onlinecalculator.me

Free online concrete block calculator

Concrete masonry units (CMU) are counted by course and run. Enter your wall dimensions and block size to find out how many blocks to buy and how much mortar to order.

About this calculator

How to use

  1. Enter the wall length and height in feet.
  2. Set the block length and height in inches (default is the standard 16×8 CMU).
  3. Adjust the mortar joint thickness if needed (default is 3/8 inch).
  4. Set a waste factor — 10% is standard for straight walls.
  5. Read off the total block count, courses, blocks per course, and mortar bags.

Formulas

blocks per course = ceil(wall length (in) / (block length (in) + mortar joint (in)))
courses           = ceil(wall height (in) / (block height (in) + mortar joint (in)))
total blocks      = blocks per course × courses
blocks with waste = ceil(total blocks × (1 + waste% / 100))
mortar bags       = ceil(blocks with waste / 25)

All lengths are converted from feet to inches before dividing.

Worked example

Wall 20 ft long × 8 ft high, standard 16×8 CMU, 3/8-inch mortar joint, 10% waste:

blocks per course = ceil(240 / 16.375) = ceil(14.66) = 15
courses           = ceil(96 / 8.375)   = ceil(11.46) = 12
total blocks      = 15 × 12            = 180
with 10% waste    = ceil(180 × 1.10)   = 198–199 blocks
mortar bags       = ceil(199 / 25)     = 8 bags

Order at least 199 blocks and 8 bags of Type S mortar.

Notes

  • Block counts assume running bond (offset half-block each course). Stack bond uses slightly different math.
  • Corners consume one extra block per course per corner to maintain bond. Add 1–2% extra for each inside or outside corner.
  • Mortar coverage varies by brand and consistency. Check the yield on the bag before ordering.
  • Always check local building codes for required block size, mortar type, and reinforcement.

Frequently asked

How do I calculate how many concrete blocks I need?
Divide the wall length (in inches) by the block length plus mortar joint to get blocks per course. Divide the wall height (in inches) by the block height plus mortar joint to get the number of courses. Multiply those two numbers for the total block count, then add your waste factor.
What is the standard concrete block size?
The most common CMU is 16 inches long by 8 inches tall by 8 inches deep (nominal). The actual face dimensions are 15.625 × 7.625 inches, with a 3/8-inch mortar joint accounting for the difference.
How much mortar do I need for concrete blocks?
A general rule of thumb is one 70-lb bag of Type S mortar per 25 blocks. The calculator uses this ratio on the total blocks including waste.
What waste factor should I use for concrete block?
A 10% waste factor is standard for straightforward walls. Add more (up to 15–20%) for walls with many corners, curves, or complex openings.
How do I share my block calculation?
Use the Share button to copy the page link, or Share with my numbers to copy a link that restores your exact dimensions when opened.
Do concrete block sizes include the mortar joint?
No. Nominal block sizes (e.g. 16×8) include one mortar joint in each direction for layout purposes, but the actual block is slightly smaller. This calculator lets you set the mortar joint thickness separately.