A wall 20 ft long and 4 ft high (80 sq ft) takes 90 blocks, or 95 with 5 percent extra.
To figure how many concrete blocks you need, divide the wall's face area in square feet by 0.889, the face of one standard 8 × 16 in block including its mortar joint, then add about 5 percent for breakage and cuts. That's 1.125 blocks per square foot, the rate our concrete block calculator uses for standard block.
For mortar, go by the bag label: Quikrete and Sakrete rate an 80 lb bag of premixed mortar at 12 to 13 standard blocks. The calculator's mortar field starts at 30 blocks per bag, so change it to your bag's number before you trust the mortar count.
How to measure a block wall
- Measure the wall length in feet. Measure along the footing. For a wall with corners, add up each run, but measure only one run at each corner to the outside face. If you measure every run to its outside corner, each corner gets counted twice and you'll order a few more blocks than you need.
- Measure the height in feet. Go from the top of the footing to the top of the finished wall. Block walls rise in 8 inch courses, so round to a multiple of 8 inches: 4 ft is 6 courses, and 3 ft 4 in is 5.
- Subtract openings. Take out the area of doors, windows, or vents. Multiply each opening's width by its height in feet and subtract it from the wall face.
- Confirm the block's nominal size. The standard face is 8 × 16 in. Block thickness (4, 6, 8, 10, 12 in and up) doesn't change the count for a single-thickness wall. Mortar yields on the bags are quoted for standard 8 in blocks, so expect a different yield with other widths.
- Choose the extra percentage. The default 5 percent suits a straight wall. Add more for many corners, openings, or cut blocks.
The concrete block formula
The calculator uses:
- Wall face (sq ft) = wall length (ft) × wall height (ft)
- Block face (sq ft) = nominal block length (in) × nominal block height (in) ÷ 144
- Blocks = wall face ÷ block face × (1 + extra % ÷ 100), rounded up
- Mortar bags = blocks ÷ blocks per bag, rounded up
The formula uses the nominal size on purpose. According to NCMA (the National Concrete Masonry Association), a block's actual dimensions are typically 3/8 inch less than nominal, so each block plus a 3/8 inch mortar joint fills exactly 8 × 16 in of wall. A standard 8 × 8 × 16 block actually measures 7 5/8 × 7 5/8 × 15 5/8 in.
Worked example (the calculator's default values)
A wall 20 ft long and 4 ft high, standard 8 × 16 in blocks, 30 blocks per bag of mortar, and 5 percent extra:
- Wall face: 20 × 4 = 80 sq ft
- Block face: 16 × 8 ÷ 144 = 0.889 sq ft
- Blocks: 80 ÷ 0.889 = 90
- With 5 percent extra: 90 × 1.05 = 94.5, rounded up to 95 blocks
- Mortar: 95 ÷ 30 = 3.2, rounded up to 4 bags
The calculator shows: "You need 95 blocks. Wall face: 80 sq ft at 0.889 sq ft per block (nominal size includes 3/8 in mortar joints), plus 5% extra. About 4 bags of mortar mix."
You can check the block count by courses. A 4 ft wall is 48 ÷ 8 = 6 courses, and a 20 ft run is 240 ÷ 16 = 15 blocks per course. 6 × 15 = 90 blocks before extra, the same answer. Counting courses is also how you'll plan the layout.
Set the mortar field to match your bag
The default of 30 blocks per bag is only a starting point, and it's high for premixed mortar. Here's what the makers list, and what each would mean for the 95-block example:
| Product | Bag | Blocks per bag (maker's figure) | Bags for 95 blocks |
|---|---|---|---|
| Quikrete Mortar Mix (Type N, premixed) | 80 lb | Up to 13 | 8 |
| Quikrete Mason Mix (Type S, premixed) | 80 lb | Up to 13 | 8 |
| Sakrete Mortar Mix Type S | 80 lb | 12 | 8 |
| Sakrete Mortar Mix Type S | 60 lb | 9 | 11 |
| Sakrete Mortar Mix Type S | 40 lb | 6 | 16 |
| Quikrete Masonry Cement (Type N) + 240 lb mason sand | 70 lb + sand | About 46 | 3 |
Premixed mortar already contains sand, so you just add water. Masonry cement is mixed with sand on site, and Quikrete says one 70 lb bag with 240 lb of mason sand yields about 2.75 cu ft of mortar. Whichever you buy, enter its number in the "Blocks per bag of mortar" field.
