What is a standard concrete block (CMU) size?

The most common US concrete masonry unit (CMU) is 8 inches tall x 8 inches deep x 16 inches long (nominal). With a standard 3/8-inch mortar joint, the actual block fits into an 8x16 inch face layout on the wall.

How many blocks per square foot?

A standard 8x16 inch CMU with a 3/8-inch mortar joint covers approximately 0.889 square feet of wall face per block, so you need roughly 1.125 blocks per square foot of wall.

How much mortar do I need per block?

A general rule of thumb is that one 70-lb bag of mortar mix covers approximately 25–30 blocks when using standard 3/8-inch joints. This calculator uses 28 blocks per bag as the default rate.

What waste factor should I use?

A 5–10% waste allowance is typical for straight walls. Use 10–15% for walls with corners, curves, or many openings where additional cuts are required.

Can I subtract door and window openings?

Yes. Enter the total opening area in the deduction field and the calculator will subtract it from the gross wall area before counting blocks.

Does this work for retaining walls?

Yes. Enter the total linear length and the desired height of the retaining wall. The block and mortar estimates are the same regardless of wall type.

Block Calculator

Name: Block Calculator
Availability: InStock
Author: Nham Vu

Estimate the number of concrete blocks (CMU) and bags of mortar needed for a wall, with mortar joint and waste factor.

Wall Dimensions

Wall Length (ft)

Wall Height (ft)

Block Size (nominal)

Mortar Joint Thickness (in)

Openings (doors/windows) area (sq ft)

Waste Factor (%)

Results

Enter dimensions and press Calculate

Summary

Estimate the number of concrete blocks (CMU) and bags of mortar needed for a wall, with mortar joint and waste factor.

How it works

Enter the wall length and height in feet and inches.
Select the block size — 8x8x16 inch is the standard CMU default.
Set the mortar joint thickness (typically 3/8 inch).
Add a waste factor percentage to cover cuts and breakage.
Click Calculate to see block count and mortar bag estimate.
Review the results panel for total blocks, mortar bags, and wall area.

Use cases

Estimating CMU block quantities for a garden wall or retaining wall.
Planning materials for a block foundation or basement wall.
Calculating blocks and mortar for a commercial or residential building.
Budgeting a masonry project before heading to the supply store.
Comparing 8x8x16 vs 4x8x16 half-block requirements.
Accounting for door and window openings to reduce waste.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu