Concrete Block Calculator (Cinder Block, CMU)
Count concrete blocks (cinder block, CMU), mortar bags, mason sand and grout fill for a wall. Subtract openings, add waste. Rates from Quikrete and CMHA.
Results
Blocks in the wall
135blocks
Order with waste
142blocks
Blocks per course
15blocks
Courses
9rows
Net wall area
120sq ft
Mortar bags
12x 80 lb
Blocks in the wall counts whole blocks per course times courses, less 1.125 blocks per sq ft of openings (rounded down). Mortar and grout are figured on the blocks laid, not the spares. Pre-mix: up to 13 blocks per 80 lb bag (Quikrete Mason Mix data sheet). Bag yields are the manufacturers' figures for standard 8 in block, so treat 6 and 12 in mortar counts as estimates.
Rates last checked . Sources are listed below.
How many concrete blocks do I need?
A standard 8x8x16 concrete block covers 0.89 sq ft of wall, so you need 1.125 blocks per square foot, or 1,125 blocks for 1,000 sq ft before waste. Cinder block, concrete block and CMU (concrete masonry unit) mean the same thing here.
Worked example: a 20 ft wall 6 ft high takes 240 ÷ 16 = 15 blocks per course and 72 ÷ 8 = 9 courses, so 135 blocks. With 5% waste you order 142. Laid in 80 lb pre-mixed mortar that is 12 bags (135 ÷ 13 × 1.10, rounded up). In 70 lb masonry cement it is 4 bags and 960 lb of sand.
| Wall | Face area | Blocks | With 5% waste | 80 lb pre-mix | 70 lb cement + sand | Grout, every cell |
|---|---|---|---|---|---|---|
| 10 ft × 4 ft | 40 ft² | 48 | 51 | 5 bags | 2 bags + 480 lb | 0.62 yd³ |
| 10 ft × 6 ft | 60 ft² | 72 | 76 | 7 bags | 2 bags + 480 lb | 0.93 yd³ |
| 20 ft × 4 ft | 80 ft² | 90 | 95 | 8 bags | 3 bags + 720 lb | 1.16 yd³ |
| 20 ft × 6 ft | 120 ft² | 135 | 142 | 12 bags | 4 bags + 960 lb | 1.75 yd³ |
| 40 ft × 4 ft | 160 ft² | 180 | 189 | 16 bags | 5 bags + 1,200 lb | 2.33 yd³ |
| 40 ft × 6 ft | 240 ft² | 270 | 284 | 23 bags | 7 bags + 1,680 lb | 3.49 yd³ |
| 60 ft × 4 ft | 240 ft² | 270 | 284 | 23 bags | 7 bags + 1,680 lb | 3.49 yd³ |
| 60 ft × 6 ft | 360 ft² | 405 | 426 | 35 bags | 10 bags + 2,400 lb | 5.24 yd³ |
Mortar and grout in the table include the calculator's 10% allowance and are figured on the blocks laid. Grout is for 8 in block. Counts round each course and the number of courses up, so a 10 ft wall (7.5 blocks per course) comes out above the per-square-foot figure.
Add each door or window to the calculator. It removes 1.125 blocks per sq ft of opening, rounded down, and leaves the cut blocks around the opening in the count. A 20 ft by 8 ft wall with one 3 x 7 ft door is 180 blocks less 23 for the door (21 sq ft × 1.125 = 23.625, rounded down), so 157 blocks.
Mortar for a block wall: pre-mix vs masonry cement
Bag counts depend on which bag you buy. Pre-mixed mortar already has the sand in it. Masonry cement is mixed on site with mason sand, so one 70 lb bag makes far more mortar than one 80 lb bag of pre-mix. A rate of about 3 bags per 100 blocks fits masonry cement plus sand. Applied to pre-mix, it runs out before half the wall is laid.
| Bag | Blocks per bag | Bags per 100 blocks | Bags per 1,000 blocks | Sand | Source |
|---|---|---|---|---|---|
| QUIKRETE Mason Mix, 80 lb pre-mixed | 13 | 7.7 | 77 | None, sand is in the bag | Mason Mix No. 1136 data sheet |
| QUIKRETE Masonry Cement, 70 lb | 46 | 2.2 | 22 | 240 lb mason sand per bag | Masonry Cement data sheet |
| Sakrete Type N Masonry Cement, 70 lb | 40 | 2.5 | 25 | Mixed with sand per bag directions | Sakrete use guide |
Figures are for standard 8x8x16 block, before waste, with bags per 1,000 rounded up. For QUIKRETE masonry cement, 1,000 blocks takes 22 bags and 5,280 lb (2.64tons) of sand. The manufacturers publish these yields for 8 in block only, so the calculator's 6 and 12 in mortar counts are estimates.
