How to use the concrete block calculator
- Enter the wall length and height and subtract door and window openings.
- Pick the nominal width — 8 inch is the usual wall unit. The face is 8 × 16 inches at every width, so the block count does not change.
- Choose a grout pattern if the wall will have grouted cells. Leave it at “none” for an ungrouted garden or veneer wall.
- Set waste — 5% for blocks and 20% for mortar is a typical starting point, in the same spirit as the Brick Industry Association’s brick allowances.
The 112.5-block rule and mortar bags
A standard stretcher is nominally 8 × 16 inches on the face, including a ⅜ inch mortar joint (the actual unit is typically 7⅝ × 15⅝ inches). There are 144 ÷ (8 × 16) = 1.125 blocks per sq ft, or 112.5 blocks per 100 sq ft of wall. Those nominal sizes are from NCMA TEK 04-01B. ASTM C90 is the product standard the units are made to (strength, absorption, dimensional tolerances); it does not itself set the 8 × 16 module.
The QUIKRETE Mortar Mix No. 1102 data sheet states that an 80 lb bag lays up to 13 standard 8 × 8 × 16 inch blocks. Bags = (net blocks ÷ 13) × (1 + mortar waste). Grout volumes are planning figures for an 8 inch wall — 1.0 yd³ per 100 sq ft when every cell is filled, 0.5 yd³ at 16 inch centers, 0.35 at 24 inch and 0.25 at 32 inch — scaled by width for 4, 6, 10 and 12 inch units. They are not a structural grouting schedule.
A worked example: 20 × 8 ft wall
A 20 × 8 ft wall with no openings is 160 sq ft. Net blocks = 160 ÷ 100 × 112.5 = 180. With 5% waste, buy 189 blocks. Mortar: 180 ÷ 13 × 1.20 = 16.62, so 17 bags of 80 lb mix. Leave grout at none unless the drawings call for filled cells. For clay brick, use the brick calculator instead — the unit, the mortar table and the waste rules are different.
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- Block count uses the NCMA TEK 04-01B 8 × 16 in nominal face, 112.5 units per 100 sq ft, any width. ASTM C90 is the product standard, not the source of the module.
- 80 lb mortar bags = (net blocks ÷ 13) × (1 + mortar waste), from the QUIKRETE 1102 data sheet (up to 13 standard blocks per bag).
- Grout volumes are planning figures for an 8 in wall (all cells 1.0 yd³/100 sf; 16 in o.c. 0.5; 24 in 0.35; 32 in 0.25), scaled by width. They are not a structural schedule.
- Default waste is 5% for blocks and 20% for mortar.
Sources
- National Concrete Masonry Association, TEK 04-01B, Typical Sizes and Shapes of Concrete Masonry Units. Nominal 8 × 8 × 16 in stretcher; actual unit typically 7⅝ × 7⅝ × 15⅝ in, with a ⅜ in joint in the module.
- ASTM International, ASTM C90/C90M, Loadbearing Concrete Masonry Units. The product standard for loadbearing CMU (strength, absorption, tolerances). It does not itself set the 8 × 16 nominal module.
- QUIKRETE, Mortar Mix No. 1102 product data sheet. An 80 lb bag lays up to 13 standard 8 × 8 × 16 in concrete blocks (or 37 standard brick).
Last reviewed . Product figures vary by brand; the label on what you buy wins.
Frequently asked questions
How many 8 × 8 × 16 concrete blocks do I need per 100 square feet?
112.5 net, because the NCMA TEK 04-01B nominal face with a ⅜ inch joint is 8 × 16 inches. Add about 5% for cuts, so a 100 sq ft wall is about 119 blocks to buy.
How many 8 inch blocks will an 80 lb bag of mortar lay?
QUIKRETE Mortar Mix No. 1102 lists up to 13 standard 8 × 8 × 16 inch blocks per 80 lb bag. This calculator divides net blocks by 13 and then adds your mortar waste percentage.
Is a cinder block the same as a concrete masonry unit?
In everyday speech, yes — “cinder block” and “concrete block” both refer to CMU. Modern units are made with concrete, not coal cinders. This page does not count clay brick.
Does the grout figure replace an engineered wall design?
No. Grout quantities here are material-planning estimates. Bar size, cell spacing and whether a wall must be grouted are set by the drawings or a licensed engineer and by local code.