Concrete Block Calculator

The most common CMU is the 8×8×16 in nominal block (actual dimensions 7.625×7.625×15.625 in). The 3/8 in mortar joint makes up the difference to the nominal size. Other common sizes include 4×8×16 in (partition walls), 6×8×16 in, and 12×8×16 in for thicker structural walls. All standard sizes share the same 8×16 in nominal face, so this calculator applies to all of them.

A 60 lb bag of Type S or Type N mortar covers approximately 35 standard CMU blocks at a 3/8 in joint. For every 35 blocks, budget one bag of mortar. A 20 ft × 8 ft wall needs about 180 blocks (with 5% waste) and approximately 6 bags of mortar. Always buy an extra bag — mortar consistency varies and running short mid-course causes joint defects.

Yes — for accurate results, calculate the gross wall area first, then subtract the area of each opening (width × height). Re-run the calculator with the net area, or calculate each section of solid wall separately and add the totals. Keep the cut blocks from openings — they can often be reused at corners or as half-blocks.

5% is standard for straightforward rectangular walls. Increase to 10% for walls with many corners, curves, openings, or angled cuts. Blocks cut for corners and openings often cannot be reused elsewhere, so err on the side of ordering slightly more rather than waiting for a second delivery.

At 8 in nominal height per course (7.625 in block + 3/8 in mortar joint), exactly 12 courses equal 8 ft (96 in ÷ 8 in = 12). Common wall heights: 4 ft = 6 courses, 6 ft = 9 courses, 8 ft = 12 courses. If your wall height is not a multiple of 8 in, plan for a custom-cut top course.

Most structural block walls require vertical rebar in the cores filled with grout, especially in seismic zones or for walls taller than 4 ft. Horizontal rebar (or joint reinforcement wire) is placed every other course for additional strength. Retaining walls, basement walls, and any wall supporting load almost always require reinforcement. Check your local building code — unreinforced CMU is generally only acceptable for small garden or landscape walls.

CMU blocks cost $2–$4 each at hardware stores or $1.50–$2.50 in bulk from a masonry supplier. A 20 ft × 8 ft wall (≈180 blocks) costs $360–$720 in block material alone. Add mortar ($8–$12 per bag × 6 bags ≈ $60), rebar if required, and labor (professional masons charge $15–$25 per sq ft installed). DIY block laying is feasible for experienced builders — expect 50–80 blocks per day for a first project.