Concrete Block Calculator

Background.

This concrete block calculator turns a wall length and height into the four numbers a masonry order actually needs: how many concrete masonry units to buy, how many bags of mortar to lay them, how much core-fill grout to pour into the cells, and what the block alone will cost. It handles ungrouted hollow walls, solid-grouted walls, and walls grouted only at the reinforced cells — 16, 24 or 32 inches on centre — because those four cases need wildly different quantities of grout and the difference is the biggest line item on the ticket.

The first thing worth getting right is the dimension convention, because it is where most block take-offs go wrong. Concrete masonry units are modular. The block everybody calls an 8 × 8 × 16 measures 7⅝ × 7⅝ × 15⅝ inches out of the cube, and it is manufactured that way on purpose: laid with the ⅜ inch mortar joint it is designed for, one unit plus its joint occupies exactly 8 × 16 inches of wall face. The joint is already inside the nominal number. That is why this calculator asks for nominal face dimensions and has no separate mortar-joint field — adding a joint on top of a nominal dimension double-counts it and under-orders the wall by about 2.3 percent. Nominal 16 × 8 works out to 144 ÷ 128 = 1.125 blocks per square foot, which is the figure every mason quotes from memory.

Grout is the part that deserves the most care, and the part this page exists for. A hollow block wall is mostly air; a solid-grouted 8 inch wall needs roughly 1.1 cubic yards of grout for every 100 square feet of wall, which at ready-mix prices costs more than the block it is poured into. Grout quantity here is not derived from an assumed percent-solid — real units vary — it is read from a published grout-quantity table for standard two-cell hollow units, indexed by nominal wall thickness and by grouted-cell spacing. Pick 8 inch and "solid grouted" and you get 1.12 cubic yards per 100 square feet; pick 24 inches on centre and you get 0.50. Be aware that published grout yields for the identical unit disagree by roughly 20 percent between suppliers, from about 0.22 to 0.27 cubic feet per 8 inch block. This calculator orders against the highest published figure, because running short of grout mid-pour leaves a cold joint in a structural wall while running long costs you a part bag. The spread is printed beside the result rather than hidden, so you can check it against your own supplier's yield sheet.

One boundary matters more than any arithmetic on this page: this is a material take-off, not a structural design. It does not choose the bar size and it does not choose the grouted-cell spacing. Vertical reinforcement in a masonry wall is set by the structural design under TMS 402 (formerly ACI 530) as adopted by the governing building code, and local amendments govern — the edition your jurisdiction has adopted may not be the current one. Enter the spacing your engineer specified. A licensed structural engineer or architect must design and sign off on any load-bearing, retaining or freestanding masonry wall before it is built, and grout lift and pour heights must follow the grout-space limits in the adopted code.

If you are laying clay brick rather than block, the brick calculator is the right page — it takes actual unit dimensions plus a separate joint, which is the brick convention. If you are dry-stacking a segmental retaining wall, that is a different product with no mortar and no cells, and the retaining wall block calculator sizes its base gravel and drainage aggregate too.

What is concrete block calculator?

A concrete block calculator is a masonry material take-off tool. Given the face area of a wall and the size of the unit, it computes how many concrete masonry units cover that area, how many courses and how many blocks per course that comes to, how much mortar is needed to bed and butter them, and how much grout is needed to fill the cells that are being grouted.

Concrete masonry units — CMU, or simply block — are hollow, load-bearing units covered by ASTM C90. They are specified by nominal dimensions in the order width × height × length, so an "8 × 8 × 16" is 8 inches thick, 8 inches tall and 16 inches long nominally. Grout is not mortar: mortar is the bedding material between units, while grout is a high-slump, ASTM C476 concrete poured into the vertical cells to bond the reinforcing steel to the masonry. A wall can be ungrouted, partially grouted (only the cells containing reinforcement) or solid grouted (every cell).

The tool is valid for straight, single-wythe walls built of standard two-cell hollow units in running bond. It does not count bond beams, lintels, pilasters, corner or sash units, control joints, cap block or ties; it does not size reinforcement; and it does not check any structural capacity.

How to use this calculator.

