🧱 Concrete Block Calculator

How to Calculate Concrete Blocks for a Wall

Concrete block is estimated by wall face area, and the arithmetic is easier than brick because block is modular by design. A standard 8 x 8 x 16 block occupies exactly 8 in by 16 in of wall face including its mortar joints, which is 0.889 sq ft.

That 1.125 figure - 1.125 blocks per square foot, or 112.5 per 100 sq ft - holds for 4, 6, 8, 10 and 12 in block, because they all share the same 8 x 16 in face. Only the wall thickness changes. Half-high (soap) block has a 4 x 16 face, so it doubles to 2.25 per square foot.

Counting by course instead

Masons usually count the other way, and it is worth cross-checking:

• Blocks per course = wall length in feet x 0.75. A 30 ft wall is 22.5, so 23 blocks per course.
• Courses = wall height in inches / 8. An 8 ft wall is 12 courses.
• Total = 23 x 12 = 276 blocks, against 240 sq ft x 1.125 = 270. The difference is the partial block at the end of each course, which is why the course method is the safer of the two.

Design the wall in 8 in increments of height and 16 in increments of length and you will cut almost nothing. Every foot that is not on the module is a saw cut and a piece of scrap.

Nominal vs actual, and why it matters

An 8 x 8 x 16 block actually measures 7-5/8 x 7-5/8 x 15-5/8 in. The missing 3/8 in on each dimension is the mortar joint. Order by nominal size, lay out by nominal size, but when you are checking whether a wall fits a foundation or an opening, measure actual. The gap catches people on the last block of every course.

Block Sizes, Weights and Coverage

"Cinder block" and "concrete block" are the same product to a supplier. True cinder block used coal ash as aggregate and has not been made in volume for decades. What you buy today is a concrete masonry unit - CMU - meeting ASTM C90, with a minimum net compressive strength of 2,000 psi.

Lightweight block is worth the premium on a tall wall. It is 20 to 25% lighter for the same strength, which matters when one person is lifting 300 of them onto a scaffold. It is also more absorbent, so it needs sealing below grade.

Do not forget the specials: corner (single-bullnose) block, bond beam block with knock-out webs, lintel block, and half blocks. A running-bond wall needs one half block at every alternate course end. Ordering only stretchers means cutting them on site.

Mortar, Grout and Rebar: Three Different Materials

People use the words interchangeably and they are not interchangeable. Getting this wrong is the fastest way to a wall that fails inspection.

Never pour ordinary concrete into block cells. It will not flow past the webs and around the bar, and you get voids exactly where the reinforcement needs encasement. Core-fill grout is deliberately wet - the block absorbs the excess water and the grout ends up at full strength.

Mortar quantities

Bag yields vary between manufacturers by 20% or more. Check the coverage printed on the bag you are actually buying and treat the table as a planning figure. Past about 300 blocks, buying masonry cement and sand separately and mixing on site is usually half the cost.

Core-fill grout quantities

An 8 in wall grouted solid takes about one cubic yard of grout per 100 square feet. That is the number to remember. If you are only filling the cells that contain rebar, the volume scales with spacing: cells sit 8 in apart, so grouting at 32 in o.c. fills a quarter of them.

Past about 1.5 cubic yards, order ready-mix grout by the truck. Below that, bagged core-fill or pea-gravel mix is simpler, at roughly 0.6 cu ft per 80 lb bag.

Reinforcement: Rebar, Bond Beams and Joint Wire

Hollow block on its own is strong in compression and weak in bending. Reinforcement is what lets a block wall resist soil pressure, wind and seismic load. The spacing must come from an engineer or from the prescriptive tables in your local code - not from a calculator. What follows is conventional practice so you know what to expect.

Block shrinks and brick grows. That single fact drives most masonry crack detailing. Concrete block loses moisture and contracts after it is laid, so it needs control joints that let it pull apart cleanly. Fired clay brick absorbs moisture and expands permanently, so it needs expansion joints that let it push. Detail them the same way and one of the two walls will crack.