Bags of mortar for 50 to 1,000 blocks
8 to 9 bags of 80 lb premixed mortar per 100 standard blocks, since Quikrete rates its Mortar Mix and Mason Mix at up to 13 blocks per bag and Sakrete its Mortar Mix Type S at 12. For bigger jobs, here is each maker's figure, rounded up to whole bags:
| Blocks | Quikrete 80 lb (13 per bag) | Sakrete 80 lb (12 per bag) | Sakrete 60 lb (9 per bag) | Masonry cement 70 lb + sand (30 per bag, NCMA) |
|---|---|---|---|---|
| 50 | 4 | 5 | 6 | 2 |
| 100 | 8 | 9 | 12 | 4 |
| 200 | 16 | 17 | 23 | 7 |
| 300 | 24 | 25 | 34 | 10 |
| 500 | 39 | 42 | 56 | 17 |
| 1,000 | 77 | 84 | 112 | 34 |
The calculator's default of 30 blocks per bag matches NCMA's figure for masonry cement: one 70 lb bag mixed with sand on site generally lays about 30 hollow units with face shell bedding (mortar on the face shells only). NCMA also counts about 240 blocks per ton of sand, so 500 blocks take about 2.1 tons and 1,000 blocks about 4.2 tons. Quikrete rates its own Masonry Cement higher, at about 46 standard blocks per bag with 240 lb of mason sand. Sakrete notes that its yields don't allow for waste and spillage, so buy a bag or two extra.
Blocks per square foot by size
| Nominal face size | Actual face (approx.) | Blocks per sq ft | Blocks per 100 sq ft (no extra) |
|---|---|---|---|
| 8 × 16 in (standard) | 7 5/8 × 15 5/8 in | 1.125 | 113 |
| 4 × 16 in (half-high) | 3 5/8 × 15 5/8 in | 2.25 | 225 |
| 8 × 8 in (half block) | 7 5/8 × 7 5/8 in | 2.25 | 225 |
NCMA notes that standard units have an 8 × 16 in nominal face and come in nominal thicknesses of 4, 6, 8, 10, 12, 14, and 16 in, with other sizes available from local producers. For a brick wall or veneer, the brick calculator uses the Brick Industry Association's bricks per square foot.
Blocks for common wall sizes
Standard 8 × 16 in blocks with 5 percent extra, straight from the calculator:
| Wall (length × height) | Wall face | Blocks with 5% extra |
|---|---|---|
| 10 × 2 ft | 20 sq ft | 24 |
| 20 × 3 ft | 60 sq ft | 71 |
| 20 × 4 ft | 80 sq ft | 95 |
| 30 × 4 ft | 120 sq ft | 142 |
| 24 × 6 ft | 144 sq ft | 171 |
| 40 × 4 ft | 160 sq ft | 189 |
| 50 × 8 ft | 400 sq ft | 473 |
Blocks per linear foot by wall height
0.75 blocks per linear foot for every 8 inch course (12 ÷ 16 in). Count the courses as the wall height in inches ÷ 8, then multiply: a 4 ft wall is 6 courses, or 4.5 blocks per linear foot, and an 8 ft wall is 12 courses, or 9 blocks per linear foot.
| Wall height | Courses | Blocks per linear foot | Blocks per 10 ft of wall | Blocks for 10 ft with 5% extra |
|---|---|---|---|---|
| 16 in | 2 | 1.5 | 15 | 16 |
| 2 ft | 3 | 2.25 | 22.5 | 24 |
| 2 ft 8 in | 4 | 3 | 30 | 32 |
| 3 ft 4 in | 5 | 3.75 | 37.5 | 40 |
| 4 ft | 6 | 4.5 | 45 | 48 |
| 5 ft 4 in | 8 | 6 | 60 | 63 |
| 6 ft | 9 | 6.75 | 67.5 | 71 |
| 8 ft | 12 | 9 | 90 | 95 |
| 10 ft | 15 | 11.25 | 112.5 | 119 |
Multiply blocks per linear foot by the wall length in feet, then add the extra. The last column is what the calculator returns for a 10 ft wall at each height with its default 5 percent extra. A 10 × 10 ft wall (100 sq ft) comes to 119 blocks, which matches NCMA's estimating figure of 119 standard units per 100 sq ft of wall, a count that includes about 5 percent waste.