1. 01Measure the wall length and finished height and enter both in feet.
2. 02Add up the area of every door, window and vent opening and enter the total in square feet — the take-off is done on net area.
3. 03Choose the nominal wall thickness: 6, 8, 10 or 12 inch. This selects which row of the published grout table is used.
4. 04Leave the face dimensions at 16 x 8 for a standard unit. These are NOMINAL — do not add a mortar joint, it is already included.
5. 05Choose the grouted-cell condition. Enter the spacing your structural engineer specified; if the wall is not reinforced, choose ungrouted.
6. 06Adjust the blocks-per-mortar-bag figure if you are not using an 8 inch unit — 13 is published for the 8 x 8 x 16 only.
7. 07Set a waste allowance. 5% covers breakage and cuts on a plain wall; go to 10% where there are many openings, corners or angled cuts.
8. 08Read the block count and the grout bag count, then check the basis note beside the result for the sources and the sign-off requirement.

The formula.

Block count is an area division. Net wall area is length × height minus openings. One unit covers (nominal face length × nominal face height) square inches, so blocks per square foot is 144 divided by that product — 144 ÷ (16 × 8) = 1.125 for a standard unit. Multiply net area by that rate to get the raw count, multiply by one plus the waste fraction, and round up, because you cannot order a fraction of a block. Courses high and blocks per course are reported separately as layout figures: wall height in inches divided by nominal face height, and wall length in inches divided by nominal face length, each rounded up so a part course is visible. The order count stays area-based rather than grid-based because that is what deducts openings correctly.

Mortar is a published yield, not a computed volume. QUIKRETE Mason Mix (Type S), Product No. 1136, states that an 80 lb bag will lay up to 13 standard 8 × 8 × 16 blocks. Bags needed is the block count divided by that figure, rounded up. The figure is an editable input because it is published for the 8 inch unit only — a 12 inch unit has a wider face-shell bed — and the page does not invent values it cannot source.

Grout is a table lookup. For standard two-cell hollow units, grout quantity is published as cubic yards per 100 square feet of wall, by nominal wall thickness and grouted-cell spacing: 0.93 / 1.12 / 1.30 / 1.73 cubic yards per 100 square feet for solid-grouted 6, 8, 10 and 12 inch walls respectively, falling to 0.35 / 0.43 / 0.53 / 0.64 at 32 inches on centre. The calculator multiplies net wall area by the rate, applies the same overage allowance, converts to cubic feet at 27 cubic feet per cubic yard, and divides by 0.65 cubic feet per 80 lb bag to get a bag count. Choosing a wall thickness outside 6, 8, 10 and 12 inches throws an error rather than interpolating, because there is no sourced row to interpolate between.

Rounding happens once. All arithmetic runs in arbitrary-precision decimal, nothing is rounded mid-calculation, and the three counts that must be whole numbers — blocks, mortar bags, grout bags — are ceilings applied to a full-precision value at the very end. A value that is mathematically exactly 378 therefore orders 378 blocks, not 379.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Westbrook Concrete Block Co., Inc. — "Fine Core Fill Grout: High Strength Masonry Grout" technical data sheet, table "Grout Calculation for Concrete Block Construction". Source of the cubic-yards-of-grout-per-100-square-feet values for 6, 8, 10 and 12 inch walls at all-cells / 16 / 24 / 32 inch on-centre spacing.
2. [2]QUIKRETE — Core-Fill Grout, Coarse, Product No. 1585-07 product data sheet. Source of the 0.65 cubic feet per 80 lb bag yield, of the ASTM C476 compliance statement, and of Table 2 grout space requirements per ACI 530.
3. [3]QUIKRETE — Mason Mix (Type S mortar), Product No. 1136 product data sheet. YIELD section: one 80 lb bag lays up to 37 standard bricks or 13 standard 8 x 8 x 16 inch blocks. Meets ASTM C270 and ASTM C1714 as Type S mortar.
4. [4]National Concrete Masonry Association — TEK 3-2A, "Grouting Concrete Masonry Walls" (2005, replaces TEK 3-2 and 3-3A). Source of the construction caveats: grout slump of 8 to 11 inches at placement, removal of mortar protrusions over 1/2 inch, and the treatment of cross webs in solid-grouted walls.
5. [5]ASTM International — ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units. Governs the modular nominal dimensions used throughout this page (nominal = actual plus the 3/8 inch mortar joint) and the minimum face-shell and web thicknesses. Standard is sold by ASTM and is not open access.
6. [6]The Masonry Society / American Concrete Institute — TMS 402 (formerly ACI 530), Building Code Requirements for Masonry Structures. The authority that sets reinforcement size and spacing for a masonry wall, which this calculator does not choose. Sold by TMS; adopted by reference in the building code, with local amendments.