How to use the cinder block calculator
- Measure the wall length and finished height in feet. For a wall with corners, add up the lengths of every straight run measured along the outside face.
- Subtract openings. Multiply the width by the height of each door, window or vent and enter the total. Leave it at 0 for a solid garden or retaining wall.
- Pick the block size. Standard concrete blocks are 8 × 8 × 16 in, but 4, 6, 10 and 12 in widths share the same 8 × 16 in face, so they all give the same count — only the wall thickness changes.
- Choose your mortar product and adjust the "blocks per bag" figure to match the bag you are buying.
Strictly speaking, "cinder blocks" made with coal cinders are rarely produced any more. What home centers sell today are concrete masonry units (CMU), and that is what this calculator assumes.
How many blocks per square foot?
A standard block's actual size is 7⅝ × 7⅝ × 15⅝ in. Add a ⅜ in mortar joint and it occupies a nominal 8 × 16 in of wall face, or 128 square inches. One square foot is 144 square inches, so a wall takes 144 ÷ 128 = 1.125 blocks per square foot (about 12.1 per m²).
Net wall area = length × (courses × 8 in) − openings
Blocks = net area × 1.125, plus waste, rounded up
Mortar bags = blocks before waste ÷ blocks per bag, rounded up
Worked example: a 30 ft × 6 ft wall with one window
The gross area is 30 × 6 = 180 sq ft. Subtract a 3 × 5 ft window (15 sq ft) to get 165 sq ft. At 1.125 blocks per square foot that's 185.6 blocks; with 5% extra it's 194.9, so buy 195 blocks. The wall is 72 in tall, which is exactly 9 courses.
For mortar, 185.6 blocks ÷ 28 blocks per bag of masonry cement = 6.6, so 7 bags, plus about 21 cu ft (0.78 cu yd) of mortar sand. Using premixed 80 lb mortar mix at 12 blocks per bag you would need 185.6 ÷ 12 = 15.5, or 16 bags.
How much mortar for concrete block?
Mortar estimates are the least precise part of any block takeoff. Joint thickness, whether you mortar only the face shells or the webs too, how much drops off the trowel and how experienced the mason is all change the result. That's why the blocks-per-bag figure is an editable field — start with the number on the bag you are buying and lean conservative.
| Product | What's in the bag | Blocks per bag (default) |
|---|---|---|
| Premixed mortar mix, 80 lb | Cement, lime and sand — add water only | 12 (labels commonly quote about 12–15) |
| Masonry cement, Type N or S (70–78 lb) | Cementitious material only — add sand and water | 28 (about 25–35 is commonly quoted) |
The big difference is the sand. A bag of masonry cement is sized to equal one cubic foot, and the ASTM C270 mortar specification calls for 2¼ to 3 parts sand per part cement by volume, so one bag ends up as several cubic feet of mortar. A premixed bag already contains its sand, so it makes much less. The calculator assumes about 3 cu ft of mortar sand per bag of masonry cement; check the mixing directions on your bag.
Type N is the general-purpose mortar for above-grade walls. Type S is stronger and is the usual choice for walls below grade, retaining walls and foundations. Follow your plans or local code where they specify a type.
Block sizes
| Nominal size (W × H × L) | Common uses | Blocks per sq ft |
|---|---|---|
| 4 × 8 × 16 in | Partition walls, veneers, caps | 1.125 |
| 6 × 8 × 16 in | Non-load-bearing walls, screening | 1.125 |
| 8 × 8 × 16 in | Most foundations, garden and garage walls | 1.125 |
| 10 × 8 × 16 in | Taller or more heavily loaded walls | 1.125 |
| 12 × 8 × 16 in | Basement and retaining walls, heavier loads | 1.125 |
Block weight and exact dimensions vary by manufacturer and by whether the block is normal-weight or lightweight, so check the supplier's spec sheet when planning delivery.
Tips for block wall estimates
- Corners and ends. Running bond needs half blocks at the ends of alternate courses. You can cut them from full blocks, or buy half and corner blocks; either way your waste allowance covers it.
- Plan heights in 8 in steps. A wall that is a whole number of courses avoids cutting the top row. A 5 ft wall is 7½ courses, and the cut top course still uses whole blocks, so the calculator counts it as 8 full courses (5 ft 4 in). Building to 4 ft 8 in (7 courses) or 5 ft 4 in (8 courses) avoids the cuts.
- Don't forget the footing. Block walls sit on a poured concrete footing — estimate it with the concrete calculator.
- Filled cells need grout, not mortar. Reinforced cores are filled with grout, and the volume depends on the cell size of the block you buy; ask your supplier for their figure per block.
Frequently asked questions
How many cinder blocks per square foot?
Standard 8 × 8 × 16 in blocks cover 8 × 16 = 128 sq in of wall each, including the mortar joint, so you need 144 ÷ 128 = 1.125 blocks per square foot. 100 sq ft of wall takes 112.5 blocks before waste.
How many blocks are in a 4 ft high wall?
A 4 ft wall is 48 in, or 6 courses of 8 in block. Each 16 in block covers 1⅓ ft of length, so a 20 ft long wall needs 15 blocks per course and 90 blocks in total, or 95 with 5% extra.
How many blocks does a bag of mortar lay?
It depends on the product. Premixed 80 lb mortar mix, which already contains sand, is commonly rated at roughly 12–15 standard blocks. A bag of masonry cement mixed with about 3 parts sand is commonly quoted at about 25–35 blocks. Joint thickness and technique change both figures.
Do 12 inch blocks need fewer blocks than 8 inch blocks?
No. Most US concrete blocks share the same nominal 8 × 16 in face regardless of width, so the count per square foot is the same. Wider blocks make a thicker, heavier and stronger wall.
Should I buy extra blocks?
Yes. About 5% covers cut ends, chipped blocks and breakage for a simple wall; go to 10% for walls with many corners, openings or cuts.