Cinder Block Calculator

Calculate how many cinder blocks (CMU) you need for a wall from length, height and openings, with courses, blocks per course, mortar bags and core fill.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To find how many blocks you need, multiply the wall area by 1.125 blocks per square foot for standard 8 x 8 x 16 blocks, then add waste. A 30 x 8 ft wall (240 sq ft) needs 270 blocks, or 284 with 5% waste, laid in 12 courses with about 22 bags of 80 lb mortar mix.

Results

Blocks to buy: 284 blocks; 270 standard 8 x 8 x 16 blocks in the wall plus 5% waste
Courses: 12 courses; 8 in per course including the joint, 8 ft tall
Blocks per course: 22.5 blocks; 30 ft at 16 in per block; half blocks fill odd ends
Mortar mix (80 lb bags): 22 bags; About 13 blocks per bag with 0.375 in joints
Core fill grout (if all cells grouted): 2.2 cu yd; 0.22 cu ft per block; most walls grout only the cells with rebar

Cinder Block formula

Wall area (sq ft) = Length x Height - openings
Blocks = Wall area x 1.125 x (1 + waste %)   (standard block with 3/8 in joints covers 16 x 8 in = 0.889 sq ft)
Courses = Height (in) / 8, Blocks per course = Length (in) / 16
Mortar bags (80 lb) = Blocks / 13

Worked example

A 40 x 6 ft foundation wall with one 3 x 7 ft door opening: wall area is 240 - 21 = 219 sq ft. 219 x 1.125 = 246.4, so 247 standard blocks, or 259 with 5% waste. The wall is 72 / 8 = 9 courses of 480 / 16 = 30 blocks each.

Mortar: 259 / 13 = 20 bags of 80 lb mortar mix. If every core is grouted, 246 x 0.22 = 54 cubic feet, or 2 cubic yards of grout.

Quick reference

Standard 8 x 8 x 16 blocks needed by wall size (no openings, no waste)
Wall length	4 ft tall (6 courses)	6 ft tall (9 courses)	8 ft tall (12 courses)
10 ft	45	68	90
20 ft	90	135	180
30 ft	135	203	270
40 ft	180	270	360
50 ft	225	338	450

How to calculate concrete blocks

Multiply the wall length by its height for the face area and subtract any openings. A standard block with its mortar joint covers 16 by 8 inches, which is 0.889 square feet, so every square foot of wall takes 1.125 blocks. A 240 square foot wall needs 270 blocks, and 5 percent waste brings it to 284.

Check the layout too. Blocks stack in 8 inch courses, so an 8 foot wall is exactly 12 courses. Along the length, each course of a 30 foot wall takes 22.5 blocks: the half block at the end of alternate courses keeps the running bond, so buy a few half blocks or plan to cut. Corners take corner blocks with one square end.

Mortar, grout and reinforcing

Block is laid with face-shell bedding, so an 80 lb bag of Type N or Type S mortar mix lays about 13 blocks. For larger walls, site-mixed mortar from masonry cement and sand is cheaper: about 3 bags of masonry cement and 1/3 cubic yard of sand per 100 blocks. Foundation and retaining walls should use Type S.

Most structural block walls have vertical #4 or #5 rebar in grouted cells every 32 to 48 inches and a bond beam at the top, with horizontal joint reinforcement every other course. Filling only the reinforced cells takes a fraction of the grout shown for a fully grouted wall. Use 12 inch block for taller retaining and foundation walls where the engineer or code table calls for it.

Key facts

A standard concrete block is 15-5/8 x 7-5/8 x 7-5/8 in; with a 3/8 in joint it lays up to a nominal 16 x 8 x 8 in.
A wall takes 1.125 standard blocks per square foot, or 112.5 blocks per 100 sq ft.
An 80 lb bag of mortar mix lays about 13 standard blocks; 100 blocks need about 8 bags.
A standard hollow 8 in block weighs 35 to 38 lb; a lightweight block weighs 26 to 30 lb.
Fully grouting an 8 in block wall takes about 0.93 cubic yards of grout per 100 sq ft of wall.

Frequently asked questions

How many cinder blocks do I need for a 100 square foot wall?

113 standard 8 x 8 x 16 blocks, or about 119 with 5% waste.

How many blocks per course?

Divide the wall length in inches by 16. A 20 ft wall is 240 / 16 = 15 blocks per course; a 30 ft wall is 22.5, so alternate courses end in a half block.

How many bags of mortar per 100 blocks?

About 8 bags of 80 lb premixed mortar, at roughly 13 blocks per bag with 3/8 in joints.

Are cinder blocks and concrete blocks the same?

Today, yes. True cinder blocks with coal-ash aggregate are rarely made; modern concrete masonry units (CMU) come in the same nominal sizes and the names are used interchangeably.

Sources and references

CMU-TEC-001 Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (Concrete Masonry & Hardscapes Association)
QUIKRETE Mortar Mix No. 1102 product data sheet (Quikrete)
Sakrete Mortar Mix Type S product page (Sakrete)
Chapter 4 Foundations, 2021 IRC (ICC)

Next steps for this project

The usual order for a concrete & masonry project. See all concrete & masonry calculators.

Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.