Cinder Block Calculator
Work out how many standard cinder blocks (CMU) and mortar bags you need based on your wall length, height, and block size.
Total blocks
135blocks
Mortar bags
5bags
Blocks per row
15blocks
Rows
9rows
Add about 7% extra blocks for waste and corner cuts — hollow CMU chips more easily than solid block. Nominal dimensions already include the standard 3/8 in mortar joint, so use 16x8 for a standard block even though its actual size is 15 5/8 x 7 5/8 in.
How the cinder block calculator works
Block walls are calculated as a grid, the same way as a retaining wall: how many blocks fit across the wall's length, times how many rows fit up the wall's height.
Formula: blocks per row = wall length (in) ÷ block width. Rows = wall height (in) ÷ block height. Total blocks = blocks per row × rows. Mortar bags = (total blocks ÷ 100) × bags per 100, rounded up.
Frequently asked questions
The standard concrete masonry unit (CMU) is nominally 16 x 8 x 8 inches. Its actual measured size is 15 5/8 x 7 5/8 x 7 5/8 inches — the missing 3/8 inch on each face is the mortar joint, so the 16x8 nominal size is exactly what you should use for this calculator.
Roughly 3 bags of 70 lb pre-mix mortar per 100 blocks for a standard 8-inch-wide wall. Thinner 4-inch block uses less (about 2 bags per 100), and thicker 12-inch block uses more (about 4 bags per 100) — adjust the field above for your wall thickness.
No — retaining wall systems use purpose-built segmental blocks (often around 18 in wide with a separate cap unit) engineered to hold back earth. Standard CMU cinder block is used for foundation walls, garden walls, and general masonry construction, and doesn't need a special cap course. Use the Retaining Wall Calculator for segmental retaining wall block instead.
About 7% is standard for hollow CMU projects — the hollow cores make corners and edges more prone to chipping during handling and cutting than solid block.