Concrete Block Calculator
Block count and mortar bags for any CMU wall
What is concrete block calculator?
A concrete block calculator answers two questions: how many blocks does my wall need, and how many bags of mortar to lay them. Both are linear functions of wall area, given block size and joint thickness.
The standard concrete masonry unit (CMU) is 8 in × 8 in × 16 in — that’s the actual block before mortar. With the conventional 3/8-in mortar joint, the effective face becomes 8.375 × 16.375 in = 137.14 in² = 0.952 ft². So one square foot of wall takes about 1.05 blocks (raw, before waste).
The mortar rule of thumb is 3 bags of 80-lb mortar mix per 100 blocks at 3/8-in joint thickness. Thicker joints burn through mortar faster — 1/2-in joints push that to roughly 4 bags per 100 blocks. The calculator ceils to the next whole bag.
Block sizing: Half-height 4×16 blocks are common for veneer and trim courses. 12×16 (12-in deep block) shows up in foundations and load-bearing applications where extra thickness matters. 6×16 is non-bearing partition block. The “Custom” option handles all of these — enter the actual block face dimensions in inches.
Important non-goal: this tool sizes blocks and mortar. It does not size grout (the concrete that fills cells inside the blocks for rebar reinforcement) or rebar (the steel reinforcement inside those grouted cells). Those are separate calculations driven by local building code and structural-engineering requirements. Anything taller than about 24 in or load-bearing should be designed with code in mind.
Use the gravel-calculator and concrete-calculator siblings to size the footing your block wall sits on — typically a 16-in-wide × 8-in-deep concrete strip footing for residential block walls.
When to use a concrete block calculator
- Garden retaining wall — Stack 8 in × 16 in CMU two or three courses high for a 16-32 in retaining wall. The calculator handles wall length × height and gives a block count plus mortar bags.
- Garage or shed foundation — Standard CMU for stem walls and foundations. The custom-size option handles 12 in CMU for thicker foundation walls.
- Outdoor kitchen or grill base — A 6-foot-long, 36-in-tall block base takes about 30 standard 8x16 blocks. Add fire-brick veneer separately.
How to use the Concrete Block Calculator
- Measure the wall — Multiply wall length by wall height in feet. For irregular walls, sum each segment separately.
- Pick a block size — Standard CMU is 8 in tall × 16 in long (with mortar joints, the effective face is 8.375 × 16.375 in). Choose half-height 4×16 for veneer or custom for foundation block.
- Set the mortar joint — 3/8 in is the universal default. Bump to 1/2 in for rough-laid block; 1/4 in for tight cavity-fill walls.
- Add waste factor — 10% covers cuts and a few cracked blocks. Bump to 15% if your wall has a lot of corners or openings.
Worked examples
10 ft × 8 ft wall, standard CMU
Input: 10 ft × 8 ft, 8 in × 16 in block, 3/8 in joint, 10% waste
Output: 93 blocks · 3 mortar bags20 ft × 4 ft retaining wall
Input: 20 ft × 4 ft, 8 in × 16 in block, 3/8 in joint, 10% waste
Output: 93 blocks · 3 mortar bags8 ft × 8 ft using half-height 4×16
Input: 8 ft × 8 ft, 4 in × 16 in block, 3/8 in joint, 10% waste
Output: 151 blocks · 5 mortar bags