Concrete Block Calculator

What the concrete block calculator does

You are building a block wall and you need to know how many blocks to buy. This works that out. You give it the wall width and height, pick your block size, and it returns the number of blocks, the bags of mortar to lay them, and the sand for that mortar. Add a price per block and it totals the cost too.

It is built for standard concrete masonry units, the grey hollow blocks most walls are made of. Below is how it counts, a worked example, and the things worth knowing before you order a pallet.

How to use it

1. Enter the wall width and height. Measure the face of the wall and choose the unit next to each box. You can mix units, and feet is usually easiest.
2. Pick the block size. The sizes are the face of the block, height by length, so a standard block is 8 by 16 inches.
3. Add a price per block. Optional. Leave it blank for just the material counts, or fill it in to get a cost.
4. Press Calculate for the breakdown, or Reset to clear it.

How the block count is worked out

The idea is simple. Work out the area of the wall, then see how many block faces it takes to cover it.

Blocks = wall area ÷ the face area of one block

So a wall is its width times its height, and a standard 8 by 16 inch block covers about 0.89 square feet of that wall. Divide one by the other and you have your block count, rounded up to a whole block.

Mortar and sand come straight off that count. The calculator allows one bag of mortar for roughly every 33 blocks, which is the long-standing mason's rule of about three bags per hundred blocks. The sand is the amount you would need to mix that many bags by hand.

Block sizes, units, and how precise this is

You can enter the wall in feet, inches, centimetres, metres, or yards, in any mix. The block sizes in the list are nominal, and that word matters here. A block called 8 by 16 inches is actually a touch smaller, around 7 and 5/8 inches, and the missing 3/8 of an inch is the mortar joint around it. Add the joint back and you get the full 8 by 16, which is why twelve courses of block come out to exactly 8 feet.

Block counts are always rounded up to a whole block, because you cannot lay most of one. The mortar and sand are estimates that move with the brand of mortar and the thickness of your joints, so treat them as a close starting point rather than an exact figure. If you add a price, the cost uses your block price plus an average price for mortar and sand, so it is a guide for budgeting, not a quote.

A worked example: a 10 ft by 8 ft wall

Say the wall is 10 feet wide and 8 feet high, built from standard 8 by 16 inch blocks.

The wall area is 10 × 8 = 80 square feet. Each block covers about 0.89 square feet, so 80 ÷ 0.89 = 90 blocks. That many blocks needs about 3 bags of mortar (90 ÷ 33), and roughly 0.4 cubic yards of sand to mix it.

So for that wall you would order 90 blocks, 3 bags of mortar, and a little under half a yard of sand, then add a few extra blocks for the reason in the next-but-one section.

How many blocks per square foot?

This is the quick number people look up, so here it is by block size. It is just 144 square inches in a square foot divided by the block's face area.

The 8 by 16 block is the workhorse, so 1.125 blocks per square foot is the figure you will reach for most. Multiply your wall area by that and you have a quick block count before you even open the tool.

Ordering, cuts, and filling the cores

The count is for a clean wall with no waste. In practice you will cut blocks at the corners and around any openings, and the odd one turns up cracked. So order about 5 to 10 percent more than the count, and lean toward 10 percent if the wall has lots of corners, doors, or windows. Returning a few spare blocks is cheaper than stopping mid-course for a second trip.

One thing this tool does not cover: if the wall is load-bearing or a retaining wall, you will usually fill the hollow cores with concrete and run rebar through them. That fill concrete is a separate order, and you can size it with the concrete calculator. Anything structural should be built on a proper footing and signed off by an engineer, so the wall stands up to what it is holding.

Questions people ask

How many blocks do I need for a wall?

Multiply the wall area by the blocks per square foot for your size. For standard 8 by 16 inch blocks that is the area times 1.125, so an 80 square foot wall needs about 90 blocks.

How many concrete blocks are in a square foot?

For a standard 8 by 16 inch block, about 1.125, since one block covers roughly 0.89 square feet of wall.

How much mortar do I need per block?

Roughly one bag of mortar for every 33 blocks, which is the usual rule of about three bags per hundred. The exact number shifts with your mortar brand and joint thickness.

How much does a concrete block weigh?

A standard 8 by 16 inch hollow block weighs around 38 pounds. Lightweight blocks are nearer 28 pounds, and solid ones can top 50.

Do I need to fill the blocks with concrete?

Only for load-bearing and retaining walls, where filled cores and rebar add the strength. For a simple garden or partition wall, hollow blocks are usually fine.

References

A quick note on where these figures come from. The blocks-per-square-foot rates and the mortar allowance follow the masonry industry's own estimating standard from the National Concrete Masonry Association. The nominal block dimensions, the 7 and 5/8 inch actual size that becomes 8 inches with the joint, are set by the ASTM specification for loadbearing concrete masonry units. The unit conversions follow the US National Institute of Standards and Technology guide.

1. National Concrete Masonry Association, TEK 14-12 (block quantity and masonry estimating). https://ncma.org
2. ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units. https://www.astm.org/c0090-22.html
3. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811