Concrete Block Calculator
Plan a concrete masonry wall end to end. Enter the wall size and block, deduct any doors or windows, and get the blocks to order plus mortar bags, sand, core grout, joint reinforcement, and an optional total cost. Works in metric or imperial units.
Formula
Worked example
A 20 ft x 8 ft wall = 160 sq ft. A standard 16 in x 8 in block covers 128 in2 = 0.889 sq ft, so 160 / 0.889 = 180 blocks. Add 5% waste -> 189 blocks. Mortar: ceil(189 / 100 x 3) = 6 bags, plus about 1.9 tons of sand.
How the block count is worked out
Estimating concrete masonry units starts from a simple ratio: the net area of the wall divided by the area of a single block face. A standard nominal block measures 16 inches long by 8 inches high, which is 128 square inches, or about 0.889 square feet, once you account for the 3/8-inch mortar joint baked into the nominal size. Dividing the wall area by that figure gives roughly 1.125 blocks per square foot of wall, a rule of thumb many masons carry in their heads. Pick a block from the preset list, or choose Custom to enter any face size, and the calculator stays accurate for 12-inch, 24-inch or metric 390 x 190 mm units, not just the common 16 x 8. Subtract the area of doors and windows in the openings field so you are not paying for blocks you will never lay.
Mortar, sand, grout and reinforcement
Laying blocks takes more than the blocks themselves. As a planning figure, about three 80-pound bags of mortar mix lay 100 standard blocks, and roughly one ton of masonry sand goes with every 100 blocks, so the calculator scales both to your block count. If your wall is reinforced, set the share of cells you will fill with grout and the tool returns the core-fill volume in cubic yards or cubic metres. Horizontal joint reinforcement, the ladder or truss wire laid in the bed joints, is estimated as linear feet from your chosen spacing: every course, every second course, or every third. These are quantities to order from, not a structural design, so always confirm reinforcement against an engineer or your local code.
Costing the wall and what is not included
Turn on the cost estimate to price the three main materials, blocks at your price each, mortar by the bag, and grout by the cubic yard or metre, for a planning total and a side-by-side breakdown. Local prices, delivery and block type all move the number, so treat it as a budget guide rather than a quote. The estimate still excludes footings and foundations, rebar steel, caps and bond beams, lintels over openings, and labor. As a quick sanity check, one 80-pound bag of standard mortar lays roughly 12 to 15 blocks, and a typical wall needs about 1.125 blocks per square foot. For footings and any solid-grouted cells, size the concrete volume on its own and confirm structural requirements before you build.
Common block sizes and coverage
| Block (nominal) | Face area | Blocks per sq ft | Blocks per 100 sq ft |
|---|---|---|---|
| 8 x 8 in | 0.44 sq ft | 2.25 | 225 |
| 12 x 8 in | 0.67 sq ft | 1.50 | 150 |
| 16 x 8 in (standard) | 0.89 sq ft | 1.125 | 113 |
| 24 x 8 in | 1.33 sq ft | 0.75 | 75 |
| 390 x 190 mm | 0.074 m2 | 13.5 / m2 | - |
Nominal face dimensions include the mortar joint.
Frequently asked questions
How many concrete blocks are in 100 square feet of wall?
For standard 16 x 8 inch blocks, about 113 blocks cover 100 square feet (1.125 blocks per square foot). Add a 5-10% waste allowance, so plan for roughly 120-125 blocks per 100 square feet of wall.
How much mortar and sand do I need for the blocks?
A common rule is about three 80-pound bags of mortar mix per 100 standard blocks, with roughly one ton of masonry sand per 100 blocks. This calculator scales both to your block count automatically. Cold weather, wide joints and porous block all increase consumption, so buy a little extra.
Should I subtract doors and windows?
Yes. Add up the area of each opening and enter the total in the openings field so it is removed from the wall before estimating. For small openings some masons let the waste allowance absorb them, but for large windows or garage doors the saving is significant.
How much grout do I need to fill the block cells?
A fully grouted standard 8-inch wall uses roughly 0.25 cubic feet of grout per block, or about one cubic yard per 100 to 110 blocks. Set the share of cells you plan to grout and the calculator returns the core-fill volume in cubic yards or cubic metres.