Size a mortared concrete masonry unit (CMU) wall — block count, 80-lb Type N mortar bags, and optional core-fill grout — from just the wall's length and height.
Quick answer: A 20 × 8 ft wall is 160 sq ft × 1.125 blocks per sq ft = 180 blocks net (198 with 10% waste). At our guide's rate of 15 eighty-pound Type N bags per 100 sq ft, it takes 24 bags of mortar net — order 27 with the standard 10% buffer.
Ballpark, not gospel. The mortar rate is published for 8-in block with 3/8-in joints — thinner or thicker walls need slightly less or more, and the grout figure is approximate, so confirm both with your supplier before ordering.
These are two different products, and the math does not transfer between them. CMU (concrete masonry units) are laid with mortar joints and sometimes reinforced with rebar and grout — that's what this calculator sizes. Segmental retaining-wall block is dry-stacked and interlocking: it takes no mortar at all and counts differently. Our retaining-wall blocks guide covers that product — do not use this page's block or mortar figures for a segmental wall.
| Nominal block size | Typical uses |
|---|---|
| 4-in | Interior partitions, light non-load-bearing walls |
| 6-in | Non-load-bearing partitions, some non-structural exterior walls |
| 8-in | Standard load-bearing exterior walls, foundations, garden walls — the most common |
| 10-in | Heavier structural walls, taller load-bearing walls |
| 12-in | Heavy load-bearing walls, tall walls, pilasters |
Sizes are nominal (actual blocks run about 3/8 in smaller to allow for joints). Every CMU has the same 8×16 face, so the block count is identical across sizes — depth only affects wall strength and core volume.
Anchor check: one 80-lb bag lays about 8 blocks, so 180 ÷ 8 ≈ 23 bags — right at the calculator's 24 (our guide rounds 14.06 up to 15 bags per 100 sq ft). Core-fill grout, if needed, runs about 1.08 yd³ for all 180 cores — approximate, confirm with your supplier.
Straight answers, no fluff. Then run your own numbers above.
Measure the wall face in square feet (length × height in feet) and multiply by 1.125 blocks per sq ft, then round up. A standard 8×8×16 CMU with 3/8-inch joints lays 1.125 blocks per square foot of wall face. A 20×8 ft wall is 160 sq ft × 1.125 = 180 blocks net — order 198 with a 10% waste buffer.
Our mortar guide's rate for standard 8-in CMU with 3/8-inch joints: about 15 eighty-pound Type N bags per 100 sq ft of wall face, net (16 with waste). A 20×8 ft wall (160 sq ft) needs 24 bags net, 27 with a 10% buffer. As a rough cross-check, one 80-lb bag lays about 8 blocks.
Type N is the general-purpose default for above-grade block work — garden walls, sheds, chimneys, mailboxes. Type S is stronger and made for structural, below-grade, or at-grade work like foundations and load-bearing walls. Note from our mortar guide: Type S yields a bit less per bag, so you need roughly 20% more bags for the same wall.
Only where the wall needs reinforcement. Fully grouted walls are for heavily loaded or tall walls; most garden and non-structural walls leave unreinforced cores hollow. The calculator's core-fill row is approximate — grout volume depends on block depth and how many cells are filled, so confirm with your supplier before ordering.
Only the reinforced cells — the ones with rebar, typically around openings and at wall ends or corners — get grout; the rest stay hollow. That means a partially grouted wall orders far less grout than the full-wall figure, and costs less while still meeting code where allowed.
Reinforced (rebar + grout) cells are what carry loads in structural block walls, and local code sets the spacing and size. For ordering purposes it just matters which cores are reinforced: only reinforced cores get grout, which is why the calculator's grout figure is approximate rather than exact.
No. Segmental retaining-wall block is dry-stacked and interlocking — it takes no mortar at all — and is a different product from mortared CMU. Use this calculator only for mortared concrete block walls; our retaining-wall blocks guide covers segmental block.
The full mortar breakdown — Type N vs S vs M, joint-thickness multipliers, and per-1,000 counts — is in our how much mortar do I need guide. Retaining-wall block is a related but different product (dry-stacked, no mortar) — see that guide instead. Filling cores or pouring a footing? The concrete calculator sizes the concrete.