How many cinder blocks do I need for a 20 x 4 ft wall?

90 standard 8 x 8 x 16 in blocks: 6 courses of 15, or 95 with 5% waste. A door or a window comes off in whole blocks; enter the total opening area.

How many blocks per square foot?

1.125 standard blocks per square foot with a 3/8 in joint, so 113 per 100 square feet before waste (the nominal face is 16 x 8 in, which is 0.89 square feet). The 440 x 215 mm block with a 10 mm joint is 9.9 per square metre.

How many bags of mortar for 100 blocks?

9 bags of 80 lb mortar mix: the 13 blocks per bag Quikrete prints on the data sheet plus 10% waste; 11 bags of 60 lb. The 90 blocks of a 20 x 4 ft wall take 8 bags of 80 lb. Site-mixed mortar is one part masonry cement to two and a quarter to three parts sand by volume (ASTM C270).

How much concrete does it take to fill a cinder block?

About 0.22 cubic feet for both cells of an 8 x 8 x 16 in block: an 80 lb bag of core-fill grout (0.65 cubic feet) fills three. Filling every cell of the 90-block 20 x 4 ft wall takes 21.8 cubic feet with 10% extra, 37 bags of 80 lb concrete; with bars every 32 in and a bond beam it is 14 bags.

How much rebar goes in a block wall?

One vertical bar in each grouted cell, at the spacing the plan or the code gives: 32 in in the form's default, which is 9 bars of 4 ft for a 20 ft wall, plus two horizontal bars in the bond beam, 40 ft. Bars usually are #4 (1/2 in) and are doweled into the footing; add the laps and dowels to the length.

Do I have to fill cinder blocks with concrete?

Not for a low garden wall in mild ground, where the cells stay empty. Retaining walls, fence walls over about 4 ft, foundation walls and anything a code calls reinforced masonry get bars in grouted cells, and the wall is only as strong as those cells. The form leaves the cells empty when you choose no grout.

Free calculator

Concrete block calculator

Enter the wall length and height, take off the doors and windows, pick the block and how the cells are filled, and get blocks, bags of mortar, bags of grout, rebar and the materials cost.

Wall lengthftAdd the runs together for an L or a foundation.Wall heightftOpeningssq ftDoors and windows, total area.

BlockWaste%5% for cuts and breakage.

Mortar

Mortar bagBlocks per bagFrom the bag: Quikrete says up to 13 for 80 lb. 10% is added.

Grout and rebar

Filled cellsGrout per block, all cellscu ft0.22 cu ft for an 8 in block: an 80 lb bag fills about 3.

Grout or concrete bagBond beam on topTop course filled solid with two horizontal bars.

Prices (optional)

CurrencyPer block$Per mortar bag$Per grout bag$

Typical US shelf prices in 2026; type your own. Set all three to 0 to hide the cost.

Result

Blocks to buy: 95 (90 + waste)
Courses × blocks per course: 6 × 15
Mortar (80 lb mortar mix): 8 bags
Cells filled: 25% of the wall + top course, 8.2 cu ft with 10% extra
Grout (80 lb bag): 14 bags
Vertical rebar: 9 bars × 4 ft = 36 ft
Bond beam rebar: 2 bars, 40 ft in total
Materials: $411.50 (blocks $237.50, mortar $76.00, grout $98.00)

Courses and blocks per course are rounded up, so the last block in a course is cut; in running bond every other course starts with a half block, cut from a whole one. Openings come off in whole blocks. Mortar and grout carry 10% extra; lintels, cap blocks, the footing, dowels and rebar laps are not counted.

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How it works

A block wall is courses. The height is divided by the course height (8 in for a standard block, 225 mm for a 440 x 215 mm block, joint included) and the length by the block length (16 in or 450 mm), both rounded up, so the last block of a course is cut and every other course in running bond starts with a half block cut from a whole one. Openings come off in whole blocks at the per-area rate; the waste you allow covers cuts and breakage.

Mortar is counted the way the bag counts it: the maker prints how many standard blocks a bag lays (13 for an 80 lb bag of mortar mix, about 12 for a 25 kg bag), a best-case figure, so 10% is added for the board, the tub and the joints that get raked out. The blocks per bag field is pre-filled from the bag and the block you pick and stays editable.

