Concrete Block Calculator

A block wall is priced and ordered off one number: the wall's square footage. Standard concrete block covers that area at a fixed rate, so once you know the area you know the block count, the mortar, and the sand. Enter the wall below, subtract any door or window openings, and the calculator returns all three.

Blocks per square foot

Standard concrete block has a nominal face of 8 inches by 16 inches. The Concrete Masonry and Hardscapes Association, the industry body formerly known as NCMA, puts it plainly in its unit specification guidance: units typically have nominal face dimensions of 8 in. by 16 in. That face is 128 square inches, and a square foot is 144, so:

Blocks = wall square footage × 1.125

Masonry supply houses publish the same figure as a flat rule. Georgia Masonry Supply's estimating guide lists "1.125 block per SF of wall area" for standard 4, 8, 10 and 12 inch block, and Riverside Brick's masonry estimating guide gives the identical number. Another way to hold it: 112.5 blocks per 100 square feet of wall.

Why wall thickness doesn't change the block count

This trips people up. A "12 inch block" and a "4 inch block" need the same number of units for the same wall, because the 4 and the 12 describe the wall's thickness, not the face you see. Every standard unit presents the same 8 by 16 face to the wall. A thicker block costs more each and weighs more, but you order the same count.

The block count only changes if the face changes. A half-height block with a 4 by 16 face covers half as much, so it takes 2.25 per square foot.

Nominal versus actual block size

Block is sold by nominal size, and nominal size includes the mortar joint. CMHA states that specified dimensions are typically 3/8 in. less than nominal, so that an 8 inch module holds once a 3/8 inch mortar joint is added. That is why an "8x8x16" block measures 7 5/8 inches by 7 5/8 by 15 5/8 when you put a tape on it.

Nominal size	Actual (specified) size	Blocks per sq ft
4 x 8 x 16	3 5/8 x 7 5/8 x 15 5/8	1.125
6 x 8 x 16	5 5/8 x 7 5/8 x 15 5/8	1.125
8 x 8 x 16	7 5/8 x 7 5/8 x 15 5/8	1.125
10 x 8 x 16	9 5/8 x 7 5/8 x 15 5/8	1.125
12 x 8 x 16	11 5/8 x 7 5/8 x 15 5/8	1.125

Nominal thicknesses run 4, 6, 8, 10, 12, 14 and 16 inches, per NCMA TEK 02-01B on typical sizes and shapes, which also notes units may vary by plus or minus 1/8 inch from the specified dimensions. Your local producer may stock shapes beyond the standard list.

The practical point: lay out the wall in nominal dimensions. A wall 16 blocks long is 16 × 16 inches = 21 feet 4 inches, and it works out because each block plus its joint occupies exactly 16 inches.

Mortar and sand

Both estimating guides above give the same mortar figure: 3 bags per 100 blocks. For sand, Georgia Masonry Supply gives 1 cubic yard per 7 bags of mortar.

Mortar bags = (block count ÷ 100) × 3
Sand (cubic yards) = mortar bags ÷ 7

These are estimating rules, not guarantees. A wide joint, a sloppy day, or a lot of cut blocks pushes mortar use up, so order a bag or two past the math.

Waste factor

Block breaks less than siding or tile, but it still breaks, and corners and openings mean cut units. Five percent is a sensible cushion on a plain wall. Push it higher on a wall with a lot of openings, corners, or a stepped footing, where more units get cut. The calculator above builds 5 percent in.

Worked example: a garage wall

A detached garage wall runs 24 feet long and 8 feet high, with a single 9 by 7 foot garage door in it.

Gross area: 24 × 8 = 192 square feet.
The door is 9 × 7 = 63 square feet, so net wall is 192 − 63 = 129 square feet.
Blocks: 129 × 1.125 = 145.1, and with 5 percent waste, 153 blocks.
Mortar: (153 ÷ 100) × 3 = 4.6, so 5 bags.
Sand: 5 ÷ 7 = about 0.7 cubic yards.

Subtracting the opening matters here. Skip it and you order 40 blocks you don't need, which is most of a pallet.

Worked example: a crawlspace foundation

A 40 by 60 foot foundation wall stands 4 feet high with no openings.

