A concrete block calculator returns the number of CMU (concrete masonry units) needed for a wall, plus the mortar volume to lay them and the grout volume to fill the cores if the wall is reinforced. The calculator above takes wall dimensions, block size, and fill mode, and returns block count, mortar quantity, and grout quantity — everything you need to place an accurate order with the supply yard.
The same tool serves several jobs depending on what you call the block. As a cinder block calculator (the older name still common in trade slang for hollow concrete blocks), the math is identical — modern "cinder blocks" are actually CMUs. As a cement block calculator, the same arithmetic applies. As a cmu block calculator (using the technical trade abbreviation), or a cement block estimator, or a concrete block estimator, the underlying math is wall area divided by block face area. As a concrete block estimator calculator returning the order quantity for a supplier, the calculator above adds waste factor on top of the base count. As a block calculator or block estimator more generally, this tool covers all standard concrete masonry products.
For wall-specific framing, the same calculator works as a block wall calculator, concrete wall calculator, concrete block wall calculator, cinder block wall calculator, and cmu block wall calculator. The terms are used interchangeably; the math is the same. A typical residential block wall (retaining wall, foundation, or garden wall) uses 8x8x16 CMU at $1.50-3.00 per block in 2026 — the calculator above gives you the count to multiply by your local price.
Concrete Block Calculator
Block count, mortar, and grout for any CMU wall — also works as a cinder block, cement block, or block wall calculator.
Inputs
Sum of door, window, and other openings to subtract
Results
How to use this concrete block calculator
The calculator above takes wall length, wall height, total opening area, block size, and fill mode and returns block count, mortar volume, and grout volume. Most users get a usable result in under a minute.
- Enter wall length in feet. For a single straight wall, this is the total length. For a multi-side wall (corner of a foundation, three sides of a retaining wall), sum each side and enter the total. Add 2 feet to the total for each corner if you want a more conservative estimate (corners use slightly more block than a straight run because of overlap and bond patterns).
- Enter wall height in feet. For a foundation or retaining wall, this is the finished height from the footing up. Standard CMU is 8 inches tall (one course = 8 inches), so wall heights work cleanly in 8-inch increments — 4 feet (6 courses), 6 feet (9 courses), 8 feet (12 courses).
- Subtract opening area for any windows, doors, or vents in the wall. Enter total opening area in square feet (a 3×7 ft door = 21 sq ft; a 4×4 ft window = 16 sq ft). The calculator subtracts this from gross wall area before counting blocks.
- Pick block size — 8x8x16 is the residential and light-commercial standard. Use 6x8x16 for non-load-bearing partitions; 12x8x16 for tall foundations or commercial walls; 4x8x16 for thin partition walls.
- Select cell fill mode. "No grout fill" for typical garden or low retaining walls without rebar. "Partial fill" for residential reinforced walls with rebar in every other cell. "Full grout fill" for high-strength commercial walls or reinforced retaining walls per engineer's spec.
- Set waste factor. Default 5% covers normal cutting and breakage; 8-10% for cut-heavy or experienced-poor jobs. Round the final block count up to whole pallets at the supplier (typical pallets are 90 or 96 blocks).
What is a concrete block? CMU sizes and types
A concrete block is a precast concrete masonry unit used for walls, foundations, retaining walls, and (less commonly today) interior partitions. The technical industry name is CMU (concrete masonry unit). The trade-slang names — cinder block, cement block, breeze block, hollow block — all refer to the same category, with minor regional and historical variations. "Cinder block" originally meant a block made with coal cinders as aggregate (lighter weight, no longer manufactured this way); modern "cinder blocks" sold at home centers are actually CMUs with standard concrete aggregate.
Standard residential CMU sizes are nominal — the actual block is 3/8 inch smaller than the nominal in each dimension to account for mortar joints. The most common size is the 8×8×16 block (nominal): actual dimensions 7-5/8 × 7-5/8 × 15-5/8 inches. Each block covers 16 × 8 = 128 sq inches = 0.889 sq ft of wall face. Other common sizes: 6×8×16 (narrower for partitions), 12×8×16 (wider for foundations), 4×8×16 (partition block).
