Nominal vs. Actual Dimensions: The Block Secret
When you buy a "8-inch cinder block," you are buying a block with nominal dimensions. The actual dimensions are exactly 3/8 of an inch smaller in all directions to account for the mortar joint.
- Nominal Size: 8" x 8" x 16" (This is how they are sold).
- Actual Size: 7-5/8" x 7-5/8" x 15-5/8" (This is the physical size of the concrete).
When building, you always design based on the nominal size. A 4-foot high wall uses exactly 6 courses (rows) of 8-inch blocks, because the 3/8" mortar joints add up perfectly to reach 48 inches.
Type S vs. Type N vs. Type M Mortar
Choosing the right mortar is critical for the longevity of your wall. The wrong mix can lead to crumbling walls or cracked blocks.
- Type S (Structural - Recommended): High strength (1,800 PSI). Best for below-grade foundations, retaining walls, and load-bearing walls. This is the most common choice for DIY projects that need to hold back dirt or weight.
- Type N (General Purpose): Medium strength (750 PSI). Good for above-grade, non-load-bearing walls (like a garden partition or a fire pit). It is softer and more flexible, which prevents cracking in non-structural applications.
- Type M (High Strength): Very high strength (2,500 PSI). Used for heavy loads and below-grade applications where compressive strength is the main concern. It is brittle and not recommended for retaining walls.
Core Filling & Rebar: When to Reinforce
Not every wall needs rebar, but if your wall is holding back dirt (retaining) or is over 4 feet tall, you must reinforce the hollow cores.
- Rebar Spacing: Place vertical #4 rebar every 32 to 48 inches. This means you need rebar in every third or fourth block core.
- Core Filling: After placing the rebar, you must fill those hollow cores with "flowable fill" or wet concrete. This turns a hollow wall into a solid, reinforced concrete wall that can withstand pressure.
- Bond Beams: For retaining walls over 4 feet, building codes often require a "bond beam" (a course of U-shaped blocks filled with concrete and horizontal rebar) every 4 feet of height.
Common Project Types & How to Measure Them
Block projects vary wildly. Here is how to use our calculator for the most common scenarios.
- Standard 20ft Retaining Wall (4ft High): Enter 20 for length and 4 for height. Select 8-inch blocks. Add a 5% waste factor. This wall will require 6 courses (rows) of blocks. Remember to account for rebar every 4 feet!
- Outdoor Fire Pit (Circular): Measure the diameter of your fire pit in feet. Enter that number for both Length and Width. Select 8-inch blocks. (Note: For a perfect circle, multiply the calculator's final block count by 0.785 to get the exact circular area). Use Type N mortar for a fire pit, as Type S is too rigid for heat expansion.
- Garden Partition Wall (3ft High): Enter your length and 3 for height. Since this is non-structural, you can use Type N mortar and skip the rebar to save money.
Frequently Asked Questions
How many blocks do I need per square foot?
For standard 8-inch blocks (nominal), you need exactly 1.125 blocks per square foot. This accounts for the block size plus the standard 3/8-inch mortar joint.
How much mortar do I need for 100 blocks?
A general rule of thumb is that one 80lb bag of mortar will lay about 12 to 14 standard 8-inch blocks. For 100 blocks, you will need approximately 7 to 8 bags of mortar.
Can I build a cinder block wall without mortar (Dry Stack)?
You can "dry stack" blocks for very short, non-structural garden walls (under 2 feet high). However, for any wall that needs to hold weight, resist wind, or retain soil, mortar is absolutely required to bond the blocks together.
What is the difference between Cinder Block and Concrete Block?
Technically, cinder blocks are made with coal cinders and are lighter but weaker. Concrete blocks are made with sand/gravel and are much stronger. Modern "cinder blocks" are almost always concrete blocks. For structural walls, always buy concrete blocks.
How do I cut a cinder block?
The best tool for cutting block is a masonry saw with a diamond blade (wet saw). For quick cuts, you can use a circular saw with a masonry blade, or score the block deeply with a chisel and hammer, then snap it.