About the Concrete Block Calculator
This concrete block calculator estimates the materials for a wall built from standard concrete masonry units (CMU), often called cinder blocks. Enter the wall length and height, choose the block width, and it works out how many blocks to buy, how many courses and blocks per course you will lay, the mortar bags required and — if you plan to fill the cores — how much grout or concrete that takes.
It is aimed at homeowners and builders planning foundation walls, retaining and garden walls, garage or shed walls and raised beds. Every common block width (6, 8, 10 and 12 inch) shares the same 8 × 16 inch nominal face, so walls take 1.125 blocks per square foot regardless of width; the width mainly changes weight, strength and how much fill the cores hold.
Mortar and grout follow the Concrete Masonry & Hardscapes Association (CMHA, formerly NCMA) estimating guide TEK 4-2A: one 70 lb bag of masonry cement, mixed with sand, lays about 30 hollow blocks with face-shell bedding (about 3.3 bags per 100 blocks, adjustable), and core fill uses its solid-grout volumes for two-core hollow block including 3% waste. Real grout use varies with block density and placement method, so check against your supplier’s data and any engineered drawings, especially for reinforced or load-bearing walls.
With the default inputs, the blocks needed (incl. waste) is 189. Change any value above to recalculate instantly.
How to use the concrete block calculator
- 1Measure the wall length and height in feet (include the footing course if it is block).
- 2Choose the block width you will use.
- 3Set a waste allowance and your mortar bags per 100 blocks.
- 4Enter the percentage of cores you will fill with grout or concrete.
- 5Review the block count, mortar bags and fill volume, then price the order.
Formula and method
A standard concrete block measures 15⅝ × 7⅝ inches on its face; with a 3/8-inch mortar joint it occupies a nominal 16 × 8 inch space, or 0.889 sq ft. That means 1.125 blocks per square foot of wall. Multiplying by the wall area and adding waste gives the order quantity, while dividing the height by 8 in and the length by 16 in gives the courses and blocks per course.
Mortar is estimated from the blocks laid (before waste) using your bags-per-100 figure; the default 3.3 comes from the CMHA guide’s one 70 lb bag of masonry cement per about 30 hollow blocks. Core fill uses the CMHA solid-grout volumes for two-core hollow block — 0.256, 0.361, 0.47 and 0.589 cu ft per sq ft of wall for 6, 8, 10 and 12 inch block, including 3% waste — scaled by the share of cores you fill, then converted to cubic yards (÷ 27) and to 80 lb bags of preblended grout that yield about 0.66 cu ft each. Scaling by the share filled is an approximation: grouting every other cell (16 in spacing) matches the table’s half volume closely.
- L, H
- Wall length and height (ft)
- 1.125
- Blocks per sq ft for 8 × 16 in nominal block
- g
- Grout per sq ft of wall when every core is filled
- fill%
- Share of cores filled
Worked examples
40 × 4 ft garden wall, unfilled
A 160 sq ft wall needs 160 × 1.125 = 180 blocks, or 189 with 5% waste — six courses of 30 blocks. At 3.3 bags per 100 blocks that is 5.94, so buy 6 bags of masonry cement; the blocks cost $472.50 at $2.50 each.
60 × 8 ft fully grouted foundation wall
A 480 sq ft wall needs 540 blocks, or 567 with waste, laid in 12 courses, and 540 × 3.3 ÷ 100 = 17.8, so 18 bags of masonry cement. Filling every core of 8-inch block takes about 0.361 cu ft per sq ft (CMHA, incl. 3% waste), or 173.3 cu ft — roughly 6.4 cu yd, which is better ordered as ready-mix grout than as 263 bags at 0.66 cu ft each.
12-inch retaining wall with half the cores filled
A 90 sq ft wall of 12-inch block needs 101.25 blocks before waste and 112 with 10% extra. Mortar is 101.25 × 3.3 ÷ 100 = 3.34, so 4 bags. Filling half the cores at 0.589 cu ft per sq ft takes 26.5 cu ft (0.98 cu yd, or 41 bags of grout), and the blocks cost $392 at $3.50 each.
Frequently asked questions
How many cinder blocks are in a square foot?+
Standard 8 × 16 inch (nominal) blocks cover 0.889 sq ft each including the mortar joint, so you need 1.125 blocks per square foot, or 112.5 blocks per 100 sq ft of wall before waste.
How many blocks do I need for a 10 × 8 wall?+
An 80 sq ft wall needs 80 × 1.125 = 90 blocks before waste. Laid out, that is 12 courses (8 ft ÷ 8 in) of 7½ blocks each; order about 95 blocks with 5% extra.
How much mortar do I need for concrete blocks?+
The CMHA (formerly NCMA) estimating guide says one 70 lb bag of masonry cement, mixed with sand, lays about 30 hollow blocks with face-shell bedding — roughly 3.3 bags per 100 blocks. If you use premixed mortar (cement and sand in one bag) you will need considerably more bags, so check the bag’s coverage chart.
How much concrete does it take to fill cinder blocks?+
CMHA’s estimating table puts solid grouting of an 8-inch two-core hollow block wall at about 36 cu ft per 100 sq ft of wall (0.361 cu ft per sq ft, including 3% waste) — about 1⅓ cubic yards per 100 sq ft. 6, 10 and 12 inch block take about 25.6, 47 and 58.9 cu ft per 100 sq ft. Filling only the rebar cells uses proportionally less.
Does block width change how many blocks I need?+
No. 6, 8, 10 and 12 inch blocks all share the same 8 × 16 inch face, so the count per square foot is the same. Wider blocks are heavier, stronger and hold more core fill.