Count the Blocks Before the Truck Leaves the Yard
A block wall looks complicated from the curb, but the estimating is some of the simplest math in construction: every standard block covers the same patch of wall, so the whole job reduces to measuring an area and dividing. Get it wrong in one direction and a crew is standing around waiting on a delivery; get it wrong in the other and you are paying to haul surplus blocks back — or finding them a permanent home in your yard.
This free concrete block calculator does the dividing for you. Enter your wall’s length and height and it returns the blocks to order with waste included, the courses the wall will take, and the mortar bags to put on the same ticket — the three numbers a supplier actually asks for.
Please enter a wall length and height greater than zero.
Estimates for planning and ordering. A 60 lb bag of mortar mix (≈ 0.5 cu ft) lays about 30 standard 8″×8″×16″ blocks; thicker blocks and wider joints need more. Always verify quantities with your supplier or mason.
How to Use This Calculator
- Enter the wall length in feet. Measure the full run of the wall. Building two walls at the same height? Add their lengths and estimate them together.
- Enter the wall height in feet. Measure from the top of the footing or slab to where the wall will finish.
- Set the waste allowance. Five percent suits a plain straight wall; 10% is the safer default once corners, ends, and openings enter the picture.
- Pick your mortar bag size. Suppliers stock 80 lb or 60 lb bags of mortar mix — the bag count adjusts to whichever you choose.
- Read off the order. Blocks to buy, courses high, blocks per course, and mortar bags, with the arithmetic shown so you can sanity-check it before you pay.
What Is a Concrete Block?
A concrete block — formally a concrete masonry unit, or CMU — is a precast rectangle of Portland cement, sand, and aggregate, usually hollow. The standard North American unit is nominally 8″×8″×16″: 8 inches thick, 8 inches high, 16 inches long. Foundations, garage and basement walls, garden and retaining walls, sheds, and commercial buildings all rise from this one module.
The key word is nominal. The block itself measures 7-5/8″ high by 15-5/8″ long; the missing fractions are the 3/8-inch mortar joint. Laid in the wall, block plus joint occupies exactly 8×16 inches of wall face — 0.889 square feet. Invert that and you get the only number you really need: 1.125 blocks per square foot of wall. The hollow cores cut the block’s weight nearly in half versus a solid unit and leave vertical channels that can take rebar and grout where a wall needs structural strength.
The Formula (With a Worked Example)
Base blocks = wall area × 1.125
Blocks to order = ceiling( base blocks × (1 + waste ÷ 100) )
Courses high = ceiling( height in inches ÷ 8 )
Mortar bags (80 lb) = ceiling( blocks to order ÷ 12 )
Worked example. A 20 ft × 8 ft garden wall, 10% waste, 80 lb mortar bags:
- Wall area: 20 × 8 = 160 sq ft
- Base blocks: 160 × 1.125 = 180 blocks
- With 10% waste: 180 × 1.10 = 198 blocks to order
- Courses: (8 × 12) ÷ 8 = 12 courses
- Blocks per course: (20 × 12) ÷ 16 = 15 blocks
- Mortar: 198 ÷ 12 per bag = 16.5 → 17 bags of 80 lb mix
The example cross-checks itself: 12 courses × 15 blocks per course = 180, exactly the base count. If you ever pencil this out and the two routes disagree, the slip is in the arithmetic, not the method.
Do the Blocks Need Grout or Rebar?
For a plain garden or partition wall, no — the blocks and mortar are the whole material list, and the counts above are complete. Structural walls are different. Where an engineer calls for reinforcement, vertical rebar drops into the hollow cores and the cores are filled with grout, either only where the bars run (partial grouting) or throughout the wall (solid grouting). Grout is a separate order from mortar — roughly half a cubic foot of grout fills the cores of one standard block — and it is not included in the mortar bag count. Retaining walls over about 4 feet, foundation walls, and anything in a high-wind or seismic area should be designed, not guessed: get the reinforcement details from an engineer or your local code before you order.