Grout to fill concrete block cores, by block width
36.1 cu ft of grout (1.34 cu yd) per 100 sq ft of 8 in block wall with every core filled, according to NCMA's estimating table for two-core hollow units, which includes 3 percent waste. If only some cores are grouted, such as the ones that hold vertical rebar, use the column for that spacing:
| Block width (nominal) | Actual width | Every core (cu ft per 100 sq ft) | Core every 16 in | Core every 24 in | Core every 48 in |
|---|---|---|---|---|---|
| 6 in | 5 5/8 in | 25.6 | 12.8 | 8.6 | 4.3 |
| 8 in | 7 5/8 in | 36.1 | 18.1 | 12.1 | 6.1 |
| 10 in | 9 5/8 in | 47.0 | 23.5 | 15.7 | 7.9 |
| 12 in | 11 5/8 in | 58.9 | 29.5 | 19.7 | 9.9 |
Divide by 27 for cubic yards: with every core filled, that's 0.95, 1.34, 1.74, and 2.18 cu yd per 100 sq ft for 6, 8, 10, and 12 in block. For bags, NCMA counts about 0.66 cu ft per 80 lb bag of preblended grout, so an 8 in wall with every core filled takes about 55 bags per 100 sq ft (39, 72, and 90 for 6, 10, and 12 in). Per standard 8 in block that's about 0.32 cu ft (36.1 ÷ 112.5 blocks), or about 2 blocks per bag, while Quikrete rates its 80 lb Core-Fill Grout at 0.65 cu ft, enough for the cores of approximately 3 standard 8 × 8 × 16 blocks. NCMA doesn't recommend grouting standard 4 in units. The calculator doesn't estimate grout, and which cores get grout and rebar comes from your plans or building department.
Tips and common mistakes
- Entering the actual size instead of the nominal size. If you enter 7 5/8 × 15 5/8, the default wall comes out at 102 blocks instead of 95, because the formula would no longer include the mortar joint.
- Trusting a generic mortar number. Premixed mortar lays far fewer blocks per bag than masonry cement mixed with sand. Use the figure on your bag.
- Picking the wrong mortar type. Quikrete Mortar Mix meets Type N, while Quikrete Mason Mix and Sakrete Mortar Mix Type S meet Type S. Sakrete lists its Type S for load-bearing walls and below-grade work. If your plans or building department name a mortar type, use that one.
- Forgetting special units. NCMA describes bond beam units, split-able half units, and bullnose corner units. Walls with open ends, corners, or a bond beam course need some of these, so plan the layout course by course before you order.
- Leaving out the footing and fill. The block count doesn't include footing concrete, grout for filled cores, or rebar. Those come from your plans. Use our concrete calculator for the footing volume, and the rebar calculator for a bar grid in a slab.
- Building a tall retaining wall without a design. Under the 2015 IRC (section R404.4), a retaining wall that isn't supported at the top and holds back more than 48 inches of unbalanced soil must be engineered, and so must one over 24 inches that also resists extra lateral loads. Check with your building department before you start, and hire a pro for structural walls.
Treat these numbers as an estimate for ordering. Footing size, reinforcement, and wall height limits are set by your local code and, for anything structural, by your designer.
Frequently asked questions
How many concrete blocks per square foot?
A standard 8 × 16 in block covers 0.889 sq ft of wall including its mortar joint, so you need 1.125 blocks per square foot, or about 113 per 100 sq ft before extra. Half-high 4 × 16 in blocks take 2.25 per square foot.
How many concrete blocks do I need?
1.125 standard 8 × 16 in blocks per square foot of wall, plus about 5 percent extra. Multiply the wall length by its height in feet, subtract doors and windows, and divide by 0.889 sq ft, the face of one block with its mortar joint. A 20 × 4 ft wall is 80 ÷ 0.889 = 90 blocks, or 95 with 5 percent extra. To check by courses, each 8 inch course takes 0.75 blocks per foot of wall.
How many cinder blocks do I need?
1.125 per square foot of wall, the same as concrete blocks, plus about 5 percent extra: 119 blocks per 100 sq ft, which matches NCMA's estimating figure. The Concrete Masonry & Hardscapes Association calls "cinder block" and "concrete block" essentially the same product, made from water, cement, and aggregate. Many early producers used cinders as a lighter aggregate, some still do, and it's nearly impossible to tell by looking, so the sizes and the count are the same. Enter the wall size and nominal block size in the calculator either way.
How many blocks do I need for a 4 ft high wall?
A 4 ft wall is 6 courses of 8 in blocks, and each foot of wall length takes 0.75 blocks per course, so it's 4.5 blocks per linear foot. A 20 ft run needs 90 blocks, or 95 with 5 percent extra.
