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Concrete Block (Cinder Block) Calculator

Wall length and height to cinder blocks needed, plus bags of mortar mix to lay them.

Your numbers
Your project

Blocks needed

Enter your numbers above.

Lay out the first course dry before you mix any mortar

Set the bottom row of blocks on the footing without mortar, spaced with 3/8 in. gaps, and see where the end falls. Adjusting the wall length a few inches to land on full or half blocks saves a lot of cutting on every course above it.

How many concrete blocks a wall takes

A standard concrete masonry unit (CMU) is 7-5/8 × 15-5/8 in. on its face. With a 3/8 in. mortar joint, each one covers a nominal 8 × 16 in., or 0.889 sq ft, which works out to 1.125 blocks per square foot of wall. That is true whatever the block's thickness, so a 6, 8 or 12 in. block wall uses the same count.

The calculator multiplies the wall's face area (length × height, less any openings) by 1.125 and adds a waste allowance for broken and cut blocks: blocks = area × 1.125 × (1 + waste %). Five percent is typical for straight walls; use more for walls with many corners, pilasters or openings.

For mortar, QUIKRETE's data sheet for its Mortar Mix says one 80 lb bag lays up to 13 standard 8 × 8 × 16 in. blocks. If you need the mortar in more detail, for brick or tile as well, the mortar mix calculator breaks it down by application.

Where these numbers come from

The formulas and default values in this calculator are based on:

  • National Concrete Masonry Association (NCMA) nominal CMU dimensions
  • QUIKRETE Mortar Mix No. 1102 data sheet (80 lb bag lays up to 13 standard blocks)

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.

Frequently asked questions

Is a cinder block the same as a concrete block?

In everyday use, yes. True cinder blocks made with coal cinders are rare today; what stores sell as cinder block is a concrete masonry unit made with cement and sand or lightweight aggregate. The count per square foot is the same.

Do I need to count the blocks in the footing?

Footings are usually poured concrete, not block, so figure them separately with the concrete footing calculator. Include any block courses below grade in the wall height if they sit on the footing.

How much extra block should I order?

About 5% covers breakage and cuts on a simple straight wall. Walls with many corners, openings or half-block courses call for 10%. Leftover full blocks are handy for steps, planters and repairs.

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