Concrete Block Calculator
Find how many concrete blocks and bags of mortar you need for a wall. Also called a cinder block or CMU (concrete masonry unit) calculator.
Measure your wall
For standard 8 × 8 × 16 in blocks. This counts a solid wall. Doors and windows mean fewer blocks.
Your answer
For a wall of 80 sq ft, with 5% extra.
This answer is a guide. For walls that hold back soil or carry weight, check your local building rules or ask a builder or engineer.
What it costs
Prices are US middle prices from Oct 2026. Ask your store for their price.
Quick answers
- Blocks per square foot
- 1.125 blocks, so 112.5 blocks per 100 sq ft. With 5% extra: 119.
- Mortar
- About 8 bags of 80 lb mortar mix per 100 blocks, plus 15% for waste. For other block widths, see our block mortar calculator.
- Block size
- Sold as 8 × 8 × 16 in. The real block is about 7⅝ × 7⅝ × 15⅝ in, to leave room for mortar.
- Block price
- About $2.84 each at our middle price (4 US stores, Oct 2026).
How to work it out
- Measure the wall length and height in feet. Multiply them.
- Multiply by 1.125 to get blocks.
- Add 5% for broken and cut blocks. Round up.
- For mortar, multiply the blocks by 8 and divide by 100. Add 15%. Round up.
20 × 4 = 80 sq ft
80 × 1.125 = 90 blocks
90 + 5% = 94.5 → 95 blocks
95 × 8 ÷ 100 + 15% = 8.74 → 9 bags of mortar
Blocks and mortar for common walls
This table shows blocks and 80 lb mortar bags for some wall sizes. Blocks include 5% extra. Mortar includes 15% for waste.
| Wall | Rows | Blocks | Mortar bags | Cost |
|---|---|---|---|---|
| 8 × 2 ft | 3 | 19 | 2 | $73 |
| 16 × 4 ft | 6 | 76 | 7 | $283 |
| 20 × 4 ft | 6 | 95 | 9 | $356 |
| 24 × 6 ft | 9 | 171 | 16 | $639 |
| 40 × 6 ft | 9 | 284 | 27 | $1,065 |
| 40 × 8 ft | 12 | 378 | 35 | $1,409 |
Cost uses our middle prices: $2.84 a block (4 stores) and $9.59 a bag of mortar (7 stores), Oct 2026.
This table counts blocks and mortar only. For walls that hold back soil or carry weight, check your local building rules. Or ask a builder or engineer.
Count the blocks row by row
A standard block is sold as 8 × 8 × 16 in. Builders call this its nominal size. It is the real block plus one mortar joint.
The real block is about 7⅝ × 7⅝ × 15⅝ in. The joint fills the last ⅜ in.
Blocks come 4, 6, 8, 10 or 12 in wide. They are all 8 in high and 16 in long. So the count is the same for every width.
To check our count by hand:
- Change the wall length to inches. Divide by 16. This gives blocks in each row.
- Change the wall height to inches. Divide by 8. This gives the number of rows.
- If either one is not a whole number, round it up. Then multiply the two.
A 20 × 4 ft wall: 240 in ÷ 16 = 15 blocks per row. Then 48 in ÷ 8 = 6 rows. So 15 × 6 = 90 blocks. That matches 80 sq ft × 1.125.
Why mortar counts differ
Ask how many blocks a bag of mortar lays, and you get different answers. Here is what three sources say for an 80 lb bag.
| Who says | Blocks per bag | Bags per 100 blocks |
|---|---|---|
| Sakrete Type S | 12 | 8.33 |
| Quikrete Mortar Mix | up to 13 | 7.69 |
| CMHA (masonry trade group) | 16 | 6.25 |
CMHA is the Concrete Masonry & Hardscapes Association, a trade group. Its figure already has some waste in it.
We plan 8 bags per 100 blocks, close to the bag makers. Then we add 15% for waste. The Brick Industry Association gives 15–25% for mortar waste, so we use the low end.
Our prices are for Type S, a strong mortar for walls that carry a load. A garden wall can usually use Type N.
For bags by block width, use our block mortar calculator. To compare mortar types, see the mortar calculator.
Filling the cores with grout
Blocks have hollow holes called cores. Some walls need these filled with grout. Your plans or local building rules say which cores to fill.
| Cores filled | Cubic feet | 80 lb grout bags |
|---|---|---|
| Every core (solid) | 36.1 | 61 |
| A core every 16 in | 18.1 | 31 |
| A core every 32 in | 9.1 | 16 |
| A core every 48 in | 6.1 | 11 |
CMHA figures for an 8 in wall. They include 3% extra. Bags are counted at 0.6 cubic feet each.
Grout also comes in 80 lb bags. Each one makes 0.6–0.68 cubic feet, depending on the brand. We plan on 0.6, the smallest, so you have enough. A solid 100 sq ft wall then takes about 61 bags.
Other block widths hold more or less grout. Our block fill calculator covers them.
The calculator above uses the "every core" figure. Use this table if your plans fill fewer cores.
Before you buy
- Measure the wall length and height in feet.
- Take off big openings, like doors and windows.
- Ask your local building office what the wall must sit on.
- Keep the 5% extra for broken and cut blocks.
- Find out which cores, if any, must be filled.
- Ask the store about delivery, and where they can drop the blocks.
Questions people ask
How many cinder blocks do I need?
Multiply the wall area in square feet by 1.125. Then add 5% for broken and cut blocks. The calculator above does it for you.
How do you calculate the volume of a block?
Multiply its real size: about 7⅝ × 7⅝ × 15⅝ in. That is about 0.53 cubic feet from the outside. The hollow cores make the solid part smaller.
How tall is a cinder block?
It is sold as 8 in tall. The real block is about 7⅝ in. The mortar joint makes up the last ⅜ in.
How many CMU blocks do I need?
CMU means concrete masonry unit, the trade name for a concrete block. A 20 × 4 ft wall has 15 blocks in a row and 6 rows. That is 90 blocks, or 95 with 5% extra.
How much do concrete blocks cost?
About $2.84 each, the middle price of 4 US stores. An 80 lb bag of Type S mortar is about $9.59, the middle of 7 stores (Oct 2026).
How much does it cost to build a cinder block wall?
For a 20 × 4 ft wall, blocks and mortar cost about $356 at our middle prices. That is for materials only. It does not cover a footing, grout or labor.
Do cinder blocks need to be filled?
Your plans or local building rules say which cores to fill. Or ask a builder or engineer. If you fill every core, the calculator above adds the grout. Our block fill calculator counts the bags.
Where our numbers come from
- Blocks, mortar, sand and core fill: Concrete Masonry & Hardscapes Association (TEK 04-02A)
- Block size: Concrete Masonry & Hardscapes Association (TEK 02-02B)
- Mortar per block: Quikrete Mortar Mix data sheet
- Mortar per block: Sakrete Type S bag (about 12 blocks per 80 lb bag)
- Which mortar type: Brick Industry Association (Technical Note 8B)
- Bricks per square foot and mortar waste: Brick Industry Association (TN 10)
- Block and mortar prices: the middle of 4 stores (blocks) and 7 stores (mortar), Oct 2026
- Blocks per bag by width: TCC Materials table (2025)
- Core-fill grout: Quikrete data sheet (coarse)
- Core-fill grout: Quikrete data sheet (fine)
Made by BuildersCount. Last checked 3 Oct 2026.
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