Cinder Block vs Concrete Block: Are They the Same?

The Short Answer

In modern construction, “cinder block” and “concrete block” refer to the same product. If you bought a block at a hardware store in the last 40 years and called it a cinder block, it was almost certainly a concrete masonry unit (CMU) made with Portland cement and aggregate — not cinders. Both terms are correct in casual use; only the material history differs.

What Were Original Cinder Blocks?

True cinder blocks were manufactured using coal cinders — the ash residue left after burning coal — as the aggregate in place of gravel or stone. This made them lighter and cheaper than concrete blocks because cinders were industrial waste material available in large quantities near coal-burning power plants and industrial facilities.

Cinder block construction peaked roughly from the 1920s through the 1960s, when coal was the dominant industrial fuel. As coal use declined and clean air regulations increased, cinder ash became scarce. Manufacturers replaced cinders with expanded clay, shale, or slate aggregate — technically making them lightweight concrete blocks, not cinder blocks.

Modern CMU Composition

Today’s concrete masonry units fall into two main weight categories:

Normal weight CMU: Portland cement + natural aggregate (gravel, crushed stone). Weight: 38–43 lbs per standard 8×8×16 block. Higher compressive strength (minimum 1,900 psi per ASTM C90).

Lightweight CMU: Portland cement + expanded clay, shale, or slate. Weight: 28–33 lbs per standard block. Slightly lower density, better thermal insulation, easier to lay. Common for above-grade walls.

Neither contains coal cinders. Both meet ASTM C90 for loadbearing applications.

Why the Name Persists

“Cinder block” persists in American English as a colloquial term because it was the dominant product for mid-20th century construction, and the vernacular name outlasted the material change. It’s the same phenomenon as calling all photocopies “Xeroxes” or all adhesive bandages “Band-Aids.”

In construction documents, specifications, and engineering drawings, “cinder block” is never used — CMU or concrete masonry unit is the correct technical term.

Does It Matter for Your Project?

No. When shopping at a hardware store, home center, or masonry supplier, any block sold as a “cinder block” will meet the same structural requirements as a CMU of the same nominal dimensions. Specify weight class (normal vs. lightweight) if it matters for your application, and verify the block meets ASTM C90 for loadbearing walls.

For estimating purposes, this calculator applies equally to any block marketed as cinder block, concrete block, or CMU.

When to Choose Each Weight Class