How much block fill (grout) do I need?
For 8 in block with every cell grouted, about 0.32 cu ft per block, or 1.18cubic yards per 100 blocks before waste. The rate comes from CMHA Tech Note CMU-TEC-002-23, Table 1: at 125 pcf units a fully grouted 8 in wall weighs 84 psf and a hollow one 34 psf. The 50 psf difference, divided by the note's grout density of 140 lb per cu ft, is 0.357 cu ft of grout per sq ft of wall. One 8x16 block face is 0.889 sq ft, so 0.357 × 0.889 = 0.317 cu ft per block.
| Grouted cells | Grout, cu ft per 100 sq ft | Grout, yd³ per 100 blocks | Added weight, 8 / spacing rule | Added weight, CMHA table |
|---|---|---|---|---|
| Every cell | 35.7 | 1.18 | 50.0 psf | 50 psf |
| 16 in on center | 17.9 | 0.59 | 25.0 psf | 25 psf |
| 24 in on center | 11.9 | 0.39 | 16.7 psf | 17 psf |
| 32 in on center | 8.9 | 0.29 | 12.5 psf | 12 psf |
| 48 in on center | 6.0 | 0.20 | 8.3 psf | 8 psf |
Partial grouting scales by 8 ÷ spacing in inches, and the CMHA table's partial rows land within 1 psf of that rule, as the last two columns show. Grout is figured before waste here; the calculator adds your allowance for grout lost into webs and joints. The CMHA table covers 8 in walls only, so the calculator does not estimate grout for 6 or 12 in block. To convert a grout volume to yards for a ready-mix order, use the Cubic Yard Calculator.
Reinforcement: ladder wire and rebar
Both are optional in the calculator and off by default. Bar size and spacing, and whether a wall needs joint reinforcement at all, are set by the engineer or the local building code. The figures below show how the calculator counts material once you know the spacing.
Joint reinforcement is welded wire (ladder or truss pattern) laid in the bed joints. CMHA TEK 12-02B gives 16 in on center as the typical spacing, which is every other course of 8 in block, and a minimum splice of 6in. Hohmann & Barnard sells it in 10 ft standard lengths. The calculator counts the bed joints between courses at each spacing step and runs wire the full wall length in each one. Pieces per joint = (wall length − 0.5 ft) ÷ 9.5 ft, rounded up, because each lapped piece adds 9.5 ft. On the 20 ft by 6 ft wall:
| Wire spacing | Reinforced joints | Linear feet | 10 ft pieces | Laps at 6 in |
|---|---|---|---|---|
| Every course (8 in) | 8 | 160 | 24 | 16 |
| Every other course (16 in) | 4 | 80 | 12 | 8 |
| Every third course (24 in) | 2 | 40 | 6 | 4 |
Vertical bars go in cores that are then grouted. The calculator places a bar at each end of the wall and one at each spacing step: bars = wall length in inches ÷ spacing, rounded up, plus 1. CMHA TEK 14-18B lists 120 in as the maximum vertical bar spacing for detailed plain and ordinary reinforced shear walls, and 48 in for intermediate and special reinforced shear walls. Each bar is the wall height plus one lap. The default lap is the minimum in CMHA TEK 12-06A for the 2009 and 2012 IBC: 12 in or 40 bar diameters, whichever is greater (20 in for #4, 25 in for #5). That is a floor. If your plans show a longer lap, enter it. On the 20 ft by 6 ft wall with #4 bars:
| Bar spacing | Bars | Length per bar | Total rebar |
|---|---|---|---|
| 24 in on center | 11 | 7.67 ft | 84.3 ft |
| 32 in on center | 9 | 7.67 ft | 69.0 ft |
| 48 in on center | 6 | 7.67 ft | 46.0 ft |
The cores with bars are grouted. If the grout setting is none, or rebar cells only, the calculator grouts one cell per bar in every course at half a block's fill: 6 bars × 9 courses × 0.159 cu ft = 8.6 cu ft before the allowance at 48 in. If you also pick a grout spacing, bars that fall between the grouted cells add their cells to the total. Order bars in stock lengths with the Rebar Calculator. For the footing under the wall, the Concrete Calculator works out the yards.
