Concrete block calculator

How many concrete blocks do I need?

Work out the wall area in square feet, take away the doors and windows, and divide by the face area of one block. For the common 8 x 8 x 16 in block that face is 16 x 8 in, which is 0.889 square feet, so you need 1.125 blocks per square foot of wall. Add a few extra for breakage and cuts, then round up, because blocks are sold whole.

A 10 ft long wall that is 8 ft high is 80 square feet. At 1.125 blocks per square foot that is exactly 90 blocks, or 95 with a 5 percent allowance.

Nominal and actual block sizes

Concrete blocks are named by their nominal size, which is the actual block plus the width of one mortar joint. Typical joints are 3/8 in. So an "8 x 8 x 16" block really measures 7 5/8 x 7 5/8 x 15 5/8 in, and once it is laid with its joint it takes up 8 x 16 in of wall face.

The calculator counts blocks on the nominal face size, because that is the space each block fills in a finished wall. The 4, 6, 8 and 12 in blocks differ only in thickness (the first number), so all four have the same 16 x 8 in face and the same count per square foot. A thicker block changes the weight and the mortar, not the number of blocks.

If your block is a different size, pick Custom size and enter its nominal length and height: the actual length and height plus one joint each.

The formula

1. Wall area: wall length times wall height. Mixed units are fine; the calculator converts them.
2. Openings: for each door or window, width times height, times how many. Subtract the total from the wall area.
3. Block face: nominal block length times nominal height. For a 16 x 8 in face, 16 x 8 / 144 = 0.889 square feet.
4. Blocks: net wall area divided by the block face.
5. Waste: add the allowance (5 percent to start), then round up to a whole block.

Worked example: a wall with a door

A garden wall 10 ft long and 8 ft high, with one door opening 3 ft wide and 7 ft high, in 8 x 8 x 16 in blocks:

1. Wall: 10 x 8 = 80 sq ft.
2. Door: 3 x 7 = 21 sq ft, so the net area is 80 - 21 = 59 sq ft.
3. Blocks: 59 / 0.889 = 66.4, which is 67 blocks with no allowance.
4. With 5 percent: 66.4 x 1.05 = 69.7, rounded up to 70 blocks.

In metric, a 16 x 8 in face works out to about 12.1 blocks per square meter of wall.

Mortar

The QUIKRETE Mortar Mix data sheet says each 80 lb bag will lay up to 13 standard 8 x 8 x 16 in blocks. Because that is an upper limit, the calculator divides your block count by 13 and rounds up, and labels the result as a minimum. For the 70 block wall above that is 70 / 13 = 5.4, so at least 6 bags. Buy a spare bag if you are new to laying block.

The sheet gives this figure only for 8 x 8 x 16 in blocks, so the mortar line appears only for that size. For other sizes, ask your supplier.

What the calculator leaves out

• Core fill: filling the hollow cores with concrete or grout is common in reinforced and retaining walls, but we have not found a manufacturer data sheet that states the core volume per block or a yield in blocks per bag. Rather than guess, the calculator does not estimate it. Your block supplier can give the core volume for the exact block they sell.
• Half blocks, corner blocks and lintels: these replace full blocks at ends, corners and over openings. Count them with your supplier if your wall needs them.
• Footings and rebar: a block wall sits on a footing. Use the concrete calculator in footing mode for the concrete, and follow local codes for reinforcement and for any wall that holds back soil.

Common mistakes

• Using the actual size instead of the nominal size. Dividing by a 15 5/8 x 7 5/8 in face counts blocks as if there were no mortar joints, and gives too many.
• Forgetting the openings. A 3 x 7 ft door and two 3 x 2 ft windows take 33 square feet out of a wall. Add each one so you do not over-order.
• Rounding down. Blocks are sold whole, so always round the final count up.

Values this calculator uses

Blocks are counted on their nominal size: the actual block plus one 3/8 in mortar joint. Your supplier's numbers win if they differ.

Sources

• archtoolbox.com: concblocksizes
• quikrete.com: data sheet mortar mix 1102