Cinder Block vs CMU Block
The terms “cinder block” and “CMU block” are often used interchangeably in everyday speech, but they refer to slightly different products. A true cinder block was made using coal cinders, the ash residue from burning coal in industrial furnaces, as the aggregate. This gave the blocks a lighter weight and a rougher texture. Cinder blocks were a popular building material from the early twentieth century through the 1950s.
A CMU block, or concrete masonry unit, is manufactured using Portland cement and aggregate such as sand, gravel, crushed stone, or industrial slag. CMU blocks are stronger, denser, and more consistent in quality than historical cinder blocks. They meet the ASTM C90 standard for loadbearing concrete masonry units and are the only type of block available at modern building suppliers.
In practice, when someone says “cinder block” today, they almost always mean a standard 8x8x16 CMU block. The calculator on this page uses the same formulas and block dimensions as our CMU block calculator, since the products are equivalent for estimation purposes.
Cinder Block Sizes and Weights
Standard cinder blocks (modern CMU) come in several nominal sizes. The table below shows the most common options along with typical weights and applications:
| Nominal Size | Actual Size | Weight | Common Use |
|---|---|---|---|
| 8" x 8" x 16" | 7⅝" x 7⅝" x 15⅝" | ~35–38 lb | General walls, foundations |
| 8" x 4" x 16" | 7⅝" x 3⅝" x 15⅝" | ~22 lb | Half block, wall caps |
| 4" x 8" x 16" | 3⅝" x 7⅝" x 15⅝" | ~20 lb | Thin partition walls |
| 12" x 8" x 16" | 11⅝" x 7⅝" x 15⅝" | ~50–55 lb | Heavy retaining walls |
How Many Cinder Blocks Do I Need?
Estimating cinder blocks is straightforward once you know the wall area. Follow these steps for a standard 8x8x16 block with a 3/8-inch mortar joint:
- Measure wall length and height in feet. A 30-foot-long wall that is 6 feet tall has an area of 180 square feet.
- Multiply by 1.125 blocks per square foot.This standard factor accounts for the mortar joint: 180 × 1.125 = 202.5, round up to 203 blocks.
- Add a 10% waste factor.Multiply by 1.10: 203 × 1.10 = 224 blocks (round up). Order at least 224 blocks to account for cuts and breakage.
- Subtract window and door openings. Measure each opening in square feet and multiply by 1.125 to find how many blocks to subtract from the total.
The cinder block calculator at the top of this page performs all four steps automatically. Enter your wall length and height, choose a block size, and the calculator returns the block count with waste, the number of courses (rows), and an optional cost estimate.
For walls with multiple openings such as windows and doors, calculate the gross wall area first (total length times total height), then subtract each opening area. Always keep a few extra blocks on hand for future repairs or wall extensions, since matching the exact texture and color of older blocks can be difficult.
Cinder block walls are durable, fire resistant, and relatively inexpensive compared to poured concrete or brick. They are a popular choice for garden walls, retaining walls, basement walls, and outbuildings. With accurate dimensions and this calculator, you can order the right quantity of blocks on the first trip to the supply yard.