Concrete block sizes
| Nominal (W x H x L) | Actual size | Common use |
|---|---|---|
| 6 x 8 x 16 in | 5 5/8 x 7 5/8 x 15 5/8 in | Partition walls, veneer backup |
| 8 x 8 x 16 in | 7 5/8 x 7 5/8 x 15 5/8 in | Foundation, garden and garage walls |
| 12 x 8 x 16 in | 11 5/8 x 7 5/8 x 15 5/8 in | Taller or more heavily loaded walls |
Each actual dimension is 3/8 in less than nominal to leave room for the mortar joint. Block also comes in 4 and 10 in widths with the same 8x16 face, so the block count above applies to them too. For a segmental wall that holds back soil, use the Retaining Wall Calculator instead.
How the concrete block calculator works
A block wall is a grid: blocks across the length times courses up the height. Nominal block dimensions include the 3/8 in mortar joint, so no separate joint allowance is needed.
- Blocks per course = wall length (in) ÷ 16, rounded up. Courses = wall height (in) ÷ 8, rounded up.
- Blocks in the wall = blocks per course × courses − (opening area in sq ft × 1.125, rounded down).
- Blocks to order = blocks in the wall × (1 + waste% ÷ 100), rounded up.
- Mortar bags = blocks in the wall ÷ blocks per bag × (1 + allowance% ÷ 100), rounded up. 13 blocks per 80 lb pre-mix bag; 46 per 70 lb masonry cement bag.
- Mason sand (masonry cement only) = mortar bags × 240 lb. Tons = lb ÷ 2,000.
- Grout (8 in block) = blocks in the wall × 0.317 cu ft × (8 ÷ cell spacing in inches) × (1 + allowance% ÷ 100). Cubic yards = cu ft ÷ 27.
- Joint reinforcement (optional): reinforced joints = (courses − 1) × 8 ÷ wire spacing, rounded down. Linear feet = joints × wall length. Pieces per joint = (wall length − 0.5) ÷ 9.5, rounded up, for 10 ft pieces lapped 6 in. Laps = joints × (pieces per joint − 1).
- Vertical rebar (optional): bars = wall length (in) ÷ bar spacing, rounded up, + 1. Length per bar = wall height + lap. Total = bars × length per bar.
- Rebar cells (8 in block): each bar not already in a grouted cell adds bars × courses × 0.159 cu ft × (1 + allowance% ÷ 100) of grout.
- Material cost, if you enter prices = blocks to order × block price + bags × bag price + sand tons × sand price + grout yards × grout price.
Rates and their sources are listed below. See the methodology page for how CalcRafter checks its calculators.
The mason sand for masonry cement can be ordered in bulk; the Sand Calculator converts it to cubic yards and tons.
Sources
- QUIKRETE Mason Mix No. 1136 data sheet (80 lb pre-mixed mortar, up to 13 blocks per bag)
- QUIKRETE Masonry Cement data sheet (70 lb bag + 240 lb sand, about 46 blocks)
- Sakrete Type N Masonry Cement use guide (about 40 blocks per bag)
- CMHA TEK 12-02B, Joint Reinforcement for Concrete Masonry (typical 16 in spacing, 6 in minimum splice)
- Hohmann & Barnard 220 Ladder-Mesh product page (10 ft standard length)
- CMHA TEK 12-06A, Splices, Development and Standard Hooks for Concrete Masonry (lap not less than 12 in or 40 bar diameters)
- CMHA TEK 14-18B, Seismic Design and Detailing Requirements for Masonry Structures (maximum vertical bar spacing)
- CMHA Tech Note CMU-TEC-002-23, Weights and Section Properties of Concrete Masonry Assemblies (Table 1)
Frequently asked questions
About 1.18 cubic yards (31.7 cu ft) for 8 in block with every cell grouted, before waste. CMHA Tech Note CMU-TEC-002-23 shows a fully grouted 8 in wall weighing 50 psf more than the hollow wall (84 vs 34 psf), and the note takes grout at 140 lb per cu ft. 50 ÷ 140 = 0.357 cu ft per sq ft of wall, or 0.317 cu ft per block. Grouting every other cell (16 in on center) takes about half: 0.59 cubic yards per 100 blocks.