Grout goes into the cells that carry vertical rebar, or into every cell. Cells sit 8 in (225 mm) apart along the wall, so bars every 32 in fill one cell in four. A bond beam fills the whole top course and carries two horizontal bars. The grout per block comes from the bag too: an 80 lb bag of core-fill grout yields 0.65 cubic feet and fills about three standard 8 in blocks, so 0.22 cubic feet each; the other widths are scaled from their cell size and can be edited. 10% is added for spills and the mortar that drops into the cells.

Rebar is one vertical bar per filled cell as tall as the wall plus the two bond beam bars: laps, dowels into the footing and horizontal joint reinforcement are extra. Footing concrete is in the concrete calculator; cap blocks and lintels over openings are not counted.

Frequently asked questions

How many cinder blocks do I need for a 20 x 4 ft wall?: 90 standard 8 x 8 x 16 in blocks: 6 courses of 15, or 95 with 5% waste. A door or a window comes off in whole blocks; enter the total opening area.
How many blocks per square foot?: 1.125 standard blocks per square foot with a 3/8 in joint, so 113 per 100 square feet before waste (the nominal face is 16 x 8 in, which is 0.89 square feet). The 440 x 215 mm block with a 10 mm joint is 9.9 per square metre.
How many bags of mortar for 100 blocks?: 9 bags of 80 lb mortar mix: the 13 blocks per bag Quikrete prints on the data sheet plus 10% waste; 11 bags of 60 lb. The 90 blocks of a 20 x 4 ft wall take 8 bags of 80 lb. Site-mixed mortar is one part masonry cement to two and a quarter to three parts sand by volume (ASTM C270).
How much concrete does it take to fill a cinder block?: About 0.22 cubic feet for both cells of an 8 x 8 x 16 in block: an 80 lb bag of core-fill grout (0.65 cubic feet) fills three. Filling every cell of the 90-block 20 x 4 ft wall takes 21.8 cubic feet with 10% extra, 37 bags of 80 lb concrete; with bars every 32 in and a bond beam it is 14 bags.
How much rebar goes in a block wall?: One vertical bar in each grouted cell, at the spacing the plan or the code gives: 32 in in the form's default, which is 9 bars of 4 ft for a 20 ft wall, plus two horizontal bars in the bond beam, 40 ft. Bars usually are #4 (1/2 in) and are doweled into the footing; add the laps and dowels to the length.
Do I have to fill cinder blocks with concrete?: Not for a low garden wall in mild ground, where the cells stay empty. Retaining walls, fence walls over about 4 ft, foundation walls and anything a code calls reinforced masonry get bars in grouted cells, and the wall is only as strong as those cells. The form leaves the cells empty when you choose no grout.

Guides

How many concrete blocks do I need for a wall?
8x8x16 blocks per square foot and for walls from 10x4 to 40x8 ft with 5% waste, mortar volume, and the metric 440 x 215 mm block per square metre.
How much concrete to fill cinder blocks? (per block, per 100, per wall)
Grout per 8 in block (0.22 cu ft, three blocks per 80 lb bag), per 100 blocks and for 10 to 40 ft walls with every cell filled or bars every 16 to 48 in, plus metric.
How much does a cinder block wall cost? (materials per wall and per square foot)
Blocks, mortar and grout priced for walls from 10x4 to 40x8 ft at 2026 shelf prices, materials per square foot, what grouting adds, and HomeGuide's installed range.
Building a block wall: courses, rebar, grout and mortar for a 20 ft wall
The material list for a CMU wall in order: footing, courses, half blocks, bars every 32 or 48 in, grouted cells, bond beam and mortar, with counts for 4 to 8 ft walls.
How much does a cinder block garage cost? (blocks, mortar, grout and slab for 1, 2 and 3 car)
Blocks, mortar and grout for 1 to 3 car block garages, 12x20 to 36x24 ft, 8 ft walls, doors and windows taken off, plus the 4 in slab, at 2026 prices.