Cinder Block Calculator -

Related Construction & Masonry Calculators

How to Use the Cinder Block Calculator

Enter Wall Dimensions: Input the total length and height of the wall you intend to build, in feet.
Select Block Size: Choose the size of the concrete masonry unit (CMU) you will be using. The calculator accounts for a standard 3/8" mortar joint.
Add a Waste Factor: It is highly recommended to add 5-10% for waste to account for blocks that may need to be cut or could break during construction.
Calculate: Get an instant estimate for the number of blocks and bags of mortar required for your project.

Understanding the Calculation

To calculate the number of blocks, we first determine the total area of the wall (Length × Height). Then, we calculate the effective area of a single block, including the mortar joint. A standard 8" x 8" x 16" block actually measures 7 5/8" x 7 5/8" x 15 5/8". With a 3/8" mortar joint, its effective area is 8" x 16", or 128 square inches (0.889 sq ft). The total number of blocks is the wall area divided by the single block area.

For mortar, a common rule of thumb is that one 80lb bag of mortar mix can lay approximately 30-35 standard 8x8x16 blocks.

Frequently Asked Questions (FAQ)

What is the difference between a cinder block and a concrete block?

Historically, "cinder blocks" were made with ash cinders as an aggregate. Today, the terms are often used interchangeably, but modern blocks are technically "concrete masonry units" (CMUs) made with sand and fine gravel. They can be hollow (with two or three cores) or solid.

Do I need to fill the hollow cores of the blocks?

For structural, load-bearing walls, it is often required by building codes to fill some or all of the block cores with concrete grout and rebar for reinforcement. This calculator estimates the blocks for the wall shell only and does not calculate the core fill material. Always consult local building codes.