Concrete Block Calculator

Home & Construction Concrete and Masonry

Blocks to order —

Net wall area (sq ft): —
Blocks per 100 sq ft: —
Courses (rows): —
Blocks per course: —
Extra blocks for waste: —

Divide the net wall area by the nominal face of one block. A standard 8×16 in block covers 0.889 sq ft, so you need 112.5 blocks per 100 sq ft. A 20 ft long, 8 ft high wall is 160 sq ft, which takes 180 blocks; add 5 to 10% for breakage and cuts.

About this tool

Masons, foundation contractors and homeowners planning a garden or garage wall use a block count to price a job and size a delivery. Enter the wall length and height in feet, pick the nominal block face, subtract door and window area, and set a waste allowance on the slider. The result is the number of blocks to order, with the net area, the number of courses, the blocks per course and the extra blocks added for waste. Nominal sizes already include the 3/8 in mortar joint, so no separate joint correction is needed. The count treats the wall as a flat panel: corners, pilasters, bond beams and lintel blocks are special units that you count separately from the drawings.

How to use it

Enter the wall size
Type the wall length and height in feet. For several walls, add their lengths if they share a height.
Pick the block face
Choose the nominal face size; 16 × 8 is the standard 8 in CMU.
Subtract openings
Enter the combined area of doors and windows in square feet.
Set waste
Drag the slider to the allowance you use, typically 5 to 10%.

Examples

20 ft by 8 ft wall, 8×16 blocks

Wall length: 20
Wall height: 8
Block face (nominal, in): 16 × 8 standard
Doors and windows area: 0
Waste allowance: 0

Result Blocks to order: 180
Net wall area (sq ft): 160
Blocks per 100 sq ft: 112.5
Courses (rows): 12
Blocks per course: 15
Extra blocks for waste: 0

Wall area = 20 × 8 = 160 sq ft
Net area = 160 − 0 openings = 160 sq ft
Block face = 16 × 8 in ÷ 144 = 0.8889 sq ft
Blocks = 160 ÷ 0.8889 = 180
With 0% waste: 180 × 1 = 180, rounded up to 180

Twelve 8 in courses of fifteen 16 in blocks give 180 blocks before any waste allowance.

24 ft by 10 ft wall with a 21 sq ft door

Wall length: 24
Wall height: 10
Block face (nominal, in): 8 × 8 half block
Doors and windows area: 21
Waste allowance: 5

Result Blocks to order: 518
Net wall area (sq ft): 219
Blocks per 100 sq ft: 225
Courses (rows): 15
Blocks per course: 36
Extra blocks for waste: 25

Wall area = 24 × 10 = 240 sq ft
Net area = 240 − 21 openings = 219 sq ft
Block face = 8 × 8 in ÷ 144 = 0.4444 sq ft
Blocks = 219 ÷ 0.4444 = 492.75
With 5% waste: 492.75 × 1.05 = 517.39, rounded up to 518

219 sq ft of net wall in 8×8 half blocks needs 492.75 blocks before waste; a 5% allowance lifts the order to 518.

How it is calculated

Blocks = ⌈ (L × H − A_open) ÷ (w × h ÷ 144) × (1 + waste ÷ 100) ⌉

L, H: Wall length and height in feet
A_open: Area of doors and windows in square feet
w, h: Nominal block face width and height in inches
waste: Allowance for breakage and cuts, in percent

Nominal block dimensions are the actual unit plus one 3/8 in mortar joint, so a 15 5/8 × 7 5/8 in block occupies 16 × 8 in of wall. Dividing by 144 turns square inches into square feet. The net area divided by that face gives the raw count, the waste factor scales it, and the total is rounded up to whole blocks.

When not to use it

Do not use it for brick veneer; standard bricks need about 6.75 units per sq ft.
It does not size reinforcement, grout fill or footings.
Curved or stepped walls need a course-by-course takeoff.

Common mistakes

Using the actual 15 5/8 in block length instead of the 16 in nominal size inflates the count.
Forgetting to subtract large openings such as garage doors.
Ordering no spare blocks, then waiting on a second delivery for a few broken units.

Frequently asked questions

How many 8×16 blocks are in 100 square feet?

112.5. Each standard block has a nominal face of 16 × 8 in, which is 128 sq in or 0.889 sq ft, and 100 divided by 0.889 is 112.5.

Does the count include mortar joints?

Yes. Nominal sizes are the real block plus a 3/8 in joint, so the face area already accounts for mortar. Using actual dimensions would overstate the number of blocks by about 5%.

How much waste should I allow for concrete blocks?

Most estimators add 5% for a straight wall with few cuts and up to 10% where there are many openings, corners or a sloped top. Blocks chip in transport, and cut pieces are often unusable elsewhere.

Why does the number of courses show a decimal?

The wall height is not an exact multiple of the block height. A 7 ft wall in 8 in courses is 10.5 courses, so the top row must be cut or the height adjusted to 6 ft 8 in or 7 ft 4 in.

Do 8 in and 12 in thick blocks need different counts?

No. Thickness does not change the face area, so a 12 in wide block with a 16 × 8 face counts the same as an 8 in one. Thickness matters for weight, grout and cost, not quantity.

How do I handle corners where two walls meet?

Measure each wall to its outside corner and total the lengths once; do not count the corner twice. Order the corner units separately, since they are a different shape.