Concrete Block Calculator
Assumes a standard 8"x8"x16" block (1.125 blocks per sq. ft.) and 3/8" mortar joints. Check the specs for non-standard blocks.
A masonry wall takes 1.125 standard concrete blocks (8"x16" face) per square foot, plus about 3 bags of 80 lb mortar for every 100 blocks. Enter your wall dimensions below and this CMU block calculator returns your block order, mortar count, and a waste allowance in one step.
Block and mortar yields by wall size
| Wall Dimensions | Square Footage | Total CMU Blocks | 80lb Mortar Bags |
|---|---|---|---|
| 10 ft x 4 ft (Retaining) | 40 sq ft | 45 blocks | 2 bags |
| 20 ft x 6 ft (Privacy Fence) | 120 sq ft | 135 blocks | 5 bags |
| 24 ft x 10 ft (Garage Wall) | 240 sq ft | 270 blocks | 9 bags |
| 50 ft x 8 ft (Foundation Side) | 400 sq ft | 450 blocks | 14 bags |
How to estimate concrete block
A standard block face works out to 1.125 blocks per square foot of wall. So instead of counting courses, most masons just measure the wall's square footage and multiply by 1.125.
Block depths and variations
Most CMUs share the same 8-by-16-inch face, but their depth varies: 4, 6, 10, and 12-inch load-bearing blocks are all common. The handy part is that the block count is the same for any block with an 8x16 face, whatever its depth. Deeper blocks just have wider cores and a wider bed joint, so they use a bit more mortar.
Mortar rules of thumb
Pre-mixed Type S or Type N mortar yields a predictable volume. The common figure for an 8x8x16 wall is three 80 lb bags per 100 blocks. With smaller 60 lb bags, plan on about 4 per 100. If you're facing the wall with stone veneer to hide the CMU, use the stone calculator for that surface material.
The standard 8x8x16 block, and cinder vs. concrete
The default here is the standard 8x8x16 CMU — the gray block you picture, with a nominal 8-inch-by-16-inch face and 8-inch depth. "Nominal" is the catch: the block actually measures 7⅝ by 7⅝ by 15⅝ inches, and the missing ⅜ inch is the mortar joint, so block-plus-joint lands on clean whole-inch dimensions and courses out evenly. That's why the count works off a 1.125-blocks-per-square-foot face figure regardless of the block's depth.
Cinder block vs. concrete block
People use the terms interchangeably, but they're not quite the same. A true cinder block is made with coal cinders or fly ash and is lighter and weaker; a modern concrete block uses sand-and-gravel aggregate and is heavier and stronger. For anything load-bearing or below grade, you want concrete block. The face dimensions and the block count are identical either way, so the estimate above holds for both — the difference is weight (about 28 lb for a cinder block versus 35 to 40 lb for a concrete block) and strength, not quantity.
Core fill and grout for reinforced walls
Mortar bonds the blocks together at the joints; grout is the separate flowable mix poured into the hollow cores to make a wall structural. Any wall holding back soil or carrying vertical load — a retaining wall, a foundation, a tall freestanding wall — needs vertical rebar in the cores and those cores filled solid with grout. This is a different material and a different quantity from the mortar the calculator above estimates.
Grout is measured by volume, not by bag count off the block total. For a fully grouted wall, the working figures are:
| Block width | Grout per 100 blocks | Blocks per cubic yard |
|---|---|---|
| 6" | 0.83 cu yd | ~120 |
| 8" | 1.0 cu yd | ~100 |
| 10" | 1.23 cu yd | ~80 |
| 12" | 1.54 cu yd | ~65 |
Most walls are only partially grouted — grout and rebar go in the cells that carry bars (often every 32 or 48 inches, plus corners and around openings) rather than every core. So take the full-fill figure above and scale it to the fraction of cores you're actually filling. A wall with bars at 48 inches on center fills roughly one core in three, so it needs about a third of the full-grout volume.
Walls, foundations, and retaining walls
The block count works the same for any straight CMU wall — measure the face area and multiply. What changes by use is the reinforcement and fill, not the block quantity.
- Freestanding and privacy walls. The most common case, and what the defaults assume. Standard 8-inch block, mortar joints, waste for end cuts and corners.
