Concrete Block Calculator
By Taro Schenker | Updated February 11, 2026
Quick Answer: To calculate concrete blocks, multiply the net wall area (sq ft) by blocks per sq ft based on block size and mortar joint. For standard 8x8x16 blocks with 3/8" mortar joints, use 1.125 blocks per sq ft. Mortar: 1 bag (80 lbs) per 30 blocks at $12/bag. Block weights: 8x8x16 = 38 lbs, 8x4x16 = 24 lbs, 12x8x16 = 55 lbs. Add 5-10% for waste and breakage. For a 20 ft x 8 ft wall (160 sq ft), you need about 198 standard blocks.
Calculate how many concrete blocks (CMU) you need for your wall, foundation, or column project. Enter your dimensions and block size to get the total number of blocks, mortar bags, weight, and estimated cost.
How to Calculate Concrete Blocks
The formula for calculating the number of concrete blocks needed:
- Measure the total wall length and height in feet.
- Multiply length × height to get the gross wall area.
- Subtract the area of any doors, windows, or other openings.
- Calculate blocks per square foot based on block size and mortar joint thickness (1.125 for standard 8×8×16 blocks).
- Multiply net wall area by blocks per square foot.
- Add 5-10% for waste, breakage, and cuts.
Block Sizes and Coverage
Common concrete block sizes and their coverage rates:
| Size | Blocks / sq ft | Weight | Typical Use |
|---|---|---|---|
| 8" × 8" × 16" | 1.125 | 38 lbs | Standard walls, foundations |
| 8" × 4" × 16" | 2.25 | 24 lbs | Half-height courses, caps |
| 12" × 8" × 16" | 1.125 | 55 lbs | Commercial, tall/retaining walls |
Mortar Requirements
Mortar is essential for binding concrete blocks together. Here is what you need to know:
- Standard joint: A 3/8-inch mortar joint is the industry standard for concrete block construction.
- Coverage: One 80 lb bag of Type S mortar will lay approximately 30 standard blocks.
- Mortar type: Use Type S mortar for below-grade and structural applications. Type N is suitable for above-grade, non-load-bearing walls.
- Mixing: Mix mortar to a peanut-butter consistency. It should hold its shape when squeezed but not crumble.
- Working time: Mixed mortar remains workable for about 90 minutes. Only mix what you can use in that window.
Formula Summary
Block Count Formula: To calculate the number of concrete blocks needed, divide the net wall area by the effective face area of a single block (including mortar joint), then multiply by the waste factor.
$$N_{\text{blocks}} = \lceil A_{\text{net}} \times \frac{144}{(H_b + J) \times (L_b + J)} \times (1 + W\%) \rceil$$
where $H_b$ and $L_b$ are block height and length in inches, and $J$ is the mortar joint thickness in inches.
Mortar: $$\text{Bags (80 lb)} = \lceil \frac{N_{\text{blocks}}}{30} \rceil$$
Weight: $$W_{\text{lbs}} = N_{\text{blocks}} \times w_{\text{block}}$$
| Block Size | Weight (lbs) | Blocks per sq ft |
|---|---|---|
| 8" x 8" x 16" | 38 | 1.125 |
| 8" x 4" x 16" | 24 | 2.25 |
| 12" x 8" x 16" | 55 | 1.125 |
Standard Assumptions: Default mortar joint is 3/8 inch. One 80 lb bag of mortar lays approximately 30 standard blocks. Mortar bag cost is $12. Default waste factor is 5-10%. For columns, gross area is calculated as column perimeter (4 sides) x height x number of columns.
Related Calculators
- Concrete Calculator — calculate concrete volume for slabs, footings, and columns
- Brick Calculator — estimate bricks for walls, patios, and walkways
- Rebar Calculator — determine rebar spacing and quantities for reinforcement
Estimates are for planning purposes only. Consult a qualified contractor for critical projects. Actual material requirements may vary based on site conditions, waste, and installation methods.
Pricing last verified: February 2026. Material prices vary by region, season, and supplier. Prices shown are national averages for planning purposes.