What this calculator does
Find out how many concrete blocks and bags of mortar to buy for a wall. Concrete blocks are also called concrete masonry units, or CMU, and are often loosely called cinder blocks. You count them from the face area of the wall, the side you see. Each block covers a fixed patch of wall. That patch is its nominal size: the block itself plus one mortar joint, in length and height. So the number you need is the wall area divided by that patch. The calculator takes the wall's length and height, subtracts doors and windows, and divides by the block face.
Then it adds extra for broken and cut blocks. It also estimates the bags of premixed mortar at a typical rate, which you can change. These numbers are a material estimate for budgeting and ordering. They aren't a structural design. Your local building code sets reinforcement, grout, footings and wall height limits. For retaining or load-bearing walls, an engineer sets them too.
Where it's used
Why this math matters outside the classroom:
- Retaining walls. A 30 ft long, 4 ft high block wall has 120 ft² of face, and at 1.125 standard blocks per square foot that is 135 blocks.
- Garage and shed walls. Builders subtract door and window openings from the gross wall area before dividing by the face of one block.
- Nominal sizes. A standard 8 × 8 × 16 in block is really 7⅝ × 7⅝ × 15⅝ in, and the 3/8 in mortar joint makes up the difference.
- Breakage allowance. Masons add a few percent to the count to cover blocks that crack in delivery or are cut at corners.
- Foundation walls. Students find the face area of each wall of a rectangular foundation and add them to estimate the total block count.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 7.G.B.6. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose US units (feet and inches) or metric units (meters and millimeters).
- Enter the wall length and height, and the total area of any doors and windows.
- Check the nominal block size. The default 16 × 8 in (400 × 200 mm) suits standard blocks of every width.
- Set the extra for breakage and the mortar rate. Then read the blocks to buy and the mortar bags.
What the inputs mean
- Area of doors and windows
- Add up the areas of all doors, windows and vents. You don't need blocks where they go, although you do need lintels (the beams over openings) and cut blocks around them.
- Block length (nominal)
- Use the nominal size, which is the real block plus one mortar joint. A standard block measures 15⅝ × 7⅝ in and is laid with ⅜ in joints. That makes it 16 × 8 in nominal. The block's width (6, 8, 10 or 12 in) doesn't change the count.
- Extra for breakage and cuts
- Blocks chip and crack when they're moved, and corners and openings need cut pieces. 5% suits a simple wall. Use more if there are many openings or corners.
- Mortar bags per 100 blocks
- A common rule of thumb is about 3 bags of 80 lb premixed mortar per 100 standard blocks. Mortaring the whole block face, thicker joints or mortar mixed on site all change that. Adjust it to how you'll build.
Formula and method
net area = wall length × wall height − openings block face = nominal length × nominal height blocks in the wall = net area ÷ block face blocks to buy = blocks in the wall × (1 + waste % ÷ 100), rounded up mortar bags = blocks in the wall × bags per 100 ÷ 100, rounded up
Net area is the wall area left after taking out openings. Block face is the patch of wall one block covers. Waste % is the extra you add for breakage and cuts, and bags per 100 is your mortar rate.
The nominal size already includes the mortar joint. So dividing the wall area by the block face counts the blocks directly, with no separate step for joints. In US units, the face in square inches is divided by 144 to give square feet. In metric, the face in square millimeters is divided by 1,000,000 to give square meters.
Mortar is figured from the blocks actually laid, before the extra for breakage, because spare blocks don't use mortar. Both counts are rounded up to whole blocks and whole bags.
Worked example
A garden wall is 30 ft long and 8 ft high, with one 3 ft × 5 ft opening (15 ft²). It uses standard 16 × 8 in blocks, 5% extra and 3 mortar bags per 100 blocks.
- Whole wall area: 30 × 8 = 240 ft².
- Take out the opening: 240 − 15 = 225 ft².
- Block face: 16 × 8 ÷ 144 = 0.8889 ft², so 1.125 blocks per ft².
- Blocks in the wall: 225 ÷ 0.8889 = 253.1.
- With 5% extra: 253.1 × 1.05 = 265.8, rounded up to 266 blocks.
- Mortar: 253.1 × 3 ÷ 100 = 7.6, rounded up to 8 bags.
Result: Buy 266 blocks and about 8 bags of 80 lb premixed mortar.
Understanding the result
Blocks are often sold by the pallet. Compare your count with your supplier's pallet size. Then decide whether to order a full extra pallet or a few loose blocks.
The courses figure shows how many rows of block the wall height makes. If it isn't a whole number, the top row will need cut blocks. Or you could adjust the wall height to fit whole blocks.
Assumptions and limitations
- The wall is a flat rectangle, one block thick (masons call this one wythe).
- Every block is the same nominal size, and openings are subtracted at their full area.
- The mortar rate is a typical figure for premixed mortar with standard joints.
- This is a material estimate, not a structural design. It doesn't size reinforcement, grout, bond beams, lintels or footings. It doesn't check wall height or loads either. Follow your local codes and the mortar maker's instructions.
- Corners, pilasters (built-in columns), half blocks and special shapes such as bond-beam or lintel blocks aren't counted separately.
- Grout or concrete fill for reinforced block holes (cores) isn't included.
Frequently asked questions
How many concrete blocks are in a square foot?
A standard 16 × 8 in nominal block covers 128 in², and a square foot is 144 in². So there are 144 ÷ 128 = 1.125 blocks per square foot, or 112.5 per 100 ft², before waste.
Does the block width change the count?
No. A 6, 8, 10 or 12 inch wide block still shows the same 16 × 8 in face. So the count for a given wall area stays the same. Width changes the weight, the mortar and the fill, not the number of blocks.
How much mortar does a block wall need?
A typical rule of thumb is about 3 bags of 80 lb premixed mortar per 100 standard blocks. The real amount depends on how much of each block gets mortar, the joint thickness and the product. Check the yield printed on the bag.
What is the difference between nominal and actual block size?
The actual block is ⅜ in smaller each way (for example, 15⅝ × 7⅝ in). Adding one ⅜ in mortar joint brings it to the nominal 16 × 8 in. That's why nominal sizes make the math come out exact.
How this calculator is checked
Method: Unit count = net wall area ÷ nominal unit face area (nominal sizes include one mortar joint); mortar from a user-editable per-100-block rate. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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