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Concrete Block Calculator

Count the concrete blocks (CMU) for a garden wall, foundation or shed base wall. Enter the wall size and any openings to get blocks, courses, bags of mortar and, if the cores will be grouted, how much grout to buy.

Units

Each course of 8 in blocks adds 8 in (with the mortar joint).

Width × height of each opening, added up. 0 = none.

Mortar and core fill

All four have the same 8×16 in face, so the block count is the same. Width only changes the core fill.

80 lb bag of pre-mixed mortar: about 13 blocks (Quikrete). 70 lb masonry cement mixed with sand: about 30 (NCMA).

Reinforced walls are grouted where the rebar runs. Your plans or local code set the spacing.

Allowances

Extra for broken and cut blocks. 5% is typical.

Cost estimate (optional)

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Blocks needed

95

Net wall area
80ft²
Before waste
90
Courses (rows)
6
Blocks per course
15

Mortar and core fill

Bags of mortar
8

Cost estimate

Enter your local prices under Cost estimate to see totals.

20 ft4 ft8 × 8 × 16 in blocks, running bond

How to use the concrete block calculator

  1. Enter the wall length and height. For several walls, add their lengths together if they are the same height.
  2. Add up the area of doors and windows (width × height of each) and enter the total.
  3. Choose the block width. 6, 8, 10 and 12 in blocks all have the same 8 × 16 in face, so the block count is the same; the width only changes the core fill.
  4. Check Blocks per bag of mortar: about 13 for an 80 lb bag of pre-mixed mortar, about 30 for a 70 lb bag of masonry cement mixed with sand.
  5. If your plans call for grouted cores, choose the spacing under Core fill.

How many blocks are in a square foot?

A standard concrete block is nominally 8 × 8 × 16 in: 7⅝ × 7⅝ × 15⅝ in actual, plus a ⅜ in mortar joint. Its face covers 8 × 16 = 128 in², or 0.889 ft², so you need 1.125 blocks per square foot of wall (NCMA lists 112.5 per 100 ft²).

  • Blocks = (wall length × height − openings) × 1.125 × (1 + waste), rounded up
  • Mortar bags = blocks ÷ blocks per bag, rounded up
  • Grout (ft³) = net wall area ÷ 100 × the NCMA TEK 4-2A table value for your block width and grout spacing
  • Courses = wall height in inches ÷ 8; blocks per course = wall length in inches ÷ 16

Worked example: a 20 ft × 4 ft wall

This is the wall loaded above, with no openings and 5% waste.

  1. Wall area: 20 × 4 = 80 ft².
  2. Blocks: 80 × 1.125 = 90; with 5% waste, 94.5, rounded up to 95 blocks.
  3. Layout: 6 courses of 15 blocks.
  4. Mortar: 95 ÷ 13 = 7.3, so 8 bags of 80 lb pre-mixed mortar.

NCMA's own figure of 119 units per 100 ft² already includes 5% for waste and breakage, which matches this result.

Mortar: why the bag count varies

Mortar is sold two ways, and they cover very different numbers of blocks. Quikrete's data sheets say an 80 lb bag of pre-mixed Mortar Mix or Mason Mix lays up to 13 standard 8 × 8 × 16 blocks (or 37 bricks). A 70 lb bag of masonry cement, mixed on site with sand, lays roughly 30 hollow units according to NCMA's estimating guide, because the sand you add makes up most of the volume. Set Blocks per bag to match the product you will buy.

Blocks and mortar for common wall sizes

Wall (length × height)Wall areaBlocks (no waste)Blocks + 5%80 lb mortar bags
10 × 2 ft20 ft²23242
10 × 4 ft40 ft²45484
20 × 3 ft60 ft²68716
20 × 4 ft80 ft²90958
20 × 6 ft120 ft²13514211
30 × 4 ft120 ft²13514211
40 × 4 ft160 ft²18018915
50 × 6 ft300 ft²33835528

8×8×16 in blocks, 1.125 per ft². Mortar at about 13 blocks per 80 lb bag of pre-mixed mortar.

Block wall tips

  • Start on a proper footing. Block walls need a level concrete footing, sized to your local code and set below the frost line. The concrete calculator's footing option gives the yards and bags.
  • Taller and retaining walls need engineering. Walls over about 4 ft, retaining walls and walls that carry a load usually need steel reinforcement, grouted cores and a permit. Your plans or local code set the bar size and grout spacing.
  • Plan heights in 8 in steps. Each course adds 8 in. A wall height that isn't a multiple of 8 in means cutting a course or adding a cap block.
  • Don't forget the extras. Corner blocks, half blocks, bond-beam blocks and cap blocks are bought separately; count them from your drawing.

Frequently asked questions

How many concrete blocks do I need for a wall?

Multiply the wall area in square feet by 1.125, then add about 5% for waste. A 20 × 4 ft wall (80 ft²) needs 90 blocks, or 95 with waste.

How many blocks are in a square foot?

1.125 standard 8 × 8 × 16 in blocks per square foot of wall face, or 112.5 per 100 ft² (about 119 including 5% waste, as NCMA lists it).

How many blocks does a bag of mortar lay?

About 13 standard blocks per 80 lb bag of pre-mixed mortar (Quikrete), or around 30 hollow units per 70 lb bag of masonry cement mixed with sand (NCMA).

How much grout do I need to fill block cores?

For an 8 in wall with every cell filled, NCMA's table gives about 36 ft³ of grout per 100 ft² of wall (with 3% waste). Grouting every 48 in takes about 6 ft³ per 100 ft². An 80 lb bag of pre-blended grout yields about 0.66 ft³.

What size is a standard concrete block?

Nominally 8 × 8 × 16 in; actually 7⅝ × 7⅝ × 15⅝ in, so that with a ⅜ in mortar joint each block takes up exactly 8 × 16 in of wall face.

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