Quick answer: a standard 8 × 8 × 16 inch concrete block covers 0.89 square feet of wall (8 × 16 = 128 in²) once you count its mortar joint, so you need 1.125 blocks per square foot of wall. A wall 20 ft long and 4 ft high is 80 ft², which is 90 blocks. Add about 5% for breakage and cuts, and plan roughly 7 bags of 80 lb mortar mix for that wall.
For your wall, use the Concrete Block Calculator and the Mortar Calculator. Here is how the numbers work, so you can check them by hand.
Block sizes: nominal vs actual
The Concrete Masonry & Hardscapes Association (CMHA) explains that concrete masonry units typically have nominal face dimensions of 8 inches high by 16 inches long, in thicknesses of 4, 6, 8, 10, 12, 14 or 16 inches. Nominal dimensions include the mortar joint. The specified (actual) dimensions are usually 3/8 inch less than nominal, so that an 8 inch module is kept with 3/8 inch mortar joints. That makes a nominal 8 × 8 × 16 block about 7⅝ × 7⅝ × 15⅝ inches in reality.
For planning, always use the nominal face size, because it already includes the joint:
Nominal face area = 8 in × 16 in = 128 in² = 0.889 ft²
Blocks per ft² of wall = 144 ÷ 128 = 1.125
Half-height blocks (4 inch tall) and other sizes exist, and CMHA notes that local manufacturers can tell you which sizes are available in your area.
The formula
Wall area (ft²) = length (ft) × height (ft)
Blocks = wall area × 1.125
Courses (rows) = height (in) ÷ 8
Blocks per course = length (in) ÷ 16
Subtract the area of door and window openings, then add an allowance (see below).
Worked example: a 20 × 4 ft wall
- Area: 20 × 4 = 80 ft².
- Blocks: 80 × 1.125 = 90 blocks.
- Check by counting: height 48 in ÷ 8 = 6 courses, and length 240 in ÷ 16 = 15 blocks per course. 6 × 15 = 90. ✔
- Add 5% for breakage and cuts: 90 × 1.05 = 94.5, so order 95 blocks.
Blocks for common wall sizes
| Wall (length × height) | Wall area | Blocks (exact) | With 5% extra |
|---|---|---|---|
| 10 × 3 ft | 30 ft² | 34 | 36 |
| 20 × 4 ft | 80 ft² | 90 | 95 |
| 30 × 6 ft | 180 ft² | 203 | 213 |
| 40 × 8 ft | 320 ft² | 360 | 378 |
Per 100 ft² of wall you need 112.5 blocks.
Walls that are not a multiple of 16 inches long or 8 inches high end with cut blocks. Plan around those numbers if you can: a 10 ft wall is 7.5 blocks per course, which means a half block in every row.
Why add 5% extra
CMHA’s unit specification says ASTM allows up to 5% of a shipment to have defects such as chips larger than 1 inch on a finished face, cracks, breakage, or dimensions outside tolerance. Add cuts at corners and ends, and 5% is a sensible minimum. If you want matching blocks for later repairs, ask your supplier about keeping a few extra from the same batch.
How much mortar?
QUIKRETE’s data sheet for its Mortar Mix says each 80 lb bag will lay up to 13 standard 8 × 8 × 16 inch blocks (or up to 37 standard bricks). It is a pre-blended Type N mortar.
Mortar bags = blocks ÷ 13, rounded up
- 90 blocks ÷ 13 = 6.9, so 7 bags.
- 95 blocks ÷ 13 = 7.3, so 8 bags if you order for the extra blocks.
QUIKRETE gives approximately 5 quarts of water per 80 lb bag, a mixing time of at least 5 minutes, and a mortar bed about ½ to ¾ inch thick. Tool the joints when they are “thumbprint hard” to make them watertight. Mortar yield depends on how thick you spread it and how much you waste, so buy an extra bag or two if you are learning.
Don’t forget the base
A wall needs a proper footing and a level, compacted base, and the stone for the base is a separate calculation. Use the Crushed Stone Calculator for base material and the Concrete Calculator for a footing. Structural, retaining and tall walls need engineered footings and reinforcement, which this guide does not cover. A short garden wall and a retaining wall are not the same job.
What the count doesn’t include
- Reinforcement and grout. Walls with rebar and grouted cells need extra material.
- Caps and corner units. Cap blocks, bond beam units and sash block are counted separately.
- Waste around openings. If your wall has a door or window, subtract the opening and add some for cut blocks at the sides.
Common mistakes
- Using actual size instead of nominal. The 7⅝ inch actual size is smaller than the 8 inch module and will overstate the blocks you need.
- Ignoring openings, or ignoring cuts. Both change the final count.
- Not rounding mortar up. Half a bag is not an option.
- Skipping the 5%. One broken pallet corner and you are a dozen blocks short.
Frequently asked questions
How many concrete blocks are in 100 square feet?
112.5 blocks, so 113 plus extra.
How many blocks for a 10 × 10 foot wall?
A 10 ft long, 10 ft high wall is 100 ft², so 112.5 blocks, about 119 with 5% extra. Note that a 10 ft high wall is above the height of a simple garden wall and may need engineering.
How many bags of mortar per 100 blocks?
100 ÷ 13 = 7.7, so 8 bags of 80 lb mortar mix.
How many blocks per pallet?
It depends on the block size and the supplier, so ask. Do not rely on a number from a different product.
How much does a standard concrete block weigh?
Weight depends on the density class and the block’s configuration. CMHA defines lightweight, medium-weight and normal-weight classes, so ask your supplier for the weight of the block you are buying.
Sources
- Concrete Masonry & Hardscapes Association, >CMU-TEC-001-23: Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications: nominal and specified unit sizes, 3/8 inch joint, defect allowance, density classes.
- QUIKRETE, >Mortar Mix Product No. 1102 data sheet: coverage per 80 lb bag, mixing water, bed thickness.
Planning estimates only. Structural and retaining walls need local code review.