Concrete Block Calculator
Build a block wall and the two numbers that matter are how many blocks and how many bags of mortar, get either wrong and you're back at the supply yard mid-pour. Enter your wall's length and height below. The calculator counts standard blocks, estimates the mortar, adds a waste allowance, and gives you a cost, so you can buy the whole job in one trip.
How many blocks you need, plus the mortar and sand
Most block calculators hand you a block count and stop. A real order needs three numbers: the block, the mortar, and the sand to mix it. This tool gives all three, and it lets you take out doors and windows so you're not buying block for a hole in the wall. Enter the wall's length and height, subtract the openings, and you'll get a count you can take to the yard.
The block math, step by step
A standard concrete block is 8 inches high and 16 inches long on the nominal face, which already includes the 3/8-inch mortar joint. That face covers about 0.89 square foot of wall. So for every square foot of wall you need about 1.125 blocks, or roughly 113 blocks per 100 square feet. Multiply your net wall area by 1.125 and you've got the count. Add 5 to 10 percent on top for breakage and cuts.
| Block face (nominal) | Blocks per sq ft | Per 100 sq ft |
|---|---|---|
| 8 x 16 (standard) | 1.125 | ~113 |
| 4 x 16 (half height) | 2.25 | ~226 |
| 12 x 16 | 0.75 | ~75 |
Take out the doors and windows
This is where most online calculators leave you over-ordering. Figure the area of each opening (width times height) and subtract it from the gross wall area before you multiply by 1.125. A single 3 by 7 foot door is 21 square feet, about 24 blocks you don't need. The tool lets you enter openings so the count lands right.
Mortar and sand
Plan on about 3 bags of mortar per 100 blocks laid with a standard 3/8-inch joint, so divide your block count by 33. By bag, an 80-pound bag of Type S mason mix sets roughly 13 blocks and a 60-pound bag about 9. If you mix your own from portland and sand, figure around 1 cubic yard of sand for every 7 bags of cement. The calculator sizes the mortar with the block so you order them together.
A real wall example
Take a 40-foot wall, 8 feet high. That's 320 square feet of gross face. Knock out a 3 by 7 door (21 square feet) and you're at 299 square feet. Times 1.125 is about 336 blocks. Add 5 percent for cuts and corners and you order 350. Mortar runs about 336 divided by 33, near 10 bags, plus the sand to mix it.
Rebar and core fill for load-bearing walls
A freestanding garden wall is one thing; a load-bearing or tall wall is another. Those get vertical rebar set in the cores at a spacing your code or engineer calls out, and the cores around it filled with grout. One standard block core holds roughly 312 cubic inches of fill, so grout adds up fast. Size the structural steel and fill to your local code, not a rule of thumb.
Where the count goes wrong
- Not subtracting openings, which over-orders block and mortar both.
- Forgetting corner and half blocks, which you cut from fulls or buy separately.
- Skipping the waste factor. Cuts, drops, and the odd cracked block mean 5 to 10 percent extra is normal.
Block wall math worth knowing
A standard 8x8x16 CMU covers 0.89 square feet of wall face, which is where the 1.125-blocks-per-square-foot figure comes from. Mortar runs about three bags of Type N or S mix per 100 blocks with 3/8-inch joints. Every course is 8 inches of height, so a 4-foot wall is six courses and the block count is always a multiple of the course length.
| Wall | Blocks (with 5% waste) | Mortar bags |
|---|---|---|
| 8 ft x 4 ft | 38 | 2 |
| 20 ft x 4 ft | 95 | 3 |
| 40 ft x 6 ft | 284 | 9 |
Quick answers
How much does a concrete block cost? $1.50 to $4 for standard gray 8x8x16 depending on region and volume. Specialty faces and colors run multiples of that.
Do block walls need filled cores? Structural walls get rebar and grout in cores at engineered spacing; garden walls often only need filled cores at corners and ends. Local code decides, not preference.
Block or poured concrete wall? Block suits DIY pace and small crews, no formwork needed. Poured wins on speed and water resistance for full basements. Cost lands close once labor is counted.
Common questions
Multiply your wall's length by its height to get square feet, then multiply by 1.125, since each standard block covers about 0.89 square feet of wall face with the joint. Enter your dimensions above and it's done, waste included.
About 3 bags of mortar mix per 100 blocks for standard 3/8-inch joints. Wider joints or rough block use more. The tool estimates bags from your block count.
Nominally 16 by 8 by 8 inches, which includes the mortar joint; the actual block is slightly smaller. That nominal 16 by 8 inch face is what the 1.125-blocks-per-square-foot figure is based on.
Yes, about 5%. Blocks crack, corners need cuts, and a few always arrive chipped. Running out mid-wall costs you a trip and a delivery fee.
Estimates for planning. Verify against your supplier's units and your local building codes before ordering or building. For anything structural, follow your engineer or local code.