When you need an engineer and a permit

Almost any structural block wall is permitted work. Get a design professional involved when:

• The wall is load-bearing - carrying floor, roof, beam or column loads.
• The wall is a foundation or basement wall with unbalanced backfill. Bar size and spacing depend on wall height, fill height and soil class, and the prescriptive tables run out fast.
• The wall retains soil. A block retaining wall is a cantilever with a heel and a footing designed for overturning and sliding. Many jurisdictions require engineering above roughly 4 ft measured from the bottom of the footing, but the trigger varies - some are 3 ft, some 30 in, and any surcharge above the wall usually forces engineering regardless of height. Check with your local building department. See our retaining wall calculator before you plan one.
• The wall is freestanding - a garden or privacy wall is a cantilever in the wind and needs a designed footing and vertical steel.
• You are in a high-wind or seismic zone, where reinforcement, grouting and anchorage are all prescribed.

This page estimates materials only. It is not a structural design and it does not replace an engineer, a stamped drawing, or a permit.

How to Lay a Concrete Block Wall

1. Pour a footing that is right. Conventionally twice the wall thickness in width and at least as deep as the wall is thick, below the local frost line, with the dowels set while the concrete is wet. Get the dowel spacing right now - drilling and epoxying later costs far more. Use our concrete calculator for the volume.
2. Snap lines and dry-lay the first course. Set the whole bottom course out with 3/8 in spacers, corner to corner, before mixing anything. Adjust the layout so you finish on a whole or half block.
3. Bed the first course in a full mortar bed on the footing, and take your time levelling it. Every course above copies the first one.
4. Build the corners and leads first, four or five courses high, checking level, plumb and diagonal. Then run a mason line and fill the field to the line.
5. Face-shell bed the field courses. Mortar on the two face shells only, both bed and head joints. Full bedding is only needed on the starting course, on piers and columns, and where the wall is solid grouted.
6. Keep the reinforced cells clear. Any mortar that drops into a grout cell has to come out. Cutting the webs on bond beam block, not chipping the face, is what keeps the horizontal steel continuous.
7. Set joint reinforcement every other course, lapped 6 in at splices and stopped short of control joints so the joint can actually move.
8. Grout in lifts of 5 ft or less, puddling or vibrating each lift, and stop the pour about 1-1/2 in below the top of the course so the next lift keys in. Taller pours need cleanout openings cut at the base of every grouted cell so an inspector can see the bar and the cell is clean.
9. Tool the joints when the mortar is thumbprint hard. A concave joint is the most weather-resistant and the easiest to strike consistently.
10. Waterproof and drain below grade. Parge coat plus a sheet or fluid-applied membrane on the soil side, a footing drain in washed stone, and backfill only after the wall is braced or the floor framing is on. Backfilling an ungrouted, unbraced block wall is how basement walls end up bowed.

Block Wall Costs

The block itself is rarely the expensive part. On a solid-grouted, fully reinforced foundation wall, grout and steel can equal the cost of the block, and labour is usually more than both. A mason lays 150 to 250 standard blocks a day on plain field work, and far fewer once corners, bond beams and grout lifts are in the sequence.

Where block genuinely beats poured concrete is on small or awkward jobs - no forms to build, strip and haul, no truck access needed, and you can stop and start. Poured concrete wins on tall walls, on long straight runs, and anywhere speed matters.

Related calculators: brick for a veneer over this wall, mortar and grout coverage for a bag count, concrete bags for small pours, gravel for the drainage stone, and square footage if the wall layout is irregular.

Frequently Asked Questions

How many concrete blocks do I need per square foot?

1.125 blocks per square foot, or 112.5 per 100 square feet. That figure holds for 4, 6, 8, 10 and 12 in block because they all have the same 8 x 16 in face including mortar joints. Half-high (soap) block has a 4 x 16 face, so it takes 2.25 per square foot.

What is the actual size of an 8 inch concrete block?

A nominal 8 x 8 x 16 block actually measures 7-5/8 x 7-5/8 x 15-5/8 in. The missing 3/8 in on every dimension is the mortar joint, so the block plus its joints occupies exactly 8 x 16 in of wall. Order and lay out by nominal size; measure by actual size when you are checking a fit.