Standard Concrete Block Sizes
All common units share the 8×16-inch face, so the 1.125-per-square-foot count never changes — only the wall’s thickness and the block’s weight do:
- 4-inch (partition) blocks — interior non-load-bearing walls, shower stalls, and space-saving partitions.
- 6-inch blocks — light-duty walls, single-story garages, and garden structures.
- 8-inch blocks — the standard structural unit for most exterior, basement, and load-bearing walls; about 35-38 lb each.
- 12-inch blocks — foundations, retaining walls, and walls that will be heavily grouted; often 50 lb or more each.
- Half blocks (8×8×8) — finish the ends of courses in running bond so vertical joints stay staggered without cutting full units.
Tips for an Accurate Block Order
- Subtract openings before you count. A 3×7 ft door is 21 sq ft — about 24 blocks. Measure every door and window, subtract their combined area from the wall area, then multiply by 1.125.
- Match the waste to the wall. Five percent covers a straight, opening-free run. Corners, piers, short walls, and lots of cutting push real waste toward 10-15%.
- Order one production run. Blocks from different batches can vary slightly in shade and dimension. A single order keeps the wall uniform and avoids a visible patchwork line.
- Round up to the pallet for big walls. Suppliers deliver by the pallet and often charge restocking fees on returns; a small surplus you keep beats a mid-job shortage every time.
- Plan where the blocks land. Two hundred standard blocks weigh about 7,000 lb. Have them set along the wall line, off the ground and covered, not in one pile at the curb.
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Frequently Asked Questions
How many concrete blocks do I need per square foot?
For standard 8×8×16 blocks laid with 3/8-inch mortar joints, you need 1.125 blocks per square foot of wall. Each block plus its joint covers exactly 8 by 16 inches of wall face (0.889 sq ft), so one divided by 0.889 gives the 1.125 figure masons use. That works out to about 113 blocks for every 100 square feet, before waste.
How many blocks do I need for a 100 square foot wall?
About 113 blocks before waste: 100 × 1.125 = 112.5, rounded up. With the standard 10% allowance for cuts and breakage, order about 125. If the wall has a door or window, subtract that opening’s area first — a standard door removes roughly 24 blocks from the count.
How many 80 lb bags of mortar do I need for 100 blocks?
About 9 bags. One 80 lb bag of mortar mix lays roughly 12 standard 8×8×16 blocks with 3/8-inch joints, so 100 ÷ 12 = 8.3, rounded up to 9 bags. If your supplier stocks 60 lb bags instead, they lay about 9 blocks each, so plan on 12 bags for the same 100 blocks.
Should I subtract doors and windows from the wall area?
Yes, always. Multiply each opening’s width by its height and subtract the total from the wall area before you count blocks. A standard 3 ft × 7 ft door removes 21 sq ft — about 24 blocks — and a 3 ft × 4 ft window removes 12 sq ft, about 14 blocks. Forgetting one door is the most common reason a block order comes up a pallet heavy.
What is the standard concrete block size?
The standard unit is nominally 8 inches thick, 8 inches high, and 16 inches long (8×8×16). The actual block measures 7-5/8 by 15-5/8 inches on its face — the nominal size includes the 3/8-inch mortar joint, so once laid, each block occupies exactly 16 by 8 inches of wall. Thinner 4-inch and 6-inch units and heavier 12-inch units share the same face size.
What is the difference between a concrete block and a cinder block?
Very little, today. True cinder blocks were made with coal cinders or ash and were lighter and weaker; they have largely disappeared from production. Nearly every block sold now is a concrete block — technically a CMU, or concrete masonry unit — made from Portland cement, sand, and aggregate. If a supplier’s yard calls them cinder blocks, you are still almost certainly buying modern concrete units.
How much does a concrete block weigh?
A standard 8×8×16 block weighs about 35 to 38 lb in normal-weight concrete. Lightweight versions of the same size weigh about 25 to 28 lb. The hollow cores are what keep the weight manageable — they also leave room for rebar and grout in reinforced walls. A full order adds up fast: 200 standard blocks weigh roughly 7,000 to 7,600 lb.
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