Standard CMU types: hollow concrete blocks (the most common — two cells inside that can be filled with grout if needed), solid concrete blocks (no cells, used where extra mass or fire resistance is required), bond beam blocks (with notches for horizontal rebar), and corner blocks (with one finished face for the wall corner). For a typical residential project, 95% of the block order will be standard hollow 8×8×16 with a small percentage of corner and bond-beam blocks.
A 6 inch CMU weighs about 25 lbs; an 8 inch CMU weighs 32-38 lbs. A standard pallet of 8x8x16 hollow CMU contains 90-96 blocks and weighs 3,000-3,500 lbs. Most residential CMU walls require only 1-3 pallets of block, with mortar and grout ordered separately.
| Nominal size | Actual size | Face coverage | Weight (each) | Typical use |
|---|---|---|---|---|
| 4×8×16 | 3-5/8 × 7-5/8 × 15-5/8" | 0.889 sq ft | ~18 lbs | Non-load-bearing partition walls |
| 6×8×16 | 5-5/8 × 7-5/8 × 15-5/8" | 0.889 sq ft | ~25 lbs | Light-duty walls, partitions |
| 8×8×16 (most common) | 7-5/8 × 7-5/8 × 15-5/8" | 0.889 sq ft | 32-38 lbs | Residential and light-commercial standard |
| 10×8×16 | 9-5/8 × 7-5/8 × 15-5/8" | 0.889 sq ft | ~42 lbs | Tall residential walls, foundations |
| 12×8×16 | 11-5/8 × 7-5/8 × 15-5/8" | 0.889 sq ft | ~50 lbs | Heavy commercial walls, deep foundations |
| Half-block (8×8×8) | 7-5/8 × 7-5/8 × 7-5/8" | 0.444 sq ft | ~16 lbs | Wall ends and openings |
| Bond-beam block (8×8×16) | 7-5/8 × 7-5/8 × 15-5/8" | 0.889 sq ft | ~30 lbs | Horizontal rebar courses |
| Corner block (8×8×16) | 7-5/8 × 7-5/8 × 15-5/8" | 0.889 sq ft | 32-38 lbs | Finished face at wall corners |
How many concrete blocks do I need?
How many concrete blocks do I need is the most-asked block-calculation question and it reduces to one formula: wall area in square feet ÷ 0.889 = block count for 8×8×16 CMU. The 0.889 comes from each block covering 16 × 8 = 128 sq inches = 0.889 sq ft of finished wall face.
How many cinder blocks do I need: same math (modern cinder blocks are CMUs with the same dimensions). For a 30-foot-long, 4-foot-high wall: 30 × 4 = 120 sq ft, ÷ 0.889 = 135 blocks. Add 5% waste = 142 blocks ordered. How many concrete blocks will I need for a foundation: same calculation per side, summed. How many cinder blocks will I need for the same wall: 142 — the answer is identical regardless of which name the supplier uses for the product. How many cement blocks do I need for a 6-foot wall: 6 ÷ 0.667 (each course = 8 inches = 0.667 ft) = 9 courses, multiplied by blocks per course (length ÷ 1.333 ft per block).
How to calculate concrete blocks needed for any wall: (1) compute wall area, (2) subtract opening area, (3) divide net area by block face coverage (0.889 sq ft for 8×8×16), (4) round up to whole blocks, (5) add 5-10% waste. For non-standard block sizes, divide by the relevant face area: 6×8×16 covers 0.889 sq ft (same length × height face); 12×8×16 also covers 0.889 sq ft (the wider dimension is depth, not face). The face area is always 16 × 8 = 0.889 sq ft for standard CMU regardless of width.
How many breeze blocks will I need (UK terminology for hollow concrete blocks): the same arithmetic. Standard UK breeze blocks are 440 × 215 × 100 mm, covering 440 × 215 mm = 0.0946 sq m of face, or about 1.018 sq ft each — slightly more than US blocks. Convert UK measurements to metric and divide by block face area in sq m for European projects. How many blocks do I need calculator: the calculator above with the math explained in this section.
Block fill calculator — grout and core fill
A block fill calculator returns the volume of concrete or grout needed to fill the hollow cores of a CMU wall. Filling the cores converts a hollow wall to a solid wall, dramatically increasing strength, weight, and resistance to lateral loads — required for load-bearing walls, retaining walls over a few feet tall, and any wall with vertical reinforcement. How much concrete to fill a cinder block, how much concrete to fill cmu block, how much concrete to fill a concrete block — all describe the same calculation.
