How many concrete blocks do I need?
A standard concrete block is 15⅝″ long and 7⅝″ high. With a ⅜″ mortar joint, each one covers 16″ × 8″ of wall, which is 1.125 blocks per square foot. The calculator counts it course by course instead: wall height ÷ 8″ gives the number of courses (rows), and wall length ÷ 16″ gives the blocks per course, both rounded up. That's more accurate for short or oddly sized walls. Then it adds 5% for breakage and cut blocks.
For example, a 20 ft × 4 ft wall is 6 courses of 15 blocks: 90 blocks laid, so buy 95.
Does block width change the count?
No. 4″, 6″, 8″, 10″ and 12″ blocks all have the same 8″ × 16″ face, so you need the same number for a given wall. Width changes the weight, the price, and how much grout the cores hold. The core-fill numbers here are for standard 8″ blocks.
How much mortar for concrete blocks?
Coverage depends on the mix brand and how the blocks are bedded, so estimates range widely. A conservative planning figure is about 12 blocks per 80 lb bag of mortar mix when mortar goes only on the outer shells of the block (face-shell bedding). Full bedding, with mortar on the webs too, uses about twice as much. Check the yield printed on your bag and change "Blocks per 80 lb mortar bag" in the options to match.
How much grout to fill the cores?
An 8″ block has two hollow cells holding about 0.12 cu ft each, so filling every core takes about 0.24 cu ft per block. Walls reinforced with vertical rebar usually grout only the cells with steel, which the calculator estimates as about half the cores. One 80 lb bag of concrete or core-fill mix makes about 0.6 cu ft. Above half a cubic yard, a ready-mix delivery or grout pump is usually cheaper and much faster than bags.
How heavy are concrete blocks?
A hollow 8×8×16 block weighs about 35 lb, and lightweight blocks are less. A pallet adds up quickly, so check your truck's payload or pay for delivery. Most yards deliver by the pallet.
Cinder block or concrete block?
Same thing in everyday use. True cinder blocks, made with coal ash, are rarely sold anymore. What stores sell as "cinder blocks" are concrete masonry units (CMU), and they're what this calculator assumes.
More concrete calculators
Pouring a footing or pad for the wall? Use the concrete slab calculator. Setting fence posts instead? The fence post concrete calculator works out how many bags each hole takes.