CONCRETE BLOCK SIZES: NOMINAL VS ACTUAL, US AND MEXICAN STANDARDS
By Virginia Viadas
A concrete block called 8 × 8 × 16 does not measure 8 × 8 × 16. It measures 7 5/8 × 7 5/8 × 15 5/8 inches. The missing 3/8 inch is the mortar joint. Once laid, the assembly returns to the nominal module — which is the entire reason the system works. Anyone dimensioning a wall from actual block size instead of nominal module will be off by a joint on every course.
US concrete masonry unit sizes
| Nominal size (in) | Actual size (in) | Common use |
|---|---|---|
| 4 × 8 × 16 | 3 5/8 × 7 5/8 × 15 5/8 | Partitions, veneer backup |
| 6 × 8 × 16 | 5 5/8 × 7 5/8 × 15 5/8 | Light load-bearing, partitions |
| 8 × 8 × 16 | 7 5/8 × 7 5/8 × 15 5/8 | Standard load-bearing wall |
| 10 × 8 × 16 | 9 5/8 × 7 5/8 × 15 5/8 | Heavier load-bearing, retaining |
| 12 × 8 × 16 | 11 5/8 × 7 5/8 × 15 5/8 | Tall walls, high load, basements |
The convention is consistent: subtract 3/8 inch from each nominal dimension to get the actual unit.
Mexican block sizes (block de concreto)
Mexican units are specified in centimeters, conventionally listed as width × height × length, and sized around a 1 cm mortar joint rather than 3/8 inch:
| Nominal (cm) | Typical actual (cm) | Common use |
|---|---|---|
| 10 × 20 × 40 | ~9 × 19 × 39 | Partitions |
| 12 × 20 × 40 | ~11 × 19 × 39 | Light partition and infill |
| 15 × 20 × 40 | ~14 × 19 × 39 | Load-bearing walls |
| 20 × 20 × 40 | ~19 × 19 × 39 | Heavy load-bearing, retaining |
Actual dimensions vary between manufacturers and regions more than they do in the US market. Verify against the supplier's ficha técnica before setting out a wall — this is the single most common source of coursing error on Mexican jobsites.
Approximate cross-reference
| US nominal | Mexican nominal | Note |
|---|---|---|
| 4 in (10.2 cm) | 10 cm | Close equivalent |
| 6 in (15.2 cm) | 15 cm | Close equivalent |
| 8 in (20.3 cm) | 20 cm | Close equivalent |
| 16 in length (40.6 cm) | 40 cm length | Close equivalent |
The two systems are dimensionally similar but not identical. The 6 mm difference across a 40 cm unit accumulates: over a 10-meter wall, that is roughly 15 cm of drift.
Unit types you need to distinguish
- Load-bearing vs non-load-bearing. Governed by different standards and different minimum compressive strengths. Substituting one for the other is a structural change, not a value engineering choice.
- Normal weight, medium weight and lightweight. Density affects strength, thermal performance, acoustic performance and — critically for scheduling — how many units a mason can lay per day.
- Hollow vs solid. Hollow units allow vertical reinforcement and grouting; solid units do not.
- Special shapes. Bond beam, lintel, corner and half units. Ordering only standard stretcher units guarantees field cutting and waste.
Estimating rule of thumb
For standard US 8 × 8 × 16 units: approximately 1.125 blocks per square foot of wall, or about 113 blocks per 100 square feet before waste. For Mexican 20 × 40 cm face units: 12.5 blocks per square meter before waste. Add 5 to 10 percent for breakage, cuts and special shapes.
Frequently asked questions
What is the actual size of an 8-inch concrete block?
7 5/8 × 7 5/8 × 15 5/8 inches. The nominal 8 × 8 × 16 designation includes a 3/8-inch mortar joint.
How many concrete blocks per square meter?
For standard 20 × 40 cm face units, 12.5 per square meter before waste.
Are Mexican and US concrete blocks interchangeable?
Dimensionally similar but not identical. A 40 cm unit is about 6 mm shorter than a 16-inch unit, which accumulates across a long wall.
Why is the block smaller than its stated size?
So that the unit plus its mortar joint equals the nominal module, keeping coursing and layout dimensions whole.
What is the difference between hollow and solid block?
Hollow units have cores that allow vertical reinforcing and grout; solid units do not and are used where full bearing or fire separation is required.