How the numbers are worked out
blocks = (length × height − openings) ÷ (8/9 ft²), plus your extra %, rounded up
mortar bags = blocks ÷ blocks per 80 lb bag, rounded up
grout (ft³) = net wall area × table value ÷ 100
A standard block is sold as “8 x 8 x 16”. Those are nominal sizes that include the mortar joint, so each block takes up 8 in by 16 in of wall face: 128 in², or 8/9 of a square foot. CMHA uses the same 8/9 ft² figure, which is where the divisor comes from. The block's width (6, 8, 10, 12 or 14 in) does not change how many you need, only how much grout the cores hold.
Mortar: two sources, two answers
CMHA's estimating table says one 80 lb bag of preblended mortar lays about 16 conventional blocks. QUIKRETE's data sheet for its Mason Mix says one 80 lb bag lays up to 13 8 x 8 x 16 blocks. We show both instead of averaging them. Planning on the lower number means fewer trips back to the store. CMHA's figure assumes face-shell mortar bedding (mortar on the outer shells only) and already includes a nominal amount for waste, so the extra % above is not added to mortar.
Filling the cores (“how much concrete for cinder blocks”)
The grout figures come from CMHA Table 3, which is per 100 ft² of wall for a given block width and core spacing. They already include 3% waste and assume two-core blocks, so the extra % above is not added to them. “Every core” means the wall is solid grouted. A partly grouted wall fills only some cores, and which ones comes from your plans or your building department, not from this page.
The bag count uses the 0.66 ft³ yield of an 80 lb bag of preblended grout given in the same TEK. If you fill cores with a different product, divide the cubic feet by that bag's yield on its own data sheet. CMHA does not list 4 in blocks in the grout table because their cells are too small to grout properly.
Worked example
A garden wall 20 ft long and 4 ft high (6 courses of 8 in) with no openings: 80 ft² ÷ 8/9 = 90 blocks. Mortar: 90 ÷ 16 = 5.6 → 6 bags by the CMHA table, or 90 ÷ 13 = 6.9 → 7 bags by the QUIKRETE sheet. Grouting every core of 8 in block: 80 × 36.1 ÷ 100 = 28.88 ft³ (1.07 yd³) → 28.88 ÷ 0.66 = 43.8 → 44 bags of preblended grout.
Check against the source: at 100 ft² with 5% extra, the calculator gives 100 ÷ 8/9 × 1.05 = 118.1 → 119 blocks, the same as CMHA's “119 units per 100 ft2”.
What this calculator is not for
It counts standard 8 in high by 16 in long blocks laid in a single wythe. Half-high units, brick-sized units, corner, pilaster and bond-beam blocks need to be counted separately, and walls with lots of corners, returns or non-modular lengths use more than the area method suggests. Horizontal bond-beam grout is not included. It does not size the footing, the rebar or how tall a wall can safely be built — for a retaining wall or any structural wall, that comes from an engineer or your local code.
Sources
- Concrete Masonry & Hardscapes Association (formerly NCMA), TEK 04-02A, Estimating Concrete Masonry Materials. Checked 2026-09-27.
“For conventional units having nominal heights of 8 in. (203 mm) and nominal lengths of 16 in. (406 mm), the exposed surface area of a single unit in the wall is 8/9 ft2 (0.083 m 2). Including a 5 percent allowance for waste and breakage, this translates to 119 units per 100 ft2 (9.29 m2) of wall area. … Preblended mortar: 1-80 lb (36.3 kg) bag 1-3,000 lb (1,361 kg) bag 16 420 50 1,550 … Values include nominal amounts for waste. Assumes face shell mortar bedding for conventional concrete masonry units … Table 3—Grout Volume Estimation for Hollow Single Wythe Concrete Masonry Walls Volume of grout, ft3 per 100 ft2 of wall (m3 per 100 m2)a Grout spacing, in. (mm) Wall width: 6 in. (152 mm) 8 in. (203 mm) 10 in. (254 mm) 12 in. (305 mm) 14 in. (356 mm) 8 (203) 25.6(7.8) 36.1(11.0) 47.0(14.3) 58.9(18.0) 74.5(22.7) 16 (406) 12.8(3.9) 18.1(5.5) 23.5(7.2) 29.5(9.0) 37.3(11.4) 24 (610) 8.6(2.6) 12.1(3.7) 15.7(4.8) 19.7(6.0) 24.8(7.6) 32 (813) 6.4(2.0) 9.1(2.8) 11.8(3.6) 14.8(4.5) 18.6(5.7) 40 (1,016) 5.2(1.6) 7.3(2.2) 9.4(2.9) 11.8(3.6) 14.9(4.5) 48 (1,219) 4.3(1.3) 6.1(1.9) 7.9(2.4) 9.9(3.0) 12.4(3.8) … a Assumes two-core hollow concrete masonry units and 3% waste. … a 80 lb (36.3 kg) bag yields approximately 0.66 ft3 (0.019 m3)”
- QUIKRETE, QUIKRETE® Mason Mix (No. 1136) — Product Data Sheet (revised November 2020). Checked 2026-09-27.
- NIST, NIST Handbook 44 (2026), Appendix C — General Tables of Units of Measurement. Checked 2026-09-24.
“1 728 cubic inches (in3) = 1 cubic foot (ft3) 27 cubic feet = 1 cubic yard (yd3)”