Concrete Block Calculator
Count the concrete blocks (CMUs) for a wall. Enter wall length and height in feet, pick a block size (standard 8x8x16, half, or half-high), and get block count with waste, courses, and 80 lb mortar mix bags.
Example: with Wall length (ft) 30 · Wall height (ft) 8 · Block size (nominal) 8 x 8 x 16 in (standard) · Waste factor (%) 5 → Blocks to buy: 284 blocks (waste included).
- Blocks (exact fit)270 blocks exact
- Course layout12 courses of 22.5 blocks
- Mortar mix24 x 80 lb bags of mortar mix
Computed by the calculator below using its default values. Change any input to see your own numbers.
A standard block's nominal 16 x 8 in face (mortar joint included) covers 0.89 sq ft, so figure 1.125 blocks per square foot of wall. Mortar: about 12 blocks per 80 lb bag of mortar mix.
Why block math uses nominal sizes
A "standard" concrete block actually measures 15-5/8 x 7-5/8 x 7-5/8 in, but with its 3/8 in mortar joint it lays up at exactly 16 x 8 in — the nominal size. That is what makes block counting clean: every standard block occupies 128 sq in (0.89 sq ft) of wall, so a square foot of wall needs 1.125 blocks. Courses land on 8 in increments, which is why block walls are designed in 8 in modules.
Mortar, waste, and what this leaves out
Five percent waste covers broken blocks and cuts on a straight wall; bump it to 10% for walls with lots of openings or corners. Preblended mortar mix lays about 12 standard blocks per 80 lb bag — masons mixing their own from masonry cement and sand get more wall per dollar, but the bag figure is the safe DIY estimate. This count does not include rebar, grout for filled cores, cap block, or ladder wire, and openings are not subtracted — leave them in as your extra margin, or subtract big openings manually.
How it’s calculated
Blocks (exact) = wall area in sq in ÷ nominal face area (16 x 8 = 128 sq in standard; 8 x 8 = 64; 16 x 4 = 64). Blocks to buy = exact × (1 + waste%/100), rounded up. Courses = wall height ÷ nominal course height; blocks per course = wall length ÷ nominal block length. Mortar = blocks ÷ 12 per 80 lb bag of preblended mortar mix, rounded up (QUIKRETE lists up to 13 blocks per bag).
Assumes running bond with 3/8 in joints and no openings subtracted; structural walls (footings, rebar, grouted cores) need an engineered design, not just a block count.
Common CMU sizes
| Nominal size | Actual size | Per 100 sq ft |
|---|---|---|
| 8 x 8 x 16 in (standard) | 7-5/8 x 7-5/8 x 15-5/8 in | 113 blocks |
| 8 x 8 x 8 in (half) | 7-5/8 x 7-5/8 x 7-5/8 in | 225 blocks |
| 4 x 8 x 16 in (half-high) | 3-5/8 x 7-5/8 x 15-5/8 in | 225 blocks |
ASTM C90 modular dimensions; nominal face includes the 3/8 in mortar joint. Counts rounded up.
Common mistakes
- Counting with actual block size (15-5/8 in) instead of nominal (16 in) — the mortar joint is part of the layout, and skipping it overbuys by about 5%.
- Forgetting half blocks at the ends of running-bond courses; buy a few half blocks or plan to cut.
- Estimating mortar by bags of concrete mix — mortar mix and concrete mix are different products; concrete mix will not bond block.
- Ignoring core fill: a reinforced wall needs grout in the rebar cells, which is a separate concrete calculation.
Frequently asked questions
How many concrete blocks per square foot of wall?
1.125 standard blocks per sq ft. The formula: wall area in square inches ÷ 128 sq in per nominal 16 x 8 in block face. A 100 sq ft wall needs 113 blocks before waste.
How much mortar do I need per block?
Plan on one 80 lb bag of preblended mortar mix per 12 standard blocks — QUIKRETE rates a bag at up to 13 blocks with 3/8 in joints. For 284 blocks, that is about 24 bags.
Why do my courses need to land on 8 inches?
Standard block plus joint stacks in 8 in increments, so an 8 ft wall is exactly 12 courses. If your height is not a multiple of 8 in, you will be cutting block or switching the top course to half-high units.
Do I subtract windows and doors?
For small openings, most estimators leave them in as extra cushion. Subtract manually if openings are a big share of the wall — say over 10% — but keep the 5% waste on the rest.
Does this calculator cover footings and rebar?
No — it counts wall units and lay-up mortar only. Footing concrete, vertical rebar, and grouted cores depend on your local code and soil; get an engineered spec for anything retaining soil or bearing loads.