The formula: net area times blocks per square foot
Block sizes are given as nominal dimensions, which include one mortar joint. That is why the count uses the nominal face and adds nothing for joints: each block plus its share of mortar covers exactly its nominal face.
- Net area = wall length × wall height − openings − other deductions.
- Blocks per square foot = 144 ÷ (face height × face length), in inches. An 8 x 16 in. face is 128 square inches, so 144 ÷ 128 = 1.125 blocks per square foot.
- Blocks = net area × blocks per square foot, then × (1 + waste percent ÷ 100), rounded up to whole blocks.
The calculator also counts the gross wall a second way, as courses up the height times blocks along the length. That number should equal the gross area times blocks per square foot, which is a quick check that the face size went in right.
Worked example 1: a wall with three windows and a door
A wall is 64 ft long and 12 ft high. It has three 4 ft by 4 ft windows and one 3 ft by 7 ft door, uses an 8 x 16 in. nominal face and takes 5% waste. These are invented study numbers.
- Gross area = 64 × 12 = 768 sq ft.
- Openings = 3 × 4 × 4 = 48 sq ft for the windows, plus 3 × 7 = 21 sq ft for the door (entered under other deductions) = 69 sq ft.
- Net area = 768 − 69 = 699 sq ft.
- Blocks = 699 × 1.125 = 786.375. With 5% waste: 786.375 × 1.05 = 825.69, so 826 blocks.
Check: one block covers 128 ÷ 144 = 0.8889 sq ft, and 699 ÷ 0.8889 = 786.4. The gross wall is 144 ÷ 8 = 18 courses of 768 ÷ 16 = 48 blocks, or 864, and 768 × 1.125 = 864 too.
Worked example 2: the same wall with a 4 x 16 in. face
Change the face height to 4 in. and keep everything else. The face is 4 × 16 = 64 square inches, so 144 ÷ 64 = 2.25 blocks per square foot, twice as many. Blocks = 699 × 2.25 = 1,572.75, and with 5% waste 1,651.39, so 1,652 blocks. Check: 2 × 786.375 = 1,572.75. These are original arithmetic examples, not exam questions.
CMU block questions
Why does an 8 x 16 block not use 7 5/8 x 15 5/8?
The actual block is smaller than nominal by one joint, and the joint fills the gap. Counting on the actual size would leave the joints out and overcount the blocks. Use the nominal face for the count.
Does this include mortar?
No. Mortar and grout are bought by a rate per number of units or per area, which a question gives or your supplier and references provide. This tool counts blocks only.
Where is this on the NASCLA exam?
Masonry is one of the twelve subject areas on the NASCLA outline, with 4 of the 115 scored questions. The Bid Bond course works this count in its Masonry lesson.
See the NASCLA exam outline for all twelve subject areas and their question counts.