How to use the block calculator
- Enter the wall. Length and height, using
20'or20' 8for feet (plain numbers are inches). For a building, add up the lengths of all the walls. - Subtract openings. Enter the total square feet of doors and windows. A 3' × 7' door is 21 ft², a 3' × 4' window is 12 ft².
- Pick the block and the grout. Most walls are 8" block. Choose "Rebar cells only" if your plans call for vertical rebar at a spacing like 32" or 48", or "All cores" for a solid-grouted wall.
The math
A standard block is 15⅝" × 7⅝" on the face; with a ⅜" mortar joint it covers 16" × 8", which is 128 square inches. A square foot is 144 square inches, so a wall takes 1.125 blocks per square foot. A 20' × 8' wall is 160 ft² × 1.125 = 180 blocks: 12 courses of 15 blocks.
Mortar: Quikrete rates an 80 lb bag of Mason Mix at up to 13 standard blocks, so 180 blocks take 14 bags. Buy a bag or two extra, since "up to" assumes no waste.
Grout: an 80 lb bag of Core-Fill Grout (about 0.68 ft³) fills the cores of about 3 blocks. Solid-grouting that same wall takes 60 bags, about 1.5 cubic yards. At that size, price a ready-mix grout truck or a pump instead of bags. Grouting only the rebar cells every 48" takes 6 cells × 12 courses, about 8 ft³ or 12 bags.
Tips before you order
- Lay out to the block. Wall lengths that are multiples of 8" (and heights in 8" courses) save cuts. A 20' 4" wall is a full number of half blocks; 20' 6" means cutting every other course.
- Corner and bond beam block: the count is total blocks. Your supplier will split it into stretchers, corners, halves and bond beam (U) block from your drawings.
- Weight: an 8" hollow block weighs roughly 35–40 lb, so 180 blocks are over 3 tons. Get them delivered on a pallet.
FAQ
How many blocks are in a square foot of wall?
1.125 standard 8" × 16" blocks per square foot, because each block covers 128 square inches including the 3/8" mortar joint. That works out to 112.5 blocks per 100 square feet.
How many cinder blocks do I need for a 20 ft wall?
A 20' wall 8' high is 160 square feet, which takes 180 blocks (12 courses of 15). Add about 5% for breakage and cuts. A 4' high wall of the same length takes 90.
How many blocks does an 80 lb bag of mortar lay?
Quikrete rates an 80 lb bag of Mason Mix at up to 13 standard 8" × 8" × 16" blocks, or about 37 bricks. Plan on roughly 8 bags per 100 blocks.
How much grout to fill block cores?
An 80 lb bag of Quikrete Core-Fill Grout yields about 0.68 cubic feet and fills the cores of about 3 standard 8" blocks, roughly 0.23 cubic feet per block. A fully grouted 8" wall takes about 0.26 cubic feet per square foot.
How many 8x8x16 blocks are on a pallet?
It varies by plant, but a pallet of 8" × 8" × 16" block is commonly 90 blocks. Ask your supplier, since pallet counts and deposits differ.