- Foundation and stem walls. Usually 8-, 10-, or 12-inch block for the added depth against the soil. Below grade, these are typically reinforced and grouted, so add the core-fill volume above to your block and mortar order. The first course sits on a poured concrete footing — size that pour with the concrete calculator.
- Retaining walls. These hold back soil, so vertical rebar and solid grout fill in the cores aren't optional. Size the steel with the rebar calculator and the fill from the grout table above; a dry-stacked, unreinforced wall won't hold back much.
About these figures: block-per-square-foot and mortar rules follow the CMHA (formerly NCMA) estimating method and the Inch Calculator reference cited below; the core-fill volumes are the Inch Calculator block-fill figures. Grout fill, rebar spacing, and wall thickness on a structural or below-grade wall are engineering calls — confirm against your plans or local code before ordering.
Frequently asked questions
How many concrete blocks are in a square foot?
You need 1.125 blocks per square foot of wall. A standard CMU has an 8-by-16-inch face, which covers 0.89 square feet including the mortar joint, so a little over one block fills each square foot. Multiply your wall's square footage by 1.125 to get the count.
How many bags of mortar do I need for 100 blocks?
About 3 bags of pre-mixed mortar (80 lb each) lay 100 standard CMU blocks. That covers standard 3/8-inch joints plus normal jobsite waste. If you're using 60 lb bags instead, plan on roughly 4 per 100 blocks.
What is the actual size of an 8x8x16 block?
An 8x8x16 CMU actually measures 7-5/8 by 7-5/8 by 15-5/8 inches. The missing 3/8 inch leaves room for the mortar joint, so the block plus its joint lands on whole-inch dimensions and courses out evenly.
How much waste should I add when ordering blocks?
Add 5 to 10 percent to your block order. That covers breakage, cutting errors, and the odd-sized pieces you need at ends and corners. Use the higher end for walls with lots of cuts or complex corners.
How much does a concrete block weigh?
A standard 8x8x16 hollow block weighs 35 to 40 pounds, depending on the aggregate. Lightweight cinder block runs closer to 28 pounds, which is worth knowing if you're hand-carrying a full pallet.
Can I use standard CMU for a retaining wall?
Yes, concrete block is common for residential and commercial retaining walls. But the hollow cores have to be reinforced with vertical rebar and filled solid with grout to handle the soil load. A dry-stacked wall without reinforcement won't hold back much.
How much grout do I need to fill concrete block cores?
For a fully grouted 8-inch block wall, plan on about one cubic yard of grout per 100 blocks. The figure scales with block width: roughly 0.83 cubic yards per 100 for 6-inch block, 1.0 for 8-inch, 1.23 for 10-inch, and 1.54 for 12-inch. Most walls are only partially grouted, with fill and rebar in the cores that carry vertical bars rather than every cell, so count only the cores you're actually filling.
What is the difference between a cinder block and a concrete block?
Both are CMU, but the aggregate differs. A true cinder block is made with coal cinders or fly ash, which makes it lighter and weaker; a concrete block uses sand and gravel aggregate, so it's heavier and stronger. In everyday use people say cinder block for any hollow gray block, but for load-bearing or below-grade work you want concrete block. Either way the face size and the block count are the same, so this calculator covers both.
How much does a concrete block wall cost?
Material runs roughly $1.50 to $3 per standard 8-inch block plus mortar, so a 100-square-foot wall of about 113 blocks lands near $200 to $400 in materials before rebar, grout, or footing concrete. Add the cost fields above to price your own block and mortar. Installed cost with a mason is several times the material figure, and regional labor rates vary widely, so get a local quote for the finished price.
Related calculators
Sources
- TEK 04-02A — Estimating Concrete Masonry MaterialsConcrete Masonry & Hardscapes Association (CMHA)Industry method for estimating block counts and mortar quantities from wall area for standard CMU.
- Concrete Block Calculator — Blocks and Mortar per Square FootInch CalculatorWorking figures used here: about 1.125 standard 8x8x16 blocks per square foot (3/8-inch joints) and roughly 3 bags of mortar per 100 blocks.