Concrete Block Calculator: How Many Blocks and Mortar Bags for Your Wall
A concrete block calculator estimates how many CMU blocks, 80-lb mortar bags, and core-fill grout your wall needs from its length, height, block size, and a waste allowance.
Concrete Block Calculator
Estimate CMU blocks, mortar bags, sand, and grout for any wall.
Common wall sizes at a glance
These figures use standard 8x8x16 CMU at 1.125 blocks per square foot, a 10% waste factor, 3 mortar bags per 100 blocks, and solid grout fill of 0.25 cubic feet per block (all cores filled). Openings are not deducted in this table.
| Wall size | Wall area | Blocks (incl. 10% waste) | 80-lb mortar bags | Sand | Grout, all cores |
|---|---|---|---|---|---|
| 10 ft x 8 ft | 80 sq ft | 99 | 3 | 990 lb | 24.8 ft³ (0.92 yd³) |
| 20 ft x 6 ft | 120 sq ft | 149 | 5 | 1,490 lb | 37.3 ft³ (1.38 yd³) |
| 50 ft x 4 ft | 200 sq ft | 248 | 8 | 2,480 lb | 62.0 ft³ (2.30 yd³) |
| 40 ft x 8 ft | 320 sq ft | 396 | 12 | 3,960 lb | 99.0 ft³ (3.67 yd³) |
| 100 ft x 8 ft | 800 sq ft | 990 | 30 | 9,900 lb | 247.5 ft³ (9.17 yd³) |
How many blocks per square foot
A standard concrete masonry unit is 8 inches high by 16 inches long on its face, which is its nominal size. The actual unit measures 7 5/8 x 7 5/8 x 15 5/8 inches; the missing 3/8 inch on each dimension is the mortar joint that sits between units. Counting at nominal size automatically accounts for that joint.
One nominal block face covers 8 x 16 = 128 square inches. Since a square foot is 144 square inches, each block covers about 0.889 sq ft, which means 144 ÷ 128 = 1.125 blocks per square foot of wall. Multiply your net wall area by 1.125 and you have the raw block count before waste. A 400 sq ft wall, for example, needs 400 x 1.125 = 450 blocks before any allowance.
Mortar and sand per 100 blocks
The long-standing masonry rule of thumb is 3 bags of mortar per 100 blocks for standard 8-inch units with 3/8-inch joints, along with roughly 1,000 pounds of sand per 100 blocks when you mix your own mortar. One 80-lb bag of Type N or Type S mortar lays about 33 to 35 standard blocks. Face-shell bedding (mortar only on the outer edges) uses a little less, closer to 2 to 2.5 bags per 100, while full bedding runs the full 3.
Mortar type matters as much as quantity: Type N is typical for above-grade walls, while Type S is specified for below-grade work and block foundations. Buying 80-lb bags rather than 60-lb bags is almost always cheaper per pound of mortar on a large wall.
Grouting cores and adding rebar
Hollow blocks have open cells that are often filled with grout and reinforced with rebar, especially on retaining walls and anything structural. The volume of grout needed depends on block width, not block count, because every size shares the same 8 x 16-inch face. Typical fill volumes for two-core units are:
- 6x8x16: about 0.17 ft³ per block
- 8x8x16: about 0.25 ft³ per block
- 10x8x16: about 0.33 ft³ per block
- 12x8x16: about 0.39 ft³ per block
Many reinforced walls use partial grouting, filling only the cells that contain rebar rather than every core, which cuts the grout volume substantially. The table above assumes every cell is filled, so treat it as an upper bound.
How block size changes your count
Because standard 4-inch, 6-inch, 8-inch, 10-inch, and 12-inch blocks all share the same 8 x 16-inch face, they all come out to 1.125 blocks per square foot — the block count stays the same and only the width, weight, and grout volume change. The exception is half-high block (4x4x16), whose 4-inch face height doubles the count to 2.25 blocks per square foot. If your wall uses a mix of standard and half-high courses, calculate each section separately.
Why you add a waste factor
Every job loses block to cuts at corners, around openings, at control joints, and from simple breakage during handling. A 5% waste allowance is reasonable for a plain rectangular wall, 10% is the common default, and 15% is safer for walls with many openings, curves, or unusual patterns. It is cheaper to return a few unused blocks than to stop mid-course and wait on a second delivery. Blocks are typically sold by the pallet, often around 90 to 108 units, so round your final number up to full pallets.
Common estimating mistakes
The biggest error is calculating from gross wall area instead of net area — always subtract the square footage of doors, windows, and other openings first. Other frequent slips: forgetting that corners consume extra block, ignoring control joints on long runs, and under-ordering mortar because the count was based on face-shell bedding when the crew uses full bedding. Double-check the block count by hand against the 1.125 figure before placing an order, since one mistyped dimension can throw the whole estimate off.
Frequently asked questions
How many concrete blocks do I need per square foot? About 1.125 standard 8x8x16 blocks per square foot of wall, which is 144 square inches divided by the block’s 128-square-inch face. A 100 sq ft wall needs roughly 113 blocks before waste.
Are cinder blocks and concrete blocks the same thing? In everyday use, yes. “Cinder block” is an older term from when units were made with coal cinders; today’s units are concrete masonry units (CMU). Both describe the same hollow blocks, and this calculator works for either term.
How many bags of mortar do I need for 100 blocks? Plan on about 3 standard 80-lb bags of mortar per 100 blocks at a 3/8-inch joint. For a quick estimate, divide your total block count by about 33.
How many blocks come on a pallet? A pallet of standard 8-inch CMU usually holds around 90 to 108 blocks, depending on the manufacturer. Rounding your order up to full pallets avoids partial-pallet surcharges.
Do I have to fill the blocks with concrete? Not always. Freestanding decorative or garden walls are often left hollow, while retaining walls and structural walls are grouted and reinforced with rebar. If you fill the cores, budget roughly 0.25 ft³ of grout per standard 8-inch block when filling every cell.
References
- Titan America Block Estimating Guide (PDF) — blocks per square foot, mortar per 100 block, and core-fill volumes by unit size.
- Omni Calculator — Concrete Block Calculator — mortar rule of thumb and block-count methodology.
- General Shale Engineering Rule-of-Thumb Bulletin (PDF) — 1.125 blocks and 3 mortar bags per 100 block.