How many bags of mortar do I need per 100 blocks?

About 9 bags of 80 lb pre-mixed mortar per 100 blocks, or 12 bags of 60 lb. Using masonry cement and sand instead, budget about 3.5 bags of 70 lb masonry cement plus 1,000 lb of sand. Bag yields differ between brands by 20% or more, so check the coverage printed on the bag.

How much grout does it take to fill concrete block cores?

An 8 in wall grouted solid takes about one cubic yard per 100 square feet, which is 0.24 cu ft per block. A 6 in wall takes 0.67 cu yd and a 12 in wall 1.67 cu yd per 100 sq ft. If you are only filling reinforced cells, the volume scales with spacing - cells sit 8 in apart, so grouting at 32 in o.c. fills a quarter of them.

Can I use concrete or mortar to fill block cores instead of grout?

No. Core fill has to be ASTM C476 grout - a fluid mix with an 8 to 11 in slump and pea gravel or no coarse aggregate at all. Ordinary concrete is too stiff to flow past the block webs and around the bar, leaving voids exactly where the steel needs encasement. Mortar is stiffer still and is only meant for joints.

How far apart should rebar be in a block wall?

Conventional residential practice is #4 or #5 vertical bars at 16 to 48 in on centre, with 32 in o.c. common on stem walls, plus a bar each side of every opening and at every corner and wall end. The real spacing depends on wall height, backfill height, soil class and wind or seismic loading, and must come from an engineer or your local prescriptive code table. Lap footing dowels at least 40 bar diameters.

Which mortar type is right for a block wall?

Type S for anything at or below grade, for foundation and stem walls, and for walls resisting lateral load - it has the flexural bond strength that matters when a wall is bending. Type N is fine for above-grade non-structural block. Type M is for masonry in contact with earth and severe frost. Type O has no place in a structural block wall.

How many blocks are in a course, and how many courses in an 8 foot wall?

Multiply the wall length in feet by 0.75 for blocks per course - a 30 ft wall is 23 blocks. Divide the height in inches by 8 for courses - an 8 ft wall is 12 courses. Designing in 8 in height and 16 in length increments means almost no cutting.

Is cinder block the same as concrete block?

In practice, yes - the terms are used interchangeably at every supply yard. True cinder block used coal ash as aggregate and has not been produced in volume for decades. What you buy today is a concrete masonry unit meeting ASTM C90, with a minimum net compressive strength of 2,000 psi.

How much does an 8 inch concrete block weigh?

A normal-weight 8 x 8 x 16 block weighs about 36 lb; the lightweight version is around 28 lb. A 12 in block runs 52 lb normal weight. On a wall of 300 blocks that difference is about 2,400 lb of lifting, which is why lightweight units are worth the premium on anything tall.

Do I need a permit or an engineer for a block wall?

For any load-bearing wall, foundation wall, or wall retaining soil - yes, expect both. Many jurisdictions require an engineered design for retaining walls above roughly 4 ft measured from the bottom of the footing, and some set the trigger at 3 ft or 30 in. Any surcharge above the wall usually forces engineering regardless of height. Freestanding garden walls often need a permit too. Thresholds vary - check with your local building department before you build.

How high can I grout a block wall in one pour?

5 ft per lift is the conventional limit for low-lift grouting. Anything taller is high-lift grouting, which requires cleanout openings cut at the base of every grouted cell so the cells can be inspected and cleaned before the pour. Stop each lift about 1-1/2 in below the top of the course so the next lift keys into it.

Where do control joints go in a block wall?

Conventionally every 20 to 25 ft, at abrupt changes in wall height or thickness, at one side of openings over about 6 ft wide, and at pilasters. Concrete block shrinks as it cures, so control joints let it pull apart cleanly. Do not copy brick spacing - brick expands and needs expansion joints, which are a different detail entirely.

Can I backfill against a new block wall right away?

No. Backfilling an ungrouted, unbraced block wall is the most common way basement and stem walls end up bowed or cracked. Wait until the grout has cured, the steel is in place, and the wall is braced or the floor framing is attached. Backfill in lifts, compact each lift, and keep heavy equipment well away from the wall.