How it works
The calculator finds the wall area, subtracts doors and windows, multiplies by the number of blocks per unit of area, and adds your waste allowance.
Wall area to block = Length × Height − Σ (Width × Height × Count of openings)
Standard: Blocks per area = 1 ÷ (16 in × 8 in) = 1.125 per sq ft
Custom: Blocks per area = 1 ÷ ((Block length + Joint) × (Block height + Joint))
Blocks = ⌈ Wall area to block × Blocks per area × (1 + Waste ÷ 100) ⌉
⌈ ⌉ means rounding up to the next whole block. Rounding happens only at the end, so 100 sq ft of standard block with 5 % waste gives 112.5 × 1.05 = 118.1, rounded up to the 119 that TEK 4-2A lists.
Units are converted internally, so you can enter the wall in feet or meters and the block in inches, centimeters or millimeters. This tool estimates material quantity only; wall thickness, reinforcement, footings and grouting come from your plans, local code or a qualified professional.
Worked example
Inputs
- Wall length
- 25 ft
- Wall height
- 4 ft
- Block size
- Standard 8 × 16 in (7⅝ × 15⅝ in, ⅜ in joint)
- Waste allowance
- 5 %
Result
- Wall area to block
- 100 sq ft
- Blocks per wall area
- 1.125 / sq ft
- Blocks
- 119
Common mistakes
- Entering the nominal size as the actual size. A standard block is sold as 8 × 16 in but measures 7⅝ × 15⅝ in; the joint makes up the difference.
- Forgetting the joint in a custom size. Each block plus its joint takes up (length + joint) × (height + joint) of wall.
- Treating the count as the full order. Corner, half, lintel and bond-beam units, mortar, grout and steel are separate items.
- Designing a retaining or load-bearing wall from a block count. Wall thickness, reinforcement and footing come from local code or a qualified professional.
Assumptions and limits
- The wall is a rectangle of length × height, laid in a single wythe; doors and windows are rectangles of width × height × count.
- Standard block is counted on its nominal 8 × 16 in module, which is 112.5 blocks per 100 sq ft before waste (CMHA TEK 4-2A).
- The count is an area estimate. It does not plan corners, cut units or the course pattern around openings.
- Blocks are rounded up after the waste allowance. This is a material quantity only, not a structural design.
Frequently asked questions
How many concrete blocks are in 100 square feet?
CMHA (formerly NCMA) TEK 4-2A gives 112.5 standard 8 × 16 in units per 100 sq ft of wall, or 119 with a 5 % allowance. Each block plus its joint covers 8 × 16 in, which is 8/9 sq ft.
Can I use this for UK or European blocks?
Yes. Choose “Custom” and enter the actual block size and joint. UK blocks are 440 × 215 mm on the face (Stowell, BS EN 771-3); with 10 mm joints each block takes up 450 × 225 mm, about 9.88 blocks per square meter.
Does the count include mortar or grout?
No. It counts blocks only. Mortar, grout fill, reinforcement and bond beams depend on the wall design – take the quantities from your plans or your supplier.
Should I subtract openings?
Subtracting doors and windows gives a closer count. Openings often need special units such as lintels or bond beams around them, which are not counted here.
Sources
- TEK 04-02A – Cost Estimating Guide for Concrete Masonry — Concrete Masonry & Hardscapes Association (formerly NCMA). Nominal 8 × 16 in face = 8/9 sq ft per unit; 112.5 units per 100 sq ft before waste, 119 with a 5 % allowance.
- 100mm Dense Concrete Block 22.5N/mm² — Stowell Concrete. UK concrete blocks 440 × 215 mm face, made to BS EN 771-3.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 2, 2026