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Concrete Block Calculator — Blocks, Mortar and Fill

Work out how many concrete or cinder blocks a wall needs, plus mortar bags, core fill, rebar and wall weight. Subtract openings, results as you type.

Cinder block and concrete block are the same thing to a supplier today, so this works for either.

Openings

Subtract doors, windows and gateways.

%

Blocks chip and crack. 5% is normal, 10% for a first wall.

Adjust prices
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These are rough national-average starting points, not quotes. Last updated October 1, 2026.

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Enter your wall size to see blocks, mortar, fill and weight.
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Blocks Needed by Wall Size

8 in block, 5% waste, no openings. Mortar bags are 80 lb. Tap any cell to load it into the form.

Wall length4 ft high6 ft high8 ft high
10 ft
20 ft
30 ft
40 ft
50 ft
100 ft

Block Size Reference

Nominal sizeActual sizeWeight (normal)Weight (lightweight)Core fill per blockTypical use
4 × 8 × 16 in3⅝ × 7⅝ × 15⅝ in26 lb19.5 lb0.11 cu ftVeneer facing and light partition walls
6 × 8 × 16 in5⅝ × 7⅝ × 15⅝ in31 lb23.25 lb0.17 cu ftGarden walls and partitions
8 × 8 × 16 in7⅝ × 7⅝ × 15⅝ in38 lb28.5 lb0.26 cu ftThe standard for foundations and structural walls
10 × 8 × 16 in9⅝ × 7⅝ × 15⅝ in45 lb33.75 lb0.35 cu ftRetaining walls and taller structural work
12 × 8 × 16 in11⅝ × 7⅝ × 15⅝ in52 lb39 lb0.45 cu ftHeavy retaining walls and tall structural walls

How to calculate concrete blocks for a wall

Every block count starts from one constant, 1.125 blocks per square foot. A standard block has a nominal face of 8 × 16 in, which is 128 sq in or 0.889 sq ft, and 1 ÷ 0.889 = 1.125. Multiply the wall area by that figure and you have the number of blocks laid, before any breakage.

Nominal size and actual size differ on purpose. A block sold as 8 × 8 × 16 really measures 7⅝ × 7⅝ × 15⅝ in, which is ⅜ in short in each direction. The missing ⅜ in is the mortar joint, so once a block is bedded the module is a clean 8 × 16 in. That is why the maths uses nominal dimensions and why a block is 15⅝ in long, not 16 in.

Here is a full worked example. A wall is 20 ft × 8 ft, so the gross area is 160 sq ft. A gateway of 3 × 7 ft removes 21 sq ft, leaving 139 sq ft net. Then 139 × 1.125 = 156.4, which rounds up to 157 blocks, and a 5% allowance takes the order to 165.

Courses and blocks per course are the cross-check. A course is 8 in tall, so an 8 ft wall is 96 ÷ 8 = 12 courses. Each block takes 16 in along the wall, so 20 ft is 240 ÷ 16 = 15 blocks per course and 12 × 15 = 180 for the full face. Both figures round up, because a wall 20 ft 4 in long still needs a sixteenth block in every course, and a partial course still means one more row to cut and lay.

Mortar is bought by the bag. Allow roughly 3 bags of 80 lb pre-mixed mortar per 100 blocks, so the 189-block wall above needs 6 bags. Mixing from scratch needs about 9 cubic feet of sand per 100 blocks, plus masonry cement in roughly a one to three ratio by volume. Buy the extra bag, because a joint that runs dry is harder to fix than one that runs over.

Core fill is concrete or grout poured into the hollow cores so a wall can carry load or hold reinforcing steel. An 8 in block holds about 0.26 cubic feet, so 104 blocks fill one cubic yard. The 180-block face grouted solid takes 180 × 0.26 = 46.8 cu ft, or 1.73 cubic yards, and you can convert that pour to cubic yards and bags to order it. Grouting only the reinforced cores at 32 in spacing fills 16 ÷ 32 of them, so the same wall needs 0.87 cubic yards. Solid grouting is normally specified for retaining and structural walls, while garden walls usually grout only the cores holding a bar.

Weight matters for what sits underneath. The unfilled face weighs 180 × 38 = 6,840 lb, about 3.4 tons, and solid grout adds roughly 7,000 lb. For any weight in tons, kg or trailer loads, use the concrete weight in lb, tons and kg. A wall that heavy needs a footing at least twice as wide as the wall is thick and as deep as the wall is thick, set below the frost line, so for an 8 in wall that is 16 in wide and 8 in deep at minimum. You can size that footing for your frost zone and add its rebar and excavation.

Rebar follows a common pattern: vertical bars in grouted cores at 32 in centres, lapped 24 in at the footing, with horizontal bond-beam steel every fourth course. For the 20 ft wall that is 8 bars × 10 ft = 80 ft vertically, and 3 runs × 20 ft = 60 ft horizontally, or 7 lengths of 20 ft bar. Local code and an engineer set the actual spacing for any structural wall.

