How to use the concrete block calculator
- Enter the wall length and height in feet.
- Keep a small allowance for broken and cut blocks; 5% is a common starting point.
The calculator assumes standard blocks with a nominal face of 8 × 16 in. The actual block is 7⅝ × 15⅝ in; the extra ⅜ in is the mortar joint. Block width (6, 8, 10 or 12 in) does not change the count, only the wall thickness. Confirm the block size with your yard before ordering. Decoratoradvice uses standard 8 × 16 in faces, and other sizes change the count.
Concrete block formula
Blocks per course = wall length (in) ÷ 16, rounded up
Courses = wall height (in) ÷ 8, rounded up
Blocks = blocks per course × courses × (1 + extra)
Per square foot of wall face, that is 144 ÷ (8 × 16) = 1.125 blocks.
Worked example
A garden wall 20 ft long and 6 ft high:
240 in ÷ 16 = 15 blocks per course
72 in ÷ 8 = 9 courses
15 × 9 = 135 blocks
With 5% extra: 141.75, so 142 blocks
Mortar for block walls
QUIKRETE states that one 80 lb bag of its Mason Mix lays up to 13 standard 8 × 8 × 16 in blocks. For 142 blocks that is at least 11 bags; the mortar calculator adds a mixing allowance and works it out for any count.
Planning around openings
For a door or window, work out the blocks the opening would have held (its area in square feet × 1.125) and subtract them. Keep the 5% allowance, because openings create more cut blocks.
Tips
- Plan wall lengths in multiples of 8 in where you can, so courses end on a half or whole block and you cut fewer.
- Block walls sit on a concrete footing. Work out the footing volume with the concrete calculator.
- Reinforced or grouted walls need bars and grout specified by your plans; the rebar calculator counts bars for slabs and footings.
- For a low garden wall without mortar, look at segmental blocks and the retaining wall calculator.
Common mistakes
- Using the actual 15⅝ in length. Use the nominal 16 in, which includes the joint.
- Rounding courses down. A 6 ft 4 in wall needs 10 courses, not 9.
- Forgetting corner and end blocks if your design uses special units; count them separately.
Frequently asked questions
How many concrete blocks are in a square foot?
1.125 standard 8 × 16 in blocks, or 112.5 per 100 sq ft.
How many blocks do I need for a 10 × 8 ft wall?
120 ÷ 16 = 7.5, rounded to 8 per course, and 96 ÷ 8 = 12 courses, so 96 blocks before any extra.
Are cinder blocks and concrete blocks the same?
People use both names for the same hollow masonry units. Check the product description for size and strength.
How much mortar does one block need?
Based on QUIKRETE's figure of up to 13 blocks per 80 lb bag of Mason Mix, at least one bag per 13 blocks, before waste.
Do I need a footing for a block wall?
Load-bearing and taller walls need a concrete footing sized to the wall and soil. Small garden walls may still need one; check local requirements.
Can I mix block and brick?
Yes, a block backing with a brick face is common. Use the brick calculator for the brick layer.
Key takeaways
- Blocks per course = length ÷ 16 in; courses = height ÷ 8 in; round both up.
- A standard block covers 0.889 sq ft, so a wall takes 1.125 blocks per sq ft.
- Add about 5% for breakage and cuts.
- One 80 lb bag of QUIKRETE Mason Mix lays up to 13 blocks.
Sources
- QUIKRETE Mason Mix (No. 1136) data sheet: blocks and bricks laid per bag.