Calculators · Concrete Block Calculator

Concrete & masonry

Concrete Block Calculator

Type the length and height of your wall, subtract doors and windows, and get the number of concrete blocks, with mortar and grout.

Wall 1

Length

Height

Door or window 1

Width

Height

How many

Block face (height × length)

Nominal size. 8 × 16 is the standard block.

Block width

Does not change the block count.

Waste to add (%)

Optional. Empty counts as 0%.

Grout in the cells

Optional. Standard 8 × 16 face, 6 to 12 in wide.

How to calculate concrete blocks for a wall

Concrete blocks, also called concrete masonry units or CMU, are counted by area. Measure the wall in feet, multiply length × height, subtract the doors and windows, and multiply by the number of blocks that fit in one square foot. A standard block has a nominal face of 8 inches high and 16 inches long. That is 8 × 16 = 128 square inches, and a square foot is 144 square inches, so:

Blocks per square foot = 144 ÷ 128 = 1.125, or 112.5 blocks per 100 square feet. The CMHA Tech Note 04-02A says the same in another way: the exposed face of one unit is 8/9 of a square foot. Since the count depends only on the face, the width of the block does not change it. Blocks that are 4, 6, 8, 10 and 12 inches wide all have the same 8 × 16 face.

Example: a 20 ft by 8 ft wall with a door

A wall is 20 ft long and 8 ft high and has one 3 ft by 7 ft door. It is built with 8 inch standard blocks, you add 5% waste and you fill the cells every 32 inches with grout. To get these same figures in the calculator, type the wall and the door, then enter 5 in the waste box and choose “Cells filled every 32 in”.

Step	Operation	Result
Wall area	20 × 8	160 sq ft
Door	3 × 7	21 sq ft
Net wall area	160 − 21	139 sq ft
Blocks before waste	139 × 1.125	156.375
With 5% waste	156.375 × 1.05 = 164.19, rounded up	165 blocks
Mortar, masonry cement	156.375 ÷ 30 = 5.21, rounded up	6 bags of 70 lb
Mortar sand	156.375 ÷ 240 = 0.65, rounded up	1 ton
Grout	139 ÷ 100 × 9.1	12.65 ft³ = 0.47 yd³
Preblended grout, 80 lb bags	12.65 ÷ 0.66 = 19.17, rounded up	20 bags

Without the door and without waste, the same wall needs 160 × 1.125 = 180 blocks. The door alone takes about 24 blocks off the count.

How many concrete blocks do I need per wall size?

This table uses standard 8 × 16 inch blocks, walls without openings and a 5% waste column for comparison. Subtract your doors and windows first if you have them.

Wall	Area	Blocks, no waste	Blocks, 5% waste
10 ft × 4 ft	40 sq ft	45	48
20 ft × 8 ft	160 sq ft	180	189
30 ft × 8 ft	240 sq ft	270	284
40 ft × 8 ft	320 sq ft	360	378
50 ft × 8 ft	400 sq ft	450	473
100 sq ft of wall	100 sq ft	113	119

The last row matches the CMHA Tech Note: 119 units per 100 square feet, including about 5% for waste and breakage.

Concrete block sizes: nominal and actual

A block called 8 × 8 × 16 is 8 inches wide, 8 inches high and 16 inches long, always in that order. Those are the nominal dimensions, which add the mortar joint to the real size. According to Angelus Block, a block manufacturer, the specified dimensions of standard precision units are as follows (checked October 2026). The difference between 8 inches and 7.625 inches is 3/8 inch, which is the joint. Actual units can vary by 1/8 inch either way.

Nominal size	Specified width	Specified height	Specified length
4 × 8 × 16	3-5/8 in	7-5/8 in	15-5/8 in
6 × 8 × 16	5-5/8 in	7-5/8 in	15-5/8 in
8 × 8 × 16	7-5/8 in	7-5/8 in	15-5/8 in
10 × 8 × 16	9-5/8 in	7-5/8 in	15-5/8 in
12 × 8 × 16	11-5/8 in	7-5/8 in	15-5/8 in

Because the nominal 8 × 16 face already includes the joint, the count of 1.125 blocks per square foot is the right figure for a finished wall. Other nominal faces work the same way. The calculator also has 4 × 16 and 8 × 8 faces, and an “Other size” option where you type the nominal height and length in inches.