The volumetric math: a standard 8×8×16 CMU has two cells totaling roughly 0.014 cubic feet of core volume per block (about 24 cubic inches per cell × 2 cells, after subtracting the web walls that separate the cells). For a fully grouted wall: total grout volume = block count × 0.014 cu ft. A 142-block wall fully grouted needs 142 × 0.014 = 2.0 cu ft of grout. To convert to cubic yards: ÷ 27. The 2.0 cu ft = 0.074 cu yd, well under the 1 cu yd minimum order most concrete suppliers require — most residential block fills are too small for ready-mix delivery and are mixed on-site from bagged grout.
How much concrete to fill 8x8x16 block per block: 0.014 cubic feet per block for full grout fill (both cells), or 0.007 cubic feet per block for partial fill (every other cell, typical for residential rebar-reinforced walls). The block core fill calculator above outputs total volume based on selected fill mode. For partial-fill walls (rebar in every 4th, 5th, or 8th cell), divide the full-fill estimate by the cell-spacing ratio.
A cmu grout calculator uses the same math but outputs in different units depending on grout supplier specifications. Bagged grout (typical 80-lb bags of dry mix) yields about 0.5 cubic feet per bag when mixed. Our 142-block wall needing 2.0 cu ft of fully-grouted concrete = 4 bags. Order 5 bags to be safe; unopened bags can be returned at most suppliers. For ready-mix delivery on commercial-scale walls, 1 cubic yard delivers 27 cubic feet of grout — enough for ~1,900 blocks of full fill, far more than most residential projects.
When to use grout vs concrete vs mortar (a common confusion): mortar bonds blocks to each other at horizontal joints; grout fills the vertical cores after the wall is laid up; concrete is the structural material in footings below the wall. All three are cement-based but have different sand:cement:water ratios and different placement methods. The mortar calculation is separate from the grout calculation and is covered in the next section.
Mortar quantity for a concrete block wall
Mortar is the thin (typically 3/8 inch) layer of cement-based binder that bonds blocks to each other at horizontal and vertical joints. The standard rule of thumb for residential block walls: 3 cubic feet of Type S mortar per 100 blocks. Type S is the standard for above-grade exterior walls and most foundation work; Type N is used for veneer and non-structural interior walls; Type M is used for below-grade and high-load conditions.
For our 142-block wall: 142 ÷ 100 × 3.0 = 4.26 cubic feet of mortar. Bagged mortar (typical 70-lb bags of dry Type S mix) yields about 0.5 cubic feet per bag mixed. So 4.26 ÷ 0.5 = 9 bags of mortar mix. Add 1 bag for safety = 10 bags. Each bag costs $6-12 in 2026 depending on brand and region — total mortar cost for this wall is $60-120, plus sand, plus mixing time and water.
The 3 cu ft per 100 blocks rule covers the visible mortar joints (10 mm × full bed thickness × full block length plus the head joints at vertical block-to-block contact). For thicker joints (1/2 inch instead of standard 3/8 inch), increase to 4 cu ft per 100. For very experienced masons or block-on-block dry-stack systems, decrease to 2 cu ft per 100. The output above assumes the standard 3/8 inch joint that 95% of residential block work uses.
Block wall heights and reinforcement
Unreinforced concrete block walls are limited to about 4 feet high before the wall begins to need horizontal or vertical reinforcement. For garden walls, low planters, and similar low-load applications, 4 feet of unreinforced 8×8×16 CMU on a proper footing is the typical maximum. Beyond 4 feet, codes and engineering practice require some form of reinforcement.
Vertical reinforcement: rebar (typically #4 or #5) placed in every cell, every other cell, or every 4th cell depending on wall height and load. Each reinforced cell is then filled with grout (the partial-fill mode in the calculator above). For a 6-foot residential block wall with rebar every 4 feet on center: 30-foot wall × every-4-foot spacing = 8 reinforced cells. Each cell takes about 0.014 cu ft of grout × 6 ft × 1.5 (full height plus development length) = roughly 0.13 cu ft per reinforced cell. Total: 8 cells × 0.13 = 1.0 cu ft of grout for partial fill.