How do I calculate how many blocks I need for a wall?
0.75 blocks per foot of wall for each 8 inch course. Divide the wall height in inches by 8 for the number of courses, multiply by 0.75 and by the wall length in feet, then add about 5 percent: a wall 30 ft long and 6 ft high is 9 courses × 0.75 × 30 = 202.5 blocks, or 213 with 5 percent extra, the same as the calculator gives. The area method agrees: 30 × 6 = 180 sq ft ÷ 0.889 = 202.5. Take doors and windows out before you add the extra.
How many bags of mortar do I need for 100 blocks?
It depends on the product. At the makers' figures, 80 lb premixed mortar at 13 blocks per bag needs 8 bags, Sakrete Type S at 12 blocks per 80 lb bag needs 9, and Quikrete Masonry Cement mixed with sand at about 46 blocks per bag needs 3 bags plus the sand.
How many bags of mortar per block?
1 bag of 80 lb premixed mortar for every 12 to 13 standard 8 × 8 × 16 blocks, or about 0.08 bag per block: Quikrete rates its Mortar Mix and Mason Mix at up to 13 blocks per 80 lb bag and Sakrete its Mortar Mix Type S at 12. Sakrete's 60 lb bag lays 9 blocks and its 40 lb bag 6. With masonry cement mixed with sand on site, NCMA figures about 30 hollow units per 70 lb bag, and Quikrete rates its Masonry Cement at about 46 blocks per bag with 240 lb of sand.
What is the actual size of an 8 × 8 × 16 block?
About 7 5/8 × 7 5/8 × 15 5/8 in. NCMA explains that actual (specified) dimensions are typically 3/8 inch less than nominal so the wall keeps an 8 inch module with 3/8 inch mortar joints.
What sizes do concrete blocks come in?
4, 6, 8, 10, 12, 14, and 16 in nominal widths, according to NCMA, all with a standard 8 × 16 in face, and other sizes are available from local producers. Each actual dimension is typically 3/8 in smaller, so an 8 in block measures 7 5/8 in wide and a 12 in block 11 5/8 in. Width doesn't change the count (1.125 standard blocks per square foot), but it changes the grout: 36.1 cu ft per 100 sq ft for an 8 in wall with every core filled versus 58.9 for 12 in. Half-high 4 × 16 in and 8 × 8 in half units take 2.25 per square foot.
How much concrete do I need to fill cinder blocks?
About 0.32 cu ft per standard 8 in block with every core filled, or 36.1 cu ft (1.34 cu yd) per 100 sq ft of 8 in wall, from NCMA's estimating table, which includes 3 percent waste. At about 0.66 cu ft per 80 lb bag of preblended grout, that's about 55 bags per 100 sq ft, or about 2 blocks per bag; Quikrete rates its 80 lb Core-Fill Grout at the cores of approximately 3 standard blocks. Use a masonry grout made for cores, such as a core-fill grout that meets ASTM C476, and fill the cores your plans call for.
Do I need an engineer for a block retaining wall?
Often, yes. The 2015 IRC (section R404.4) requires retaining walls that aren't supported at the top and hold back more than 48 inches of unbalanced soil to be designed by accepted engineering practice. The limit drops to 24 inches when the wall also resists lateral loads beyond the soil, such as vehicles parked above it or a fence on top that catches wind. Your local code may differ, so ask your building department.
What type of mortar should I use for a block wall?
Quikrete Mortar Mix meets Type N, while Quikrete Mason Mix and Sakrete Mortar Mix Type S meet Type S, and Sakrete lists its Type S for load-bearing walls and below-grade work. If your plans or building department name a mortar type, use that one.
Sources
- NCMA TEK 02-01B: Typical Sizes and Shapes of Concrete Masonry Units (PDF)
- QUIKRETE Mortar Mix No. 1102: Product Data Sheet (PDF)
- QUIKRETE Mason Mix No. 1136: Product Data Sheet (PDF)
- QUIKRETE Masonry Cement (Type N): Product Data Sheet (PDF)
- Sakrete Mortar Mix Type S: Technical Data Sheet (PDF)
- ICC: 2015 IRC Significant Changes, R404.4 Retaining Walls (PDF)
- NCMA TEK 04-02A: Estimating Concrete Masonry Materials (CMHA)
- CMHA: What Is the Difference Between a Cinder Block and a Concrete Block?
- QUIKRETE Core-Fill Grout Coarse No. 1585-07: Product Data Sheet (PDF)
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