Concrete Block Calculator
Calculate how many concrete blocks a wall needs, plus the mortar to lay them.
Enter the wall length and height, pick a block size, and get the block count, blocks per square foot, mortar volume and bag count, with a waste allowance and an optional cost.
Supports standard 16 × 8 in CMU, 12 × 8 in, 16 × 4 in and metric 390 × 190 mm blocks.
How to use the concrete block calculator
Enter the wall length and height.
Choose your block size.
Read the block count, mortar volume and cost.
Why use the Build Tally concrete block calculator
Four block sizes
Standard CMU, 12 in, 4 in and metric 390 × 190 mm.
Mortar included
Cubic feet and 80 lb bag count.
Waste allowance
Adjustable, 5% by default.
Cost estimate
Price per block in any currency.
Why 1.125 blocks per square foot
A standard CMU is nominally 16 × 8 inches, but that nominal size already includes a 3/8 inch mortar joint — the block itself is 15⅝ × 7⅝. Working in nominal sizes, each block covers 16 × 8 = 128 square inches, and 144 ÷ 128 gives 1.125 blocks per square foot. That is the number every masonry estimator uses, and it is why a 100 square foot wall takes about 113 blocks.
Mortar and grout
As a working figure, one cubic foot of mortar lays about 33 standard blocks, so an 80 lb bag of mortar mix covers roughly 36 blocks. If the cores will be grouted — common in reinforced walls — that is a separate and much larger volume: a standard block has about 0.26 cubic feet of core space, so grouting every core in a 113-block wall takes close to a cubic yard. Grout is not included in this calculator.
Openings, corners and bond
The calculator gives the gross wall area. Subtract door and window openings yourself, but do not subtract the full opening — you will need extra blocks for the jambs and any bond beam above. Corners consume a few extra blocks because of the overlap in running bond, and half blocks are usually needed at the ends of alternate courses.
Read the guide: How Much Concrete Do I Need? A Straight Answer for Slabs, Footings and Posts
Frequently asked questions
How many blocks do I need for a wall?
About 1.125 standard blocks per square foot of wall. Enter your length and height and the calculator adds a waste allowance.
How many blocks in 100 square feet?
Roughly 113 standard 16 × 8 in blocks, before waste.
How much mortar per block?
About one cubic foot of mortar per 33 blocks, or one 80 lb bag per 36 blocks.
How many blocks does an 80 lb bag of mortar lay?
Roughly 36 standard blocks, though it varies with joint thickness and how much is wasted.
Should I subtract windows and doors?
Yes, but add back blocks for the jambs and any bond beam over the opening.
How high can I build a block wall?
Height limits depend on thickness, reinforcement and wind load, and are set by code. Freestanding walls above about 4 ft usually need engineering.
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