Concrete Block Calculator
Estimate blocks needed, mortar bags, and total cost for any CMU (concrete masonry unit) wall project. Standard 8x8x16 blocks cover 0.889 sq ft each.
Reviewed & updated for 2026 by Rakesh Choudhary, PhDMethodology & sources · How we calculate
Concrete block sizes reference
| Block size | Face area | Blocks/100 sq ft | Typical use |
|---|---|---|---|
| 8 × 8 × 16 (most common) | 0.889 sq ft | 113 blocks | Foundation, garage, retaining walls |
| 4 × 8 × 16 | 0.889 sq ft | 113 blocks | Partition walls, half-thickness |
| 6 × 8 × 16 | 0.889 sq ft | 113 blocks | Medium walls |
| 12 × 8 × 16 | 0.889 sq ft | 113 blocks | Large foundation walls |
| Half-high (8 × 4 × 16) | 0.444 sq ft | 225 blocks | Adjusting course height |
Block counts for common projects
Most homeowners are not building skyscrapers. The five projects below are typical residential jobs; use them as a sanity check after running the calculator. Garage and shelter rows count all four walls and deduct nothing for openings.
| Project | Typical dimensions | Blocks (8x8x16) | Mortar bags |
|---|---|---|---|
| Garden retaining wall | 20 ft × 3 ft | 75 blocks | 3 bags |
| Detached garage walls | 24 × 24 × 8 ft | 951 blocks | 36 bags |
| Basement foundation (single side) | 40 ft × 8 ft | 396 blocks | 15 bags |
| Privacy wall around yard | 100 ft × 6 ft | 743 blocks | 28 bags |
| Storm shelter walls | 8 × 8 × 8 ft | 317 blocks | 12 bags |
All counts are worked by the calculator above with a 10% waste allowance and the rule of thumb that one 80-pound bag of pre-mixed mortar lays roughly 27 standard CMUs. Both are conventions, not standards; walls with many corners, openings or curves need a larger allowance.
Mortar, rebar, and grout: what you actually need
A finished CMU wall is not just blocks. For any wall taller than three or four courses, you will also need mortar to glue them together, rebar for vertical reinforcement, and (often) grout to fill the cores around the rebar. Forgetting any one of these turns a weekend job into a return trip to the supplier.
- Mortar uses Type N for above-grade walls (chimneys, garden walls) and Type S for below-grade or load-bearing walls (foundations, retaining walls). As a rule of thumb, one 80-lb bag of pre-mix lays 25 to 30 standard blocks; the bag's own coverage figure governs.
- Rebar vertical bars go in selected cores. Bar size and spacing come from your local code or an engineer's design, and are closer together for taller and retaining walls.
- Grout fills the rebar-containing cores. How much depends on the block's core size and how many cores are filled; block suppliers publish grout volumes for their units.
- Bond beam blocks (U-shaped CMU) run horizontally at intervals set by the design, filled with horizontal rebar and grout. They tie the wall together and resist cracking.
- Sand and cement if you mix mortar yourself instead of buying pre-mix: Portland cement, lime and mason sand in the proportions specified for the mortar type you need. Pre-mix removes the risk of ratio mistakes.
For scale: a 30 ft × 8 ft wall is 270 blocks, 297 with a 10% allowance, and about 11 bags of mortar at the rule-of-thumb rate. Rebar and grout come on top, in quantities set by the wall's design.
CMU vs poured concrete: which is right for your project
CMU and poured concrete walls behave differently and suit different projects. The decision usually comes down to project size, DIY skill level, and how the wall will be loaded.
| Factor | Concrete block (CMU) | Poured concrete |
|---|---|---|
| DIY friendly | Yes, with practice | No, needs forms and pumping |
| Speed for small projects | Faster (no forms needed) | Slower setup, faster pour |
| Speed for large projects | Slower (every block by hand) | Faster (single pour) |
| Waterproofing | Mortar joints can leak | More watertight |
| Insulation | Hollow cores can be filled or foam-injected | Solid; needs exterior insulation |
CMU suits garden walls, garages, storm shelters, low retaining walls and projects you want to do yourself. Poured concrete suits full house foundations, tall walls, basement walls in high water-table areas, and any project where speed of pour matters more than DIY accessibility. Either way, retaining and load-bearing walls may need an engineer's design under your local code.
FAQs
How do I calculate concrete blocks needed?
Divide the wall's area by the face area of one block. A standard 8x8x16 unit has a nominal face of 8 by 16 inches, or 0.889 sq ft, so a wall 30 feet long and 8 feet high (240 sq ft) needs 270 blocks before any allowance for breakage and cuts.
How many concrete blocks per square foot?
1.125 for any unit with an 8 by 16 inch face, whatever its thickness, because 144 square inches divided by 128 is 1.125. The 4, 6, 8 and 12 inch thick units all share that face. So 100 sq ft of wall takes 113 blocks, or 125 with a 10% allowance the way this calculator rounds.
How much mortar do I need for concrete blocks?
A common trade rule of thumb is one 80 lb bag of premix mortar for every 25 to 30 blocks, and this calculator uses 27. On that range a 200-block wall takes 7 to 8 bags. Beginners tend to use more, and the coverage printed on your mortar bag is the figure to trust.
What's the cost of concrete blocks?
Block and mortar prices vary by region, supplier and quantity, so this page does not quote them. Enter your supplier's price per block and per bag of mortar and the calculator multiplies them by the counts. Labour, footings, rebar and grout are extra and are not included.
What sizes of concrete blocks are available?
Sizes are quoted nominally, thickness by height by length, and the actual unit is made smaller by the width of the mortar joint, commonly 3/8 inch. Common sizes are 8x8x16 for most load-bearing walls, 4x8x16 and 6x8x16 for thinner walls, 12x8x16 for heavier foundations, and 8x4x16 half-high units for adjusting course height.
How tall can a concrete block wall be?
There is no single answer. Safe height depends on reinforcement, grout, soil, wind and whether the wall holds back earth, and your local building code sets the limits. Retaining walls and load-bearing walls are where an engineer's design is most often required, so check with your building department before you build either.