Concrete Block Calculator – CMU & Cinder Blocks Needed

Concrete Block Calculator (CMU / Cinder Block)

A standard 8×8×16 concrete block covers 8" × 16" of wall face, so you need 1.125 blocks per square foot — 112.5 blocks per 100 sq ft. Enter your wall length and height, subtract any openings, and get the block count, mortar bags and cost below.

Wall dimensions

Wall length

Wall height

Block size (nominal face 8″ × 16″)

Openings (doors / windows)

No openings. Add a door or window to subtract its area.

Options

Waste / extra (%)

Price per block (optional)

Concrete blocks needed

142blocks

Includes 5% waste. 9 courses × 15 per course.

Net wall area

120 sq ft

Courses

Mortar bags (80 lb)

Show the math

net area = 120 sq ft (after openings)

blocks = 120 × 1.125 = 135

+ 5% waste → 142 blocks

mortar = ceil(142 ÷ 13) = 11 bags

How to use this calculator

Enter the wall length and height in feet and inches.
Pick your block size (8×8×16 is the standard).
Add any door or window openings to subtract their area.
Add a waste allowance (5% is typical for cuts and breakage).
Optionally enter a price per block to estimate material cost.

Formula

net area = (length × height) − openings
blocks per sq ft = 144 ÷ (8 × 16) = 1.125
blocks = ceil(net area × 1.125 × (1 + waste%))
mortar bags ≈ ceil(blocks ÷ 13)

Worked example

A 20 ft × 6 ft wall, no openings, 5% waste:

net area = 20 × 6 = 120 sq ft

blocks = 120 × 1.125 = 135

with 5% waste = 135 × 1.05 = 141.75 → 142 blocks

Concrete block calculator diagram of a CMU wall with courses and openings

Reference tables

Blocks by common wall size (8×8×16, before waste)

Wall (L × H)	Area	Blocks
10 × 4 ft	40 sq ft	45
20 × 4 ft	80 sq ft	90
20 × 6 ft	120 sq ft	135
30 × 8 ft	240 sq ft	270
40 × 8 ft	320 sq ft	360

112.5 blocks per 100 sq ft. Add about 5% for waste.

Tips & buying advice

Order about 5% extra for breakage and cut blocks at corners and openings.
A standard CMU weighs roughly 30–38 lb; a lightweight block is lighter. Plan for help lifting.
One 80-lb bag of mortar mix lays about 13 standard blocks — buy a spare bag.
Blocks are usually sold on pallets; ask your supplier how many per pallet to save on handling.

Frequently asked questions

How many blocks are in 100 square feet?

You need 112.5 standard 8×8×16 blocks per 100 square feet of wall, because each block covers 8" × 16" of face (1.125 blocks per square foot). Round up and add about 5% for waste.

What are the actual dimensions of an 8×8×16 block?

The nominal size is 8×8×16 inches, but the actual block measures about 7⅝ × 7⅝ × 15⅝ inches. The missing ⅜ inch on each side leaves room for a standard mortar joint.

What is the difference between a cinder block and a concrete block?

Both are concrete masonry units (CMU). "Cinder block" is an older term for lighter blocks made with cinders or fly ash; modern blocks are usually heavier aggregate concrete. For estimating counts, treat them the same.

How much mortar do I need per block?

About one 80-lb bag of mortar mix per 13 standard blocks laid with a ⅜-inch joint. For 142 blocks you would plan on roughly 11 bags. Mix in small batches so it does not set before use.

How much does a concrete block weigh?

A standard 8×8×16 block weighs about 30–38 pounds depending on whether it is normal-weight or lightweight aggregate. Solid and larger 12-inch blocks weigh more.

How many blocks are on a pallet?

It varies by block size and supplier — typically 90 to 144 standard blocks per pallet. Confirm the exact count with your yard so your delivery and totals line up.

Built and maintained by Sikander Mushtaq. Formulas checked against CMU nominal-face geometry (144 ÷ 128 = 1.125), Quikrete mortar coverage. See our methodology and sources, or learn about YardageMath.

Last updated: October 7, 2026

Estimates only. Confirm quantities with your supplier or contractor. Read the full disclaimer.