Estimates only. Check measurements, product labels and local building codes before you buy or build.
How the Concrete Block Calculator works
This calculator answers how many concrete blocks (CMU, often called cinder blocks) and how many 80 lb bags of mortar you need to lay up one wall, after taking out doors and windows.
Wall area = length × height, minus openings. Blocks per sq ft = 144 ÷ the nominal face area in square inches: an 8" × 16" face is 128 sq in, so 144 ÷ 128 = 1.125 blocks per sq ft (112.5 per 100 sq ft). Multiply by the net area, add waste and round up. For mortar, one 80 lb bag of mortar mix lays about 13 standard 8×8×16 blocks (Quikrete); other face sizes are scaled by joint length.
Example: a wall 20' long and 6' high is 120 sq ft; take out 20 sq ft of openings and 100 sq ft remain. 100 × 1.125 = 112.5 blocks, plus 10% waste = 123.75, so 124 blocks. Mortar: 124 ÷ 13 = 9.54, so 10 bags of 80 lb.
Nominal sizes include the 3/8" joint: an 8×8×16 block actually measures 7 5/8" × 7 5/8" × 15 5/8". Block thickness (6", 8", 12") does not change the count; only the face size does. Planning lengths and heights in 8" steps saves cuts. The waste field starts at 5%; raise it for a wall with many cuts. The count does not separate corner, bond-beam or lintel blocks, and does not include grout, rebar or the footing.
Frequently asked questions
How many concrete blocks per square foot?
For a standard 8" × 16" face, 1.125 blocks per square foot, or 112.5 per 100 sq ft (144 ÷ 128). An 8" × 8" face takes 2.25 per sq ft and a 12" × 16" face 0.75. Add a few percent for breakage and cuts.
How many blocks does a bag of mortar lay?
One 80 lb bag of mortar mix lays about 13 standard 8×8×16 blocks according to Quikrete (Sakrete says about 12 for its Type S). That is roughly 8 bags per 100 blocks. Real use varies with joint size and how much mortar gets dropped, so treat it as an estimate.
What are the actual dimensions of an 8x8x16 block?
7 5/8" wide × 7 5/8" high × 15 5/8" long. The nominal size adds one 3/8" mortar joint, so each block takes up 8" × 16" on the wall face. That is why planning lengths and heights in 8" steps keeps cuts to a minimum.
Does wall thickness change how many blocks I need?
No. A 6", 8" or 12" thick block with an 8" × 16" face lays up at 1.125 per square foot, because the count depends only on the face. Thickness changes the weight, the grout fill and the price, not the number of blocks.