Horizontal reinforcement: bond-beam blocks (CMU with cut-out tops) every 4-8 feet vertically, with a horizontal rebar laid into the cut-out and filled with grout. For walls over 6 feet tall or commercial applications, horizontal reinforcement at every other course or every fourth course is standard. The bond-beam blocks are special-order from most suppliers and add 5-10% to total block count.
For retaining walls, basement walls, or any structural application: get the wall designed by an engineer or use prescriptive tables from your local code. The calculator above estimates material quantities for ordering; it does not size reinforcement or replace structural design. For wood-frame components above the block wall — lintels above doors and windows, or ledger plates supporting floor joists — see the LVL beam calculator on this site for engineered lumber sizing.
Worked example: 4-foot retaining wall, 30 feet long
A homeowner is building a 4-foot-tall garden retaining wall, 30 feet long, with no openings, using standard 8×8×16 CMU. The wall will have rebar in every 4 feet of run for stability. Here is the complete material calculation.
Wall area: 30 ft × 4 ft = 120 sq ft. Block face coverage: 0.889 sq ft per 8×8×16 block. Block count: 120 ÷ 0.889 = 134.97, round up to 135 blocks. Add 5% waste: 135 × 1.05 = 142 blocks (rounded up). Order 144 blocks for two full pallets of 72 each.
Mortar: 142 blocks × 3 cu ft per 100 = 4.26 cu ft, ÷ 0.5 cu ft per bag = 9 bags of Type S mortar. Order 10 bags. Sand: about 0.5 cu yd for a project this size. Total mortar cost: 10 × $9 = $90 plus sand at $30 = $120.
Grout (partial fill, 8 reinforced cells per the rebar layout): 8 cells × 0.13 cu ft per cell = 1.0 cu ft. Two 80-lb bags of grout mix (each yielding ~0.5 cu ft) = $25 total.
Rebar: 8 × 4-foot lengths of #4 rebar = 32 lineal feet. At $0.85/lf = $28.
Total material cost (2026): 142 blocks × $2.25 = $320; mortar $120; grout $25; rebar $28; total = $493 in materials. Add labour for a DIY weekend or roughly $1,200-1,800 for a contractor build, and the finished wall comes in at $1,700-2,300 turnkey. The same arithmetic with different inputs (longer wall, taller wall, full grout fill, special-order block) scales the calculation linearly.
For larger block-foundation projects scaled to a full house build, the foundation and masonry portion typically runs 8-12% of total construction cost. The cost to build a house calculator on this site applies this share automatically when you specify a foundation type — useful when the block work is just one line item in a larger budget.
How we sourced these numbers
The block-count math (wall area ÷ block face area) and mortar/grout volume formulas are accepted construction estimating practice, not sourced to a single citation. The 3 cu ft per 100 blocks mortar rule is the long-standing industry standard documented in masonry industry references including the Brick Industry Association technical notes and the National Concrete Masonry Association (NCMA) TEK manuals. Block size specifications (8×8×16 nominal, 7-5/8 × 7-5/8 × 15-5/8 actual) are ASTM C90 standard for hollow load-bearing CMU.
Pricing reflects current 2026 ranges at U.S. building suppliers (Home Depot, Lowe's, regional masonry suppliers). Where two retailers price the same block size differently by more than 15%, the broader range is published. Cost ranges for small/residential projects assume DIY or small-crew labour; commercial-scale block work in larger U.S. markets runs 20-40% higher per block due to logistics and labour overhead. Recommendations on this page are reviewed annually and updated whenever NCMA technical notes are revised.
For related framing work above the foundation, this site has dedicated tools. The stud calculator handles wood-framed wall sections above CMU foundations. The floor joist calculator covers floor framing supported by CMU walls. The LVL beam calculator handles engineered headers over openings in CMU walls.
For project-scale budgeting and finish-stage references, related tools cover the surrounding workflow. The cost to build a house calculator handles the full project budget. The siding calculator handles exterior cladding once framing is complete.
For roof framing on CMU buildings, the rafter length calculator handles roof framing math. The roofing calculator covers roof area and material ordering separately from the foundation work below.
Need to run the numbers?Use the free roof pitch calculator on the home page to convert pitch to angle, calculate rafter length, or estimate roof area in any unit.