Concrete block calculator for a wall

How to use the concrete block calculator

1. Measure the wall: its length and its height in feet and inches, or choose Meters.
2. Count the doors and windows and set their sizes. Their area is taken off the wall.
3. Pick the block size. Standard 8 x 8 x 16 in is filled in. There are also half-high blocks, the UK 440 x 215 mm block, and Other size for any block you type in.
4. Set the waste. 5% is filled in. It is a common allowance for broken and cut blocks, not a rule.

For several walls of the same height, add their lengths and type the total. If the walls differ in height, work each one out and add the counts, or find the total area with the square footage calculator and multiply by the blocks per square foot.

Blocks per square foot

A standard concrete block is sold by its nominal size, 8 x 8 x 16 in, but the block itself is a little smaller. The Concrete Masonry and Hardscapes Association (formerly NCMA) says Specified dimensions of concrete masonry units are typically 3/8 in. (9.5 mm) less than nominal dimensions, so that the module is kept with 3/8 in. (9.5 mm) mortar joints. A block is 7 5/8 in high and 15 5/8 in long, and with its joint it takes up 8 x 16 in of wall.

That gives the rule:

• One block plus joint covers 8 x 16 = 128 sq in.
• A square foot is 12 x 12 = 144 sq in.
• 144 / 128 = 1.125 blocks per square foot.

Half-high blocks (4 x 16 in nominal face) cover half as much, so 2.25 per square foot. The thickness does not change the count: CMHA lists nominal thicknesses from 4 to 16 in, and an 8 in or 12 in wide block with an 8 x 16 in face is still 1.125 per square foot.

Concrete block calculator for a wall

Here is a garden or garage wall 20 ft long and 8 ft high with one 3 x 7 ft door, in standard blocks with 5% waste. Press Try an example to see it in the calculator.

Without the door the wall is 12 courses of 15, which is 180 blocks, the same as 160 x 1.125. When the height is not a multiple of 8 in, the calculator shows part of a course and warns you: the top row then needs cut blocks or half-high blocks.

How many cinder blocks do I need?

Work it out the same way: a standard 8 x 8 x 16 in cinder block is 1.125 per square foot of wall. "Cinder block" is an older name. CMHA's answer to this question calls them essentially the same product produced with the same three basic constituent materials: water, cement, and aggregate. The name dates from the early 1900s, when cinders were used as a light aggregate in the mix. So this calculator works as a cinder block calculator too: pick the block size you are buying.

How much mortar for blocks?

Mortar needs vary with the joint, the mix and the mason, so the calculator uses one maker's published figure. QUIKRETE's data sheet for its Mortar Mix (No. 1102) says each 80 lb bag will lay up to 37 standard bricks or 13 standard 8 x 8 x 16 in blocks. Because that is an upper figure, blocks / 13, rounded up, is the fewest bags you are likely to need.

For the example wall, 165 / 13 = 12.7, so at least 13 bags of 80 lb. Other mortars state different yields, so check the bag you buy. We found no maker's figure for half-high or UK blocks, so the calculator gives none for those.

Corner, half and cap blocks

The count is by area, so it does not split blocks by type. In practice:

• Corners and wall ends often use corner blocks with a finished end. CMHA's note shows single and double corner units, and units made to be easily split on the jobsite, producing two half-length units for openings, ends and corners. Ask your supplier which they stock and swap some standard blocks for them.
• Cap blocks finish the top of a wall. You need about the wall length divided by the length of one cap, for example 240 in / 16 in = 15 caps for a 20 ft wall with 16 in caps. Check the length of the caps you buy.
• Over doors and windows a lintel or bond beam carries the blocks above. That is a structural detail for your plans or your local building department.

Concrete block calculator UK

The common UK block has a face of 440 x 215 mm and is laid with 10 mm mortar joints, so each block takes up 450 x 225 mm of wall. H+H's datasheet for its Celcon blocks gives Blocks/m2 laid 9.88 (10mm joints) for the 440 x 215 mm size, which is 1 / (0.45 x 0.225) = 9.877.

A garden wall 4.5 m long and 1.8 m high is 8.1 m²: 8.1 x 9.877 = 80 blocks, 84 with 5% waste. That is 8 courses of 10 blocks. Choose Meters and the UK block to work in metres. Blocks laid with a thin joint need more per square metre: use Other size and type the joint you will use.

Filling the cores with concrete

Hollow block walls often have some or all of their cores filled with grout or concrete, and reinforced walls need it where the steel runs. The volume depends on the block's core size, which varies by maker, so it is not worked out here. Once you know the volume, the cubic yard calculator turns it into cubic yards and bags of concrete.

In Excel or Google Sheets

If the wall area in square feet is in A2 and the waste percent is in B2, the standard blocks to buy are:

=ROUNDUP(A2*1.125*(1+B2/100), 0)

Microsoft's help page says ROUNDUP Rounds a number up, away from 0 (zero), and Google's says it rounds always rounding up to the next valid increment. For the mortar bags, divide the blocks by 13 and round up the same way. For UK blocks with the area in m², use 9.877 in place of 1.125.

Questions and answers

Sources

Checked on 13 October 2026. If a rule has changed, please tell us.

• Concrete Masonry and Hardscapes Association: CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications
• Concrete Masonry and Hardscapes Association: CMU-FAQ-014, cinder block and concrete block
• QUIKRETE: Mortar Mix (No. 1102) data sheet (yield per 80 lb bag)
• H+H UK: Celcon pack information datasheet (blocks per m² with 10 mm joints)
• Microsoft Support: ROUNDUP function
• Google Docs Editors Help: ROUNDUP