Free Concrete block calculator
Estimate everything a concrete-block wall needs in one place — enter the wall's length and height, subtract any door or window openings, and get the count of standard 8×8×16 CMU blocks at about 1.125 per square foot, the bags of mortar to lay them, and the core-fill grout for a reinforced wall, all with a waste margin built in and updated live, as you type.
On this page16 sections
| Item | Quantity | Est. cost |
|---|---|---|
| Blocks (with waste) | 173 | $346 |
| Mortar (80 lb bags) | 14 bags | $126 |
Prices are regional estimates: blocks ≈ $2.00 each, mortar ≈ $9.00/bag, core-fill grout ≈ $13.00/bag. Coverage: ≈1.125 blocks/ft², ~13 blocks per mortar bag, ~3 blocks per grout bag.
Block, mortar and grout coverage vary by product and joint. How accurate is this?
Results are estimates. Consult a professional.
How the concrete block calculator works
Concrete block is estimated by wall area, not volume. Each block covers a fixed slice of the wall face, so the concrete block calculator finds the area of your wall — length times height — subtracts any door and window openings, then divides by the face area one block covers. A standard 8×8×16 CMU, sitting on its 3/8-inch mortar joint, covers about 0.888 square feet of wall, which works out to roughly 1.125 blocks for every square foot. From the block count it then estimates the mortar bags and, optionally, the grout to fill the cores.
What the result means
The block count is the headline — it is what you order from the masonry yard. The mortar figure tells you how many bags of mason mix to buy to lay them, and the optional fill figure tells you how much grout to pour into the cores if the wall is reinforced or load-bearing. The calculator adds a waste margin on top of the block count, because cuts at corners and openings, plus the occasional cracked block, mean a few never make it into the wall.
What goes into your block estimate
A block wall estimate is built from three pieces: the blocks themselves, the mortar that bonds them, and — for reinforced walls — the grout that fills the cores. Get the area right and the rest follows.
Block count — the base number
Net wall area divided by the face area of one block. For the standard 8×8×16 unit that is area ÷ 0.888, or area × 1.125. This is the geometry, and it is the part the calculator nails exactly. Subtracting openings matters: a single 3 ft × 7 ft doorway removes 21 square feet of wall, or about 24 blocks.
Mortar — how many bags to lay them
Mortar is the bedding between courses and the head joints between blocks. An 80 lb bag of pre-mixed Type S mason mix lays about 13 standard blocks at a 3/8-inch joint, so you divide the block count by 13. Estimating guides quote roughly three bags of straight masonry cement (mixed with sand on site) per 100 blocks; the pre-mixed figure is higher because the bag already includes the sand.
Core fill — grout for reinforced walls
Hollow blocks have two open cells. On a load-bearing or reinforced wall those cells get filled with grout, often around steel rebar. Each 8×8×16 block holds about 0.318 cubic feet of void, and an 80 lb bag of coarse core-fill grout (about 0.65 cubic feet yield) fills roughly three blocks. The calculator only counts fill on the un-wasted block count, since you grout the cells you lay, not the ones lost to breakage.
A worked example using the block calculator
Maria is building a 20 ft long, 8 ft high garage wall from standard 8×8×16 blocks, with one 3 ft × 7 ft service door. The wall is reinforced, so she wants the block count, the mortar bags, and the grout to fill the cores — with a 10% waste margin on the blocks.
Step 1 — Find the net wall area
Gross area is 20 × 8 = 160 ft². The door removes 3 × 7 = 21 ft², leaving 139 ft² of wall to fill.
Step 2 — Convert area to blocks
139 ÷ 0.888 = 156.5, rounded up to 157 blocks. With a 10% waste margin, 156.5 × 1.10 = 172.2 → 173 blocks to order.
Step 3 — Work out the mortar
At about 13 blocks per 80 lb bag, 173 ÷ 13 = 13.3 → 14 bags of Type S mason mix.
Step 4 — Estimate the core fill
Grouting all 157 blocks at 0.318 ft³ each is 49.9 ft³ of grout. Divided by the 0.65 ft³ yield of an 80 lb core-fill bag, that is 77 bags — or about 1.85 cubic yards if you order ready-mix grout instead.
How many blocks do I need?
If you want a ballpark before you measure openings, this table gives the block count for common wall sizes using standard 8×8×16 units at 1.125 blocks per square foot. These are gross figures — subtract your openings and add 5–10% for waste before you order.
| Wall size | Wall area (ft²) | Blocks (8×8×16) | Mortar bags (80 lb) |
|---|---|---|---|
| 10 ft × 8 ft | 80 | 91 | 7 |
| 20 ft × 8 ft | 160 | 181 | 14 |
| 30 ft × 8 ft | 240 | 271 | 21 |
| 40 ft × 8 ft | 320 | 361 | 28 |
| 50 ft × 10 ft | 500 | 564 | 44 |
| 100 ft × 10 ft | 1,000 | 1,127 | 87 |
Blocks at 1.125 per ft² (area ÷ 0.888, rounded up); mortar at ~13 blocks per 80 lb bag. Figures are gross (no openings subtracted) and exclude the waste margin.
When to use this block calculator
Reach for it whenever a project is built up in courses of concrete masonry units, and you need to order the right quantity of block, mortar and grout in one trip.
- Foundation and stem walls — below-grade walls that carry the house, usually reinforced and grouted.
- Garage, shed and outbuilding walls — above-grade structural walls in standard 8-inch block.
- Retaining and boundary walls — where openings are rare but length runs long.