How much waste to add

The CMHA Tech Note builds about 5% for waste and breakage into its 119 units per 100 square feet. It also warns that non-modular layouts and walls with many returns and corners can need considerably more. This calculator starts at 0% so you see the exact count first. Type your own percentage in the waste box.

Mortar and grout

The calculator gives mortar and grout for the standard 8 × 16 face, using the rules of thumb in the same CMHA Tech Note. They are guides, and the Tech Note itself says mortar and grout are among the most commonly misestimated materials.

Masonry cement mortar. With face shell bedding, one 70 lb bag lays about 30 hollow blocks, and one ton of masonry sand goes with every 8 bags, which is about 240 blocks per ton of sand. The Tech Note advises adding half a ton to the sand if you use more than 3 tons. For smaller amounts, round up. The calculator applies both rules, and it counts the mortar on the blocks you lay, before any waste you typed.
Grout. Its Table 3 gives the volume to fill the vertical cells of a wall, in cubic feet per 100 square feet, for walls 6 to 14 inches wide with the filled cells spaced from 8 to 120 inches apart. It assumes two-core hollow units and includes 3% waste. The Tech Note advises against grouting conventional 4 inch units, so the calculator gives no grout figure for them.
Preblended grout. The Tech Note says an 80 lb bag yields about 0.66 cubic feet.

What this calculator does not count

The Tech Note says that corner, pilaster and bond beam units are calculated separately and subtracted from the total of standard units. This calculator counts the wall as if it were all standard units, so adjust the count if your plans call for special pieces. It does not size reinforcing steel or lintels, and it does not show prices, because they change by region and supplier. Take the block count to your supplier and ask for a quote.

Frequently asked questions

How do I calculate how many concrete blocks I need?

Multiply the length of the wall by its height to get the area in square feet, subtract the area of doors and windows, then multiply by the number of blocks per square foot. A standard 8 × 16 inch block face covers 128 square inches, so there are 144 ÷ 128 = 1.125 blocks per square foot. A 20 ft by 8 ft wall with a 3 ft by 7 ft door has (160 − 21) × 1.125 = 156.375 blocks, which rounds up to 157.

How many concrete blocks do I need for 1000 square feet?

For 1,000 square feet of net wall with standard 8 × 16 inch blocks you need 1,000 × 1.125 = 1,125 blocks before waste. With 5% waste that is 1,181.25, which rounds up to 1,182 blocks. The CMHA Tech Note lists 119 units per 100 square feet including about 5% waste, which gives about 1,190.

How many concrete blocks are in a square foot, and does the width change it?

A standard block has 1.125 blocks per square foot of wall, or 112.5 per 100 square feet. The width does not change the count. Blocks that are 4, 6, 8, 10 and 12 inches wide all share the same 8 × 16 inch nominal face, so a wall of 4 inch blocks and a wall of 12 inch blocks of the same area need the same number of units.

How much sand and cement do I need for 100 blocks?

Using the CMHA rule of thumb for masonry cement mortar with face shell bedding, one 70 lb bag lays about 30 hollow blocks and one ton of sand goes with every 8 bags, about 240 blocks. For 100 blocks that is 100 ÷ 30 = 3.33, so 4 bags, and 100 ÷ 240 = 0.42 tons of sand. The Tech Note says to add half a ton if you use more than 3 tons of sand, and for smaller amounts to round up, so 100 blocks call for 1 ton. The Tech Note warns that mortar is one of the most commonly misestimated materials, so treat this as an estimate.

What is the actual size of an 8x8x16 concrete block?

The nominal size is 8 × 8 × 16 inches, width by height by length. The specified size of a standard precision unit is 7-5/8 × 7-5/8 × 15-5/8 inches (7.625 × 7.625 × 15.625). Nominal size is the specified size plus the mortar joint, and 8 − 7.625 is 3/8 inch. Actual units can vary by 1/8 inch either way.

How much waste should I add to a concrete block order?

The CMHA Tech Note includes an allowance of about 5% for waste and breakage in its figure of 119 units per 100 square feet. Layouts that are not modular and walls with many corners and returns can need more. Type the percentage in the waste box and the calculator adds it for you. If you leave it empty, it counts 0%.

How much grout do I need to fill the block cells?

It depends on the wall width and on how often the cells are filled. For an 8 inch wall with cells grouted every 32 inches, the CMHA Tech Note gives 9.1 cubic feet per 100 square feet of wall, with 3% waste included. For 139 square feet of net wall that is 12.65 cubic feet, or 0.47 cubic yards. Pick the spacing in the calculator to get the figure for your wall.