so be careful about adding loads or cutting large openings. Bulging, horizontal cracks, or water seepage in a foundation wall call for a structural engineer, whatever the block type.

Common sizes

The nominal 8x8x16 inch block is actually about 7-5/8 by 7-5/8 by 15-5/8 inches, so with a 3/8 inch mortar joint it lays out at 8 by 16. Other common widths are 4, 6, 10, and 12 inches, plus half blocks, corner blocks, and bond-beam blocks.

Garden and DIY uses

People use blocks for raised beds, fire pits, steps, and garden walls. For a dry-stacked raised bed, standard blocks work fine. For a fire pit, use blocks rated for heat or line the inside with fire brick, since ordinary concrete block can crack or spall when it gets very hot and wet. For walls that hold soil, use a block system made for retaining walls or a reinforced design.

Buying tips

Ask whether the yard stocks normal-weight or lightweight block.
Order corners, halves, and caps along with stretchers.
Check the pallet count to plan delivery. See how many concrete blocks on a pallet.

Bottom line

Cinder block is the old name for blocks made with coal cinders. What you buy today is concrete block, a stronger, standardized product made with sand and gravel or lightweight aggregate. The name lives on, but for new projects the choice is really between normal-weight and lightweight concrete block.

Questions people ask

Are cinder blocks and concrete blocks the same?

Not technically. True cinder blocks used coal cinders; modern blocks are concrete. People still use the names interchangeably.

Is cinder block weaker than concrete block?

Historic cinder block is generally weaker and more porous than modern concrete masonry units.

How much does a cinder block weigh?

A standard 8x8x16 concrete block weighs about 35 to 40 pounds; lightweight block is less.

Can I use concrete blocks for a fire pit?

Line the inside with fire brick or use heat-rated block, since ordinary block can crack under high heat.

Can I still buy real cinder blocks?

They are rarely made now. Most stores sell concrete block even when it is labeled cinder block.

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