- Fence and privacy walls — freestanding masonry, often capped, where you size by length and height.
Standard CMU sizes and coverage
Blocks are named by their nominal size, which includes the 3/8-inch mortar joint; the actual unit is 3/8 inch smaller in each direction. The 8-inch-high by 16-inch-long face is shared across most blocks, so the wall coverage is the same — what changes is the wall thickness and the void you grout.
| Nominal size (W×H×L) | Actual size | Face coverage | Blocks per ft² |
|---|---|---|---|
| 8 × 8 × 16 in | 7⅝ × 7⅝ × 15⅝ in | 0.888 ft² | 1.125 |
| 6 × 8 × 16 in | 5⅝ × 7⅝ × 15⅝ in | 0.888 ft² | 1.125 |
| 12 × 8 × 16 in | 11⅝ × 7⅝ × 15⅝ in | 0.888 ft² | 1.125 |
| 4 × 8 × 16 in | 3⅝ × 7⅝ × 15⅝ in | 0.888 ft² | 1.125 |
All standard blocks share the 8×16-inch face, so wall coverage is 1.125 blocks/ft² regardless of width — only the wall thickness and the void you grout change. Half-height and other non-standard units change the face math.
How much mortar for a block wall
Mortar is sold two ways: as straight masonry cement you mix with sand, or as a pre-mixed mason mix that already has the sand in the bag. The pre-mixed bags are simpler for small jobs; bulk cement-and-sand is cheaper at scale.
| Product | Coverage | Note |
|---|---|---|
| 80 lb pre-mixed Type S mason mix | ~13 blocks per bag | Sand included; the figure this calculator uses |
| Masonry cement + sand | ~3 cement bags per 100 blocks | Mixed on site; one cubic yard of sand per ~7 bags of cement |
| Type N mortar | Above-grade, non-load-bearing | Lower strength; common for veneer and partition walls |
| Type S mortar | Foundation and load-bearing | Higher bond and compressive strength; the usual structural choice |
Pre-mixed coverage from QUIKRETE Type S Mason Mix data; cement-per-100-blocks from standard masonry estimating guides. Joint thickness changes usage — thicker joints use more mortar.
The ~13 blocks per 80 lb bag figure is the published coverage for QUIKRETE Type S Mason Mix; the ~3 cement bags per 100 blocks rule is from standard masonry estimating guides such as the Georgia Masonry Supply estimating guide.Grout to fill block cores
Reinforced walls have grout poured into the cells, usually around vertical rebar. A standard 8×8×16 block holds about 0.318 cubic feet of void in its two cells. That sets the grout volume — but only the cells you fill count toward it.
| Blocks filled | Grout volume | 80 lb core-fill bags |
|---|---|---|
| 100 blocks | ~32 ft³ | ~49 |
| 157 blocks (worked example) | ~50 ft³ | 77 |
| 250 blocks | ~80 ft³ | ~123 |
| Per cubic yard of grout | 27 ft³ | ~42 |
Void at 0.318 ft³ per block; bags at the 0.65 ft³ yield of QUIKRETE Core-Fill Grout Coarse (~3 blocks per 80 lb bag). Many walls grout only the rebar cells, not every cell — check your drawings.
The 0.65 ft³ yield and ~3 blocks per 80 lb bag come from the QUIKRETE Core-Fill Grout Coarse product data.Past about a cubic yard of grout, ready-mix or pump grout is far easier than bags. Fully grouted walls also weigh much more — confirm the footing and the wall design carry the load before you fill every cell.
Waste factor: how many extra blocks to order
Order the exact calculated count and you will run short. Blocks crack in handling, corners and openings need cut units, and a few are simply unusable out of the cube. A modest overage covers it and saves a second trip to the yard mid-course.
The 5–10% rule
For a plain rectangular wall, 5% is usually enough. For walls with several openings, corners, or curves — where you cut more blocks — 10% is safer, and tight, complex layouts can justify up to 15%. The calculator's waste field defaults to 10% and adjusts up to 20%.
Why running short stalls the job
Masonry goes up course by course, and a wall half-laid can sit waiting for blocks while mixed mortar goes off in the tub. The overage keeps the crew moving. Leftover whole blocks are easy to return or reuse; a stalled wall is not.
Concrete block definitions
How accurate is this concrete block calculator?
The block count is exact for the geometry you enter. Net wall area divided by 0.888 square feet per block is the precise number of standard units the wall takes, and subtracting openings is simple arithmetic. If your length, height and openings are right, the block count is right.
The mortar and grout figures are estimates, on purpose. Bag coverage — about 13 blocks per mortar bag, about 3 blocks per core-fill bag — shifts with joint thickness, how full you tool the joints, mixing water, and the exact product. Counts can drift a bag or two either way. Treat the block count as firm and the mortar and fill as planning figures. Confirm the coverage printed on your specific bags, and when in doubt order to the high side — a stalled course costs far more than a returned pallet.
Frequently asked questions about the free Concrete block calculator
About this concrete block calculator
This concrete block calculator runs entirely in your browser — nothing you type is sent anywhere or stored. It estimates the blocks, mortar and optional core-fill grout for a CMU wall using standard masonry coverage rates (about 1.125 standard blocks per square foot, ~13 blocks per mortar bag, ~3 blocks per core-fill bag), and recomputes the moment you change a dimension, an opening or the waste factor.
It is one of the material estimators on our construction calculators shelf, alongside the concrete and brick calculators. Browse the full set on the calculators directory to plan the rest of your build.