Cinder Block Calculator
Calculate how many cinder blocks, mortar bags, and sand you need for walls, foundations, and retaining walls.
Cinder Block Estimator
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How to Calculate Cinder Blocks
The standard calculation for an 8×8×16 cinder block wall uses 1.125 blocks per square foot of wall area. This accounts for the 3/8-inch mortar joint between each block, which slightly reduces the effective coverage area compared to the block's nominal dimensions.
To calculate manually: multiply your wall length by wall height to get total square footage, then multiply by 1.125. Add a 5% waste factor for cuts and breakage. For windows and door openings, subtract those square footages from the total before calculating.
Don't forget openings: If your wall has windows or doors, measure those rough openings and subtract that square footage from your total wall area before plugging into the calculator. A 3×7 door opening = 21 sq ft you don't need blocks for.
Mortar Estimating Rule of Thumb
A standard 60lb bag of mortar mix covers approximately 25-30 blocks when used as bed and head joints on a standard 8×16 block. This calculator estimates mortar at 30 blocks per 60lb bag, which is a conservative middle estimate. Buy a few extra bags — mortar is inexpensive compared to running short mid-project.
Block Size Reference Table
| Block Size (nominal) | Blocks per Sq Ft | Common Use |
|---|---|---|
| 8×8×16 (standard) | 1.125 | Most walls, foundations, garages |
| 6×8×16 | 1.125 | Non-load-bearing partition walls |
| 12×8×16 | 1.125 | Heavy load-bearing, retaining walls |
| 4×8×16 (half block) | 1.125 | Caps, fillers, veneer walls |
Cinder Block vs. Concrete Block
Technically, "cinder block" and "concrete block" are not the same thing, though the terms are used interchangeably in everyday speech. True cinder blocks are made with coal cinder aggregate and were common through the mid-20th century. Modern "cinder blocks" are almost always concrete masonry units (CMU) made with Portland cement, sand, gravel, and water — not actual cinders.
For practical purposes, when people ask for "cinder blocks" at a hardware store or lumberyard, they're getting CMU concrete blocks. This calculator works for both.
Frequently Asked Questions
- How many cinder blocks do I need per square foot?Standard 8×8×16 cinder blocks require 1.125 blocks per square foot of wall area, accounting for mortar joints.
- What is the standard size of a cinder block?The nominal size is 8"×8"×16". The actual (manufactured) dimensions are 7-5/8"×7-5/8"×15-5/8" — the 3/8" difference on each side accounts for a mortar joint.
- How much does a cinder block weigh?A standard 8×8×16 cinder block (CMU) weighs approximately 30-35 pounds for a standard block. Lightweight blocks can weigh as little as 22 pounds; heavy-duty solid blocks can exceed 50 pounds.
- Do I need rebar in a cinder block wall?For structural walls, retaining walls, and anything over 4 feet tall, yes — rebar is typically required by building codes. Vertical rebar runs through the hollow cores, which are then filled with grout. Always check local building codes and pull a permit for structural block work.