Perimeter: 2 × (40 + 60) = 200 feet.
Wall area: 200 × 4 = 800 square feet.
Blocks: 800 × 1.125 = 900, plus 5 percent, 945 blocks.
Mortar: (945 ÷ 100) × 3 = 28.4, so 29 bags.
Sand: 29 ÷ 7 = about 4.1 cubic yards.

Perimeter times height is the same move the wall square footage calculator uses for drywall and siding. Block is just a different divisor on the same area.

Cinder block or concrete block?

People search both and usually mean the same thing. Strictly, a cinder block was made with coal cinders as the aggregate, which made it lighter and weaker, and it is largely a historical product in new construction. What you buy today is a concrete masonry unit, or CMU, made with sand and gravel aggregate. If a supplier says "cinder block," they almost certainly mean standard CMU. The estimating math is identical either way.

Retaining wall block is a different product

Segmental retaining wall units are not CMU. They are solid, they interlock or pin together rather than being mortared, and their face dimensions vary by manufacturer, so there is no universal blocks-per-square-foot figure. Get the coverage number off the specific product's data sheet and divide your wall face area by it. The 1.125 rule on this page applies to mortared CMU only.

Where block counts come up

Foundation and crawlspace walls. The most common residential block job, and the one where perimeter times height does all the work.

Garage and shed walls. Usually a simple rectangle with one big opening to subtract.

Retaining and garden walls. Mortared CMU walls use the rule on this page. Interlocking retaining wall units do not, as above.

Firewalls and commercial partitions. Block is common in commercial interiors, where the wall area often comes off a plan rather than a tape. The gross floor area guide covers reading area off plans.

Common mistakes

Assuming a thicker block means fewer blocks. It does not. All standard units share the 8 by 16 face, so the count is the same and only the cost and weight change.

Measuring actual block size instead of nominal. Dividing by 15 5/8 inches rather than 16 overstates your count, because the nominal size already includes the mortar joint.

Forgetting to subtract a garage door. A single 9 by 7 door is 63 square feet, which is about 70 blocks. That is worth taking out.

Using the 1.125 rule on retaining wall block. Different product, different coverage. Read the data sheet.

Ordering exactly what the math says. Blocks crack and corners need cuts. Five percent is the floor.

Frequently asked questions

How many concrete blocks do I need?

Multiply the wall's square footage by 1.125. A standard block has a nominal 8 by 16 inch face, which covers 0.89 square feet, so it takes 1.125 blocks to cover a square foot. A 129 square foot wall needs about 145 blocks before waste, or roughly 153 with a 5 percent cushion.

How many blocks are in 100 square feet of wall?

About 112.5, which in practice means ordering 113 before any waste factor. That figure holds for 4, 6, 8, 10 and 12 inch block, because all of them present the same 8 by 16 inch face.

Does a 12 inch block cover less area than an 8 inch block?

No. The 8 and the 12 describe the wall's thickness, not the face. Both present a nominal 8 by 16 inch face, so both take 1.125 blocks per square foot. The thicker unit costs more and weighs more, but you need the same count.

Why is an 8x8x16 block actually 7 5/8 inches?

Because nominal sizes include the mortar joint. Specified block dimensions are 3/8 inch under nominal, so a 15 5/8 inch block plus a 3/8 inch joint occupies exactly 16 inches and the modular layout works out.

How much mortar do I need per 100 blocks?

About 3 bags, with roughly 1 cubic yard of sand per 7 bags of mortar. Those are the figures masonry suppliers publish for estimating. Wide joints and lots of cut units push the real number higher, so order a little past the calculation.

Is a cinder block the same as a concrete block?

For estimating, yes. A true cinder block used coal cinders as aggregate and is rare in new work. What suppliers sell today is a concrete masonry unit, and it uses the same 1.125 blocks per square foot.

How do I calculate blocks for a wall with a door or window?

Work out the gross wall area, subtract the opening's area in square feet, then multiply the net figure by 1.125. A 9 by 7 foot garage door removes 63 square feet, which is about 70 blocks you would otherwise have bought.

Does this work for retaining wall blocks?

No. Interlocking segmental retaining wall units are a different product with manufacturer-specific face dimensions and no mortar joint. Take the coverage figure from the product data sheet and divide your wall's face area by it.

For the same wall in drywall, siding or paint, use the wall square footage calculator. For the footing underneath it, the concrete calculator converts area and depth into cubic yards, and the price per square foot calculator turns the finished area into a materials cost.

Concrete Block Calculator