Concrete Block Calculator
Calculate exactly how many CMU (concrete masonry unit) blocks and mortar bags you need for your wall project. Accounts for door and window openings.
How to Calculate Concrete Blocks
Concrete blocks, also called CMU or cinder blocks, are the backbone of many residential and commercial construction projects. The standard block is 8×8×16 inches (nominal), which actually measures 7-5/8 × 7-5/8 × 15-5/8 inches to allow for a 3/8" mortar joint. Our calculator uses the nominal dimensions plus mortar joint to compute the wall area each block covers.
Standard Block Sizes
The most common CMU is the 8×8×16 standard block. Other sizes include 4×8×16 (half-width for veneer walls), 8×4×16 (half-height), and 12×8×16 (wider for foundation walls). Always verify which size your project spec calls for before ordering.
How Much Mortar Do I Need?
As a rule of thumb, you need about 7 bags of mortar mix per 100 standard blocks. This assumes standard 3/8" joints with face-shell bedding. Thicker joints or full mortar bedding will increase the amount needed. Our calculator includes this estimate automatically.
Accounting for Openings
Doors and windows reduce the number of blocks needed significantly. A standard 3×7 foot door opening eliminates about 24 blocks, and a 3×3 foot window opening removes about 10 blocks. Always subtract openings before ordering — but keep the 5% waste factor for cuts around openings and corners.
Frequently Asked Questions
How much does a concrete block weigh?
A standard 8×8×16 hollow block weighs about 28–35 pounds. Solid blocks of the same size weigh around 43 pounds. Lightweight blocks made with lighter aggregates weigh 22–28 pounds.
How much does a concrete block cost?
Standard 8×8×16 blocks typically cost $1.75–$3.00 each at home improvement stores. Prices vary by region and quantity. Buying pallets (usually 72–108 blocks) usually offers a discount.
Do I need rebar in a block wall?
Most block walls over 4 feet tall require vertical rebar every 32–48 inches, with the cells grouted solid. Horizontal reinforcement (bond beam blocks with rebar) is typically required every 48 inches vertically. Check your local building codes.
What type of mortar should I use?
Type S mortar is the most common for structural block walls. Type N is suitable for non-load-bearing walls and veneer. Type M is used for below-grade applications like foundations and retaining walls.
How many blocks can a mason lay per day?
An experienced mason can lay 400–500 blocks per day. For DIYers, expect 50–100 blocks per day depending on experience. Plan your project timeline accordingly.
Related Calculators
- Concrete Calculator — volume calculator for slabs, footings, columns
- Concrete Slab Calculator — with rebar and materials
- Concrete Driveway Cost Calculator — driveway cost estimator
Estimates only. Consult a licensed professional for your specific project requirements.