How to Estimate Concrete Blocks and Mortar for a Wall

By Shakeel Muzaffar, reviewed by Prof. Dr. Khalil Mudassar, PhD · Last updated 2026-10-04

To make a concrete block estimate, find the wall area in square feet, subtract any openings, then multiply by 1.125 blocks per square foot for standard 8x8x16 units. Add 5 to 10 percent for waste. For mortar, plan one 80 lb bag per 13 blocks, and buy extra.

What Is the Fastest Way to Estimate Blocks for a Wall?

The fastest method is area times blocks per square foot. Measure the wall length and height in feet, multiply them for the gross area, and subtract the area of any doors or windows. Then multiply the net area by 1.125 for standard 8x8x16 units.

Here is a worked case. A wall 10 feet long and 8 feet high with no openings is 80 square feet. Multiply 80 by 1.125 and you get 90 standard blocks before waste. That single number drives your whole order, so it pays to get the module right first.

You do not need to count blocks one row at a time. The per square foot shortcut already bakes in the mortar joints, which is why it uses the nominal size rather than the smaller actual block.

Why Is It 1.125 Blocks per Square Foot?

The figure comes from the block’s nominal face, not its actual size. A standard unit has a nominal face of 16 inches long by 8 inches high, which is 128 square inches. A square foot is 144 square inches. Divide 144 by 128 and you get 1.125 blocks per square foot.

This number is a derived standard. The Concrete Masonry and Hardscapes Association (CMHA) publishes the nominal module, and the 1.125 value is simple arithmetic on that verified face. The association’s technical note does not print the figure itself.

Because the nominal face already includes space for mortar, you plan the wall on the nominal grid. That keeps your estimate honest without measuring every joint by hand.

How Does Nominal Size Differ From Actual Size?

A nominal 8x8x16 block is not actually 8 by 8 by 16 inches. CMHA states that specified dimensions are typically 3/8 inch less than nominal. So the real unit measures about 7-5/8 by 7-5/8 by 15-5/8 inches.

The missing 3/8 inch is the mortar joint. When you add one standard joint to each actual dimension, the block returns to its nominal 8 and 16 inch grid. That is how courses line up cleanly across a long wall.

This matters for your estimate in one key way. You plan on nominal sizes, because the joints are part of the module. If you used the smaller actual size, your block count would come out too high.

How Much Mortar Do You Need per Block?

Plan on one 80 lb bag of masonry mix for about every 13 standard blocks. Quikrete lists that each 80 lb bag of its Mason Mix (No. 1136, Type S) or Mortar Mix (No. 1102, Type N) will lay up to 13 standard 8x8x16 blocks. That works out to about 7.7 bags per 100 blocks.

Honesty check: The phrase “up to 13” is a lab maximum. Real walls with full bed joints, spillage, and cleanup use more mortar than the sheet suggests. A safe habit is to order roughly 10 to 15 percent extra mortar on top of the bag count.

Type S and Type N are different mixes for different jobs. Type S has higher compressive strength and suits below-grade or structural work. Type N is a general purpose mix for above-grade walls. Both data sheets list the same per bag block yield.

What Do the Numbers Look Like at Different Wall Sizes?

The table below scales the same method to three common wall areas. It uses 1.125 blocks per square foot and 13 blocks per 80 lb bag, with a 10 percent waste column for a safer order. Treat every figure as an estimate and order extra.

These are benchmark estimates, not a materials list. Openings, bond pattern, half blocks, and jobsite waste all shift the real count. Use them to sanity check a quote, then let the tool handle your exact wall.

What Mistakes Throw Off a Block Estimate?

The most common error is forgetting waste. Cuts at corners, broken units, and partial courses all add to the count. A 5 to 10 percent allowance covers most jobs, and cut-heavy layouts may need more.

A second error is trusting the mortar yield as a promise. The 13 blocks per bag figure is a maximum under ideal conditions, so planning exactly to it leaves you short. Round bags up and keep a spare.

A third slip is ignoring the hollow cores. If your design calls for grout in some cells, that is a separate volume you estimate on its own with the CMU grout fill calculator. Block count and grout fill are not the same number.

Structural note: This article covers quantity estimating only. Load-bearing and retaining walls need an engineer’s specification and must follow your local building code. Reinforcement, grout spacing, and footing design are not something to guess from a square foot rule.

How Does Block Estimating Compare to Brick or Concrete?

The method is the same idea, but the numbers differ by material. Blocks use a large nominal face, so you need few per square foot. Brick is much smaller, so a square foot of brick wall takes several units.

If your project is a brick veneer instead, the count per square foot changes entirely. Our guide on how many bricks per square foot walks through that figure. For a poured footing or slab under the wall, you switch from counting units to measuring volume, which the concrete calculator handles.

For other material estimates on the same project, the construction calculators hub groups the tools you are likely to need next.

Concrete Block Estimating: Frequently Asked Questions

Sources and Further Reading

Educational estimate, not professional construction or engineering advice. Follow the manufacturer’s current data sheet and your local building codes; waste allowance is your own assumption. Not affiliated with any brand, and trademarks belong to their owners. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated October 4, 2026.