Cost is the last step. Blocks at $2.20 and mortar at $7.50 a bag put the same wall at about $461 in materials, or $2.88 per sq ft of face. That excludes the footing, fill, steel, tools and labour, so price the whole project line by line before you commit to a budget.

How to use this calculator

  1. 01Choose By wall size if you know the dimensions, or By block count if a plan or quote already gives you a number. Pick the block width and weight.
  2. 02Enter the wall length and height with a unit for each. Mixed units are fine, so a length in metres and a height in feet works.
  3. 03Tap + Add opening for every door, window or gateway and enter its width and height. Each opening is subtracted from the wall area before blocks are counted.
  4. 04Set the waste allowance, then tick Fill cores or Rebar if the wall is grouted or reinforced and choose the spacing.
  5. 05Read the blocks, mortar, fill, rebar and weight, which update as you type. Open the Adjust prices panel to match your local block, mortar, fill and rebar costs.

Common questions

How many concrete blocks do I need per square foot?
A standard block has a nominal face of 8 × 16 in, which is 0.889 sq ft once the mortar joint is included, so a wall needs 1.125 blocks per square foot. Multiply the wall area by 1.125, subtract any doors and windows first, then add 5% for breakage. A 100 sq ft wall therefore needs 113 blocks laid and about 119 to buy.
How many blocks are in a 20 foot wall?
Each course is 15 blocks long, because 20 ft is 240 in and each block takes 16 in with its joint. A wall 4 ft high is 6 courses, or 90 blocks laid and 95 with 5% waste. At 6 ft it is 9 courses and 135 blocks, and at 8 ft it is 12 courses and 180 blocks, or 189 with waste.
What is the difference between a cinder block and a concrete block?
Cinder blocks were made with coal cinders as the aggregate, which made them light but weaker. Modern blocks use Portland cement, sand and gravel or lightweight stone, and meet ASTM C90 strength standards. Suppliers sell only concrete blocks today, so a request for cinder blocks gets you the same 8 × 8 × 16 unit.
How many bags of mortar do I need per 100 blocks?
Plan on about 3 bags of 80 lb pre-mixed mortar per 100 blocks laid. A 189-block wall therefore needs 6 bags. If you mix from scratch, allow roughly 9 cubic feet of sand per 100 blocks plus masonry cement in a ratio of about 1 part to 3 of sand by volume.
How much concrete does it take to fill a block core?
An 8 in block holds about 0.26 cubic feet of fill across its cores, so roughly 104 blocks fill one cubic yard. A 4 in block holds 0.11 cu ft, a 6 in block 0.17, a 10 in block 0.35 and a 12 in block 0.45. Fill only the reinforced cores and the volume drops in proportion, for example to half at 32 in spacing.
Do I need to fill concrete block cores with concrete?
Not always. Cores are grouted where a bar passes through them, at wall ends, and along openings, and a retaining or structural wall is often specified fully grouted. Many freestanding garden walls fill only the reinforced cores, which cuts the grout volume by half or more at 32 in spacing. Local code and any engineer's drawing decide it.
How much does an 8 inch concrete block weigh?
A normal-weight 8 × 8 × 16 block weighs about 38 lb, and the lightweight version about 28.5 lb. Filling its cores adds around 39 lb of grout, since 0.26 cu ft of concrete weighs roughly 150 lb per cubic foot. A 20 × 8 ft wall of 180 blocks therefore weighs about 3.4 tons unfilled.
What are the actual dimensions of an 8x8x16 block?
An 8 × 8 × 16 block is a nominal size that includes the mortar joint. The unit itself measures 7⅝ × 7⅝ × 15⅝ in, so a ⅜ in joint brings each block back to a clean 8 × 16 in module. That is why courses stack to a multiple of 8 in and the layout works without cutting.
How much does a concrete block wall cost?
Materials for a 20 × 8 ft wall in 8 in block come to roughly $460: 189 blocks at $2.20 is $416 and 6 bags of mortar is $45. That is about $2.90 per sq ft of wall face before fill, rebar and footing. Hired and installed, a block wall commonly runs $15 to $35 per sq ft, depending on region, height and grouting.
Does a concrete block wall need rebar?
Most structural and retaining walls do, and freestanding walls above about 4 ft usually need it too. A common pattern is a vertical bar in grouted cores at 32 in centres, lapped 24 in at the footing, with a horizontal bar in a bond beam every fourth course. Spacing is set by local code and an engineer for anything structural.
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Quantities are planning estimates. Block, mortar and fill prices are user-editable national-average defaults rather than quotes. Rebar spacing, footing size and whether cores must be grouted are set by local building code, and by an engineer for any structural or retaining wall, not by this tool. Last updated: October 1, 2026.