Cinder Block Calculator — Blocks and Mortar Bags for Any Wall
Concrete blocks are sold individually but you need to account for mortar joints to get an accurate count. This calculator works out blocks per row, the number of rows, and the total block count — plus an estimate of mortar bags.
How we calculate
Effective block area = (block length + joint) × (block height + joint). Blocks = ⌈(wall area ÷ effective block area) × (1 + waste %)⌉. Mortar bags = ⌈blocks ÷ 15⌉.
Worked example
4 m × 2 m wall (8 m²). 400 × 200 mm blocks with 10 mm joints: effective area = 0.41 × 0.21 = 0.086 m². Raw blocks ≈ 93. With 5% waste: 98 blocks. Mortar: ⌈98 ÷ 15⌉ = 7 bags.
Common mistakes
- Counting blocks without accounting for mortar joints — this significantly underestimates the number needed.
- Not ordering a few extra blocks — chipped corners during delivery or cutting are common.
- Forgetting lintels over door and window openings — these replace a run of standard blocks.
- Using the wrong mortar mix for the application — below-ground blocks need a stronger mix.
Frequently asked questions
How many cinder blocks do I need per square metre?+
Standard 400 × 200 mm blocks with 10 mm mortar joints have an effective face area of about 0.086 m², so you need roughly 11–12 blocks per square metre of wall. Adjust for your specific block size using the calculator.
What is the difference between a cinder block and a concrete block?+
Historically, cinder blocks used coal ash as aggregate, making them lighter. Modern "cinder blocks" are almost always dense or lightweight concrete blocks. Both terms are used interchangeably in DIY contexts — the size and mortar calculation is the same.
How much mortar do I need for a block wall?+
A rough rule is one 25 kg mortar bag for every 15 blocks. For a precise count, use the Mortar Calculator — it accounts for wall area and your specific bag coverage.
Do I need to add a waste allowance for blocks?+
Yes — allow at least 5%. Blocks chip during delivery and cutting, and corner and doorway cuts produce unusable offcuts. Buying a few extra is much cheaper than a second delivery.