How to calculate concrete blocks
Concrete masonry units (CMU, often called cinder blocks) are sold by nominal size. A standard block is called 8 × 8 × 16 in., which includes a 3/8 in. mortar joint. Its face covers 8 × 16 = 128 sq in., or 0.889 sq ft, so a wall takes 1.125 blocks per square foot. That number is the same for every block in the 8 × 16 family, whatever its width.
Blocks = net wall area × 1.125 × (1 + waste %)
Courses = height (in) ÷ 8 · Blocks per course = length (in) ÷ 16
Worked example: a 20 ft wall, 4 ft tall
- Wall area: 20 × 4 = 80 sq ft, with no openings.
- Blocks: 80 × 1.125 = 90 blocks (15 per course × 6 courses).
- Add 5% waste: 90 × 1.05 = 94.5, round up to 95 blocks.
- Premixed mortar at about 12 blocks per 80 lb bag: 95 ÷ 12 = 7.9, round up to 8 bags.
- Or masonry cement at about 30 blocks per 70 lb bag: 95 ÷ 30 = 3.2, so 4 bags, plus about 3.2 ÷ 8 = 0.4 tons of mason sand.
- If the wall is fully grouted: 90 blocks × 0.27 cu ft = 24.3 cu ft, or about 0.9 cu yd of grout.
Nominal vs. actual block sizes
Every dimension of a concrete block is 3/8 in. smaller than its name, so that block plus mortar joint lands on an even 8 or 16 in. An "8 × 8 × 16" block actually measures 7-5/8 × 7-5/8 × 15-5/8 in. Lay it with 3/8 in. joints and every course rises exactly 8 in., and every block advances the wall exactly 16 in.
| Nominal size (W × H × L) | Actual size | Blocks per 100 sq ft (no waste) | Grout if fully grouted (approx.) | Common uses |
|---|---|---|---|---|
| 4 × 8 × 16 in. | 3-5/8 × 7-5/8 × 15-5/8 in. | 112.5 | Not usually grouted | Veneer, partitions, cap courses |
| 6 × 8 × 16 in. | 5-5/8 × 7-5/8 × 15-5/8 in. | 112.5 | About 0.22 cu ft per block | Interior partitions, garden walls |
| 8 × 8 × 16 in. | 7-5/8 × 7-5/8 × 15-5/8 in. | 112.5 | About 0.27 cu ft per block | Foundations, garages, most walls |
| 10 × 8 × 16 in. | 9-5/8 × 7-5/8 × 15-5/8 in. | 112.5 | About 0.32 cu ft per block | Taller or heavier-loaded walls |
| 12 × 8 × 16 in. | 11-5/8 × 7-5/8 × 15-5/8 in. | 112.5 | About 0.42 cu ft per block | Basement and retaining walls |
Grout volumes are typical figures for two-core hollow block and include a small allowance for loss. Core shapes vary by plant, so if the manufacturer lists a grout volume, enter it as an override. For 100 sq ft of fully grouted 8 in. wall, that's about 30 cu ft, or roughly 1.1 cu yd.
Standard blocks are only part of the order. Free wall ends need half blocks or end (corner) blocks with a finished end, typically one at each free end of every other course in a running bond. Door and window openings may need bond beam or lintel blocks. These replace standard blocks one for one in the count, so the total stays about the same.
How much mortar for concrete block?
Mortar use varies more than block count does, so treat every mortar figure as an estimate and keep the calculator inputs editable.
- Premixed mortar mix (80 lb bags): cement, lime and sand already blended; just add water. Bag labels typically say one bag lays up to about 13 standard 8 × 8 × 16 blocks. The calculator defaults to 12 for a small margin. Best for small walls and repairs.
- Masonry cement (70 lb bags) plus sand: what most masons use on bigger walls. A common industry estimate is about 30 hollow units per bag, and about 1 ton of mason sand for every 8 bags. Usually costs less per block than premix, but you need a mixer and a sand pile.
- Type S or Type N? Type N is a general-purpose mortar for above-grade walls. Type S is stronger and is usually specified for foundations, retaining walls and other walls below grade or under heavy load. Your plans or local code may specify the type.
Grout and rebar
Hollow block walls are often reinforced: steel rebar runs vertically through some cores, and those cores are filled solid with grout. Some walls (many retaining and basement walls) are grouted in every core. Spacing, bar size and which cores get filled come from the structural design or local code, not from a rule of thumb. Common details also include a bond beam (a course of U-shaped block with horizontal rebar and grout) at the top of the wall and ladder-type joint reinforcement in the mortar every other course.
If only some cores are grouted, estimate grout from the grouted cores, not the whole wall. For example, one core in each 16 in. of wall is roughly half the full-grout figure for that length. Our concrete calculator covers the footing under the wall; a traditional rule of thumb is a footing about twice as wide as the wall is thick, but local code sets the actual size and depth.
Tips before you order
- A standard 8 × 8 × 16 hollow block weighs about 35–40 lb; lightweight block is less. A pallet is a heavy load, so arrange delivery for anything more than a few dozen.
- Plan wall lengths in 8 in. increments and heights in 8 in. steps when you can. It saves cutting.
- Store block off the ground and covered. Wet block shrinks as it dries and can crack the wall.
- Building with brick instead? Use the brick calculator. For a dry-stacked landscape wall, the retaining wall calculator handles blocks, caps and drainage stone.
Frequently asked questions
How many concrete blocks are in a square foot?
About 1.125 standard 8 × 16 in. blocks per square foot of wall, or 112.5 per 100 sq ft. Each block plus its mortar joint covers 8 × 16 = 128 sq in., which is 0.889 sq ft.
How many blocks do I need for a 20 ft long, 8 ft tall wall?
The wall is 160 sq ft, so 160 × 1.125 = 180 blocks (15 per course × 12 courses). With 5% waste, buy 189. Subtract 1.125 blocks for every square foot of door or window opening.
How many bags of mortar do I need for 100 blocks?
About 8 or 9 bags of 80 lb premixed mortar mix, since labels typically say each bag lays up to about 13 blocks. If you mix your own with masonry cement and sand, plan on roughly 3–4 bags of 70 lb masonry cement plus about 1/2 ton of mason sand. Actual use varies with joint size and technique.
What is the actual size of an 8x8x16 concrete block?
7-5/8 × 7-5/8 × 15-5/8 in. The nominal size includes a 3/8 in. mortar joint, so each course rises 8 in. and each block covers 16 in. of wall length.
How much grout does it take to fill concrete block?
For fully grouted 8 in. block, plan on about 0.27 cu ft per block, or about 1.1 cu yd per 100 sq ft of wall. Wider block takes more: roughly 0.32 cu ft for 10 in. and 0.42 cu ft for 12 in. block. Core shapes vary, so check the manufacturer's figure if available.
How much does a cinder block weigh?
A standard 8 × 8 × 16 in. hollow concrete block typically weighs about 35–40 lb. Lightweight block is less, and solid or wider block weighs more. A full pallet of 8 in. block (often around 90 blocks, depending on the supplier) can top 3,000 lb.
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