Sawhorse Calc

Concrete block calculator

Add each wall as its length and height. You get the number of blocks to buy, the bags of mortar to lay them, and the cost. It starts with standard 8 by 8 by 16 in blocks.

Areas to add up

Lengths in feet. 12.5 means 12 ft 6 in. You can also type 12' 6" or 6 in.Lengths in metres.

Block size, mortar and openings

Starting figures are typical. Type the ones on your product or quote if you have them.

Cost (optional)

189 blocks

Wall area, sq ft
160.0
Blocks per sq ft
1.125
Bags of mortar
16

How to work it out

  1. Measure each wall's length and height, and enter them as length and width. Add each wall as its own area.
  2. Add up doors, windows and other openings and enter the total to subtract.
  3. Check the block size. A standard block is 15⅝ by 7⅝ in, which is 16 by 8 in once you add a ⅜ in mortar joint. Enter the size with the joint.
  4. Keep about 5% extra for cuts and broken blocks, and check the bag of mortar for how many blocks it lays.

Quick reference

ItemValue
Standard block, with joint16 × 8 in, 0.89 sq ft
Blocks per 100 sq ft of wall112.5
Metric block, with joint450 × 225 mm, about 9.9 per m²
One 80 lb bag of mortar laysAbout 12 standard blocks

These are common starting points. Your site, climate and supplier may call for something different.

A worked example

A garden wall 20 ft long and 4 ft high is 80 sq ft. Each standard block covers 0.889 sq ft with its joint, so you need 80 ÷ 0.889 = 90 blocks, or 95 with 5% extra. At about 12 blocks a bag, that is 8 bags of mortar.

What this leaves out

Footings, rebar, grout for filled cores, wall ties and lintels over openings are worked out separately. Block walls that hold back soil, carry a load or stand over about 4 ft tall usually need a design and a permit, so check with your local building department first.

Common questions

How many concrete blocks are in a square foot?

1.125 standard 8 by 8 by 16 in blocks per square foot, or 112.5 per 100 sq ft, counting the mortar joint.

How many blocks does a bag of mortar lay?

An 80 lb bag of mortar mix lays about 12 standard blocks with normal joints, according to typical bag yields. Full mortar bedding uses more, so check the bag.

Are cinder blocks and concrete blocks the same?

People use both names for the same hollow building block, also called a CMU (concrete masonry unit). Most blocks sold today are made with concrete, not cinders.

What size is a standard concrete block?

The real size is 7⅝ by 7⅝ by 15⅝ in. With a ⅜ in mortar joint it takes up 8 by 8 by 16 in, which is the size people quote and the one to enter here.

How do I count blocks for a wall with a door?

Work out the full wall, then subtract the door and window areas. Enter their total in the openings box and the calculator takes it off before counting.

Doing this job for a customer? Turn your numbers into a free written estimate on QuoteToPaid.

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