Softonic

Concrete Block Calculator

Step 1 / 3 Blocks needed
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Ordering concrete masonry units is easy to get wrong, guess low and the job stalls waiting on a second delivery, guess high and a half-pallet of CMU blocks sits in the yard until next spring. This calculator turns a wall’s length and height into the exact block count for whatever unit size you’re laying, adds the bags of mortar needed to bed them, and folds in a waste factor so cuts, breakage and the odd dropped block are already covered before you place the order.

How to calculate how many concrete blocks you need

  1. 1

    Measure the wall

    Enter the wall length and height in feet. For multiple walls, add the lengths together at the same height.

  2. 2

    Set the block size and waste

    Enter the block face length and height in inches (a standard CMU is 16 in × 8 in), then set a waste percentage: 5% is typical.

  3. 3

    Read the result

    You get the total blocks needed, the wall area in square feet, and the number of mortar bags to buy.

The formula

Block counts come from the face area of the wall and how many blocks of your chosen size fill one square foot of that face, mortar joints included in the nominal dimensions:

Wall area        = length × height                       (ft²)
Block face area  = (block length × block height) / 144   (ft²)
Blocks per ft²   = 1 / block face area
Blocks (final)   = ceil( wall area × blocks per ft² × (1 + waste / 100) )
Mortar bags      = ceil( blocks final / 33 )

Dividing by 144 converts the block face from square inches to square feet. A standard 16 in × 8 in block has a nominal face of 0.889 ft², so it takes about 1.125 blocks per square foot of wall.

Worked example

A garden wall 20 ft long and 8 ft high in standard 16 in × 8 in block, with 5% waste:

  • Wall area = 20 × 8 = 160 ft²
  • Block face = (16 × 8) / 144 = 0.889 ft²
  • Blocks per ft² = 1 / 0.889 = 1.125
  • Raw blocks = 160 × 1.125 = 180
  • With 5% waste = 189 → 189 blocks
  • Mortar = 189 ÷ 33 = 5.7 → 6 bags
Wall (ft) Block size (in) Waste Blocks Mortar bags
20 × 8 16 × 8 5% 189 6
30 × 8 16 × 8 5% 284 9
20 × 8 12 × 8 10% 264 8

Common pitfalls

  • No waste factor. Cuts at corners and openings, plus the occasional cracked block, eat 5–10% on a straightforward job and more on anything curved or detailed.
  • Mixing nominal and actual sizes. Use the nominal face (16 in × 8 in for a “standard” block), it already includes the 3/8 in mortar joint. Entering the actual 15 5/8 in size double-counts the joint.
  • Ignoring openings. Doors, windows and vents reduce the count. Subtract their area from the wall area before ordering on large jobs.
  • Forgetting the bond beam and lintels. Solid-grouted courses and lintel blocks change both block and mortar needs on structural walls, treat this count as the field-stretcher baseline.

A rule of thumb: about 1.125 standard blocks per square foot with a 3/8 in joint. The calculator is just that rule, made exact for your wall and padded for waste.

Frequently Asked Questions

About 1.125 standard 16 in × 8 in blocks per square foot, since one block face covers roughly 0.889 ft² once the mortar joint is included. The calculator works this out from whatever block length and height you enter, so it adapts to half-blocks and metric sizes too.

Five percent is the usual allowance for a straightforward wall, it covers cuts and the occasional broken block. For walls with lots of corners, openings or curves, bump it to 10–15%.

A common field figure is one bag of mortar mix per about 33 standard blocks laid. The calculator divides your final block count by 33 and rounds up, so the bag total already includes your waste allowance.

The numbers you enter are sent to our server only to compute your results. They are not stored, saved or shared. In the step-by-step view, the values also travel in the page link between steps, so avoid sharing links that contain numbers you want to keep private.

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