1,125 standard 8x8x16 blocks (1,000 × 1.125), or 1,182 with 5% waste. Subtract door and window openings first, at 1.125 blocks per sq ft of opening.
About 22 bags of 70 lb masonry cement (1,000 ÷ 46 = 21.7, rounded up), mixed with 5,280 lb (2.64 tons) of mason sand at 240 lb per bag. That uses QUIKRETE's yield of about 46 standard blocks per bag. Sakrete's Type N sheet gives about 40 blocks per bag, which is 25 bags. With 80 lb pre-mixed mortar (up to 13 blocks per bag) it is 77 bags. Add about 10% for waste.
Work it course by course. Blocks per course = wall length in inches ÷ 16, rounded up. Courses = wall height in inches ÷ 8, rounded up. Multiply the two. A 20 ft by 6 ft wall is 240 ÷ 16 = 15 blocks per course and 72 ÷ 8 = 9 courses, so 135 blocks. Take out 1.125 blocks per sq ft of openings, then add 5% waste: 142 to order.
For ordering, yes. CMU stands for concrete masonry unit. Older cinder blocks used coal cinders as aggregate; blocks sold today under that name are concrete block made with sand, gravel or lightweight aggregate. The sizes and the counts on this page are the same for all three names.
Nominally 8 x 8 x 16 inches. Measure one and you get 7 5/8 x 7 5/8 x 15 5/8. The missing 3/8 in on each face is the mortar joint, so the nominal size already includes the mortar. Use the nominal size when you figure a wall.
1.125. One block covers 16 × 8 = 128 sq in nominal, and a square foot is 144 sq in, so 144 ÷ 128 = 1.125. The 6, 8 and 12 in blocks all have the same 8x16 face, so the count is the same for each.
It depends on the bag. 80 lb pre-mixed mortar lays up to 13 standard blocks per bag (QUIKRETE Mason Mix), so 100 ÷ 13 = 7.7 bags, about 8. 70 lb masonry cement mixed with 240 lb of sand lays about 46 blocks (QUIKRETE), so 100 ÷ 46 = 2.2 bags, about 3, plus 720 lb of sand for 3 bags. Sakrete's Type N sheet gives about 40 blocks per bag, or 2.5 bags per 100. Add about 10% for waste.
About 5% on a straight hollow CMU wall. The thin webs chip in handling and crack when cut. On the 135-block wall in the table that is 7 extra, 142 in all. A wall with many corners, openings or cut courses can need 10%.
Because each course is counted in whole blocks. A 10 ft wall is 120 inches, or 7.5 blocks, so each course takes 8 and the last one gets cut. At 4 ft high that is 48 blocks against 45 from the per-square-foot rule.
Bars = wall length in inches ÷ bar spacing, rounded up, plus 1, and each bar is the wall height plus any lap. A 20 ft by 6 ft wall with #4 bars at 48 in on center takes 240 ÷ 48 = 5, plus 1, so 6 bars. With the 20 in minimum lap from CMHA TEK 12-06A (40 bar diameters for #4), each bar is 7.67 ft and the total is 46.0 ft. At 32 in on center it is 9 bars and 69.0 ft. The engineer or local code sets the bar size, spacing and lap, and the cores with bars are grouted.
CMHA TEK 12-02B gives 16 in on center as the typical spacing for joint reinforcement, which is every other course of 8 in block. On a 20 ft by 6 ft wall (9 courses) that is 4 reinforced joints and 80 ft of wire, or 12 pieces of the 10 ft standard length with 8 laps of at least 6 in. Your plans or local code may call for wire in every course or at 24 in.
No. Segmental retaining wall systems use their own block, often about 18 in wide with a separate cap unit, stacked without mortar. Concrete block (CMU) is laid in mortar for foundation walls, garden walls and general masonry. For a segmental wall holding back a slope, use the Retaining Wall Calculator.
Add up blocks to order × price per block, mortar bags × price per bag, sand by the ton if you mix masonry cement, grout by the cubic yard, rebar, the footing, and labor. Block and bag prices vary by region and supplier, so enter your own prices in the calculator for the material total.
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