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Reviewed · Prices checked October 2026 · How we calculate

Concrete Block Calculator: How Many Blocks and Bags of Mortar

Enter the wall length and height. You get the number of standard 8×8×16 concrete (cinder) blocks to buy and the 80 lb bags of mortar mix to lay them, plus what it costs at your store's price.

Wall size

Units
ft
ft
sq ft

Total area of doors and windows to leave out.

Extra for waste

5% covers a few chipped blocks and cuts. Use 10% for lots of corners or cut blocks.

Block size is fixed at the standard 8×8×16 in (nominal). Half blocks, corners and bond beam blocks are counted as whole blocks.

Shopping list

Updates as you type
95blocks

8×8×16 concrete blocks for 80 sq ft of wall

Plus 8 bags of 80 lb mortar mix.

  • 95 × 8×8×16 concrete blocks$118.75–$380
    90.0 blocks for the wall plus 5% extra, rounded up.

    Typical price (estimate, checked Oct 2026): $1.25–$4.00 per 8×8×16 concrete block

    95 blocks × $1.25–$4.00 = $118.75–$380Estimate
  • 8 × 80 lb bags of mortar mix$64–$128
    95 blocks ÷ 12 blocks per bag = 7.92, rounded up.

    Typical price (estimate, checked Oct 2026): $8.00–$16.00 per 80 lb bag of mortar mix

    8 bags × $8.00–$16.00 = $64–$128Estimate
Project total (estimate where no price is entered)$182.75–$508
Checked Oct 2026

Not included: rebar, grout for the cores and footings. They are structural, so this page does not size them. A reinforced or load-bearing wall needs an engineer or your local building code.

Show the math
20 ft × 4 ft = 80 sq ft of wallNo openings to subtract: 80 sq ft80 sq ft × 1.125 blocks per sq ft = 90.0 blocks90.0 × (1 + 5% waste) = 94.5 → 95 blocks95 blocks ÷ 12 per 80 lb bag = 7.92 → 8 bags of mortar

Your prices are saved on this device only. Blocks are sold by the piece and mortar mix by the 80 lb bag (36.3 kg), even when you type meters.

How it works

The formula

wall area = length × height − openings
blocks per sq ft = 144 ÷ (8 × 16) = 1.125
blocks = area × 1.125 × (1 + waste), round up
mortar bags = blocks ÷ 12, round up

Assumptions

  • Blocks are the standard "8×8×16", which is a nominal size. A real block measures 7-5/8 × 7-5/8 × 15-5/8 in. With a 3/8 in mortar joint, each block takes up 8 × 16 in of wall, so a sq ft of wall (144 sq in) holds 1.125 blocks.
  • One 80 lb bag of mortar mix lays about 12 standard blocks. That is the QUIKRETE Mortar Mix (Type N) figure; their Mason Mix (Type S) says up to 13. We use 12 so you do not run short, and we work from the blocks you buy, so waste mortar is covered too.
  • Waste of 5% (default) or 10% is added to the block count. Openings are subtracted before waste.
  • Metric: type meters and square meters. The wall is converted to feet for the math (1 m² needs 12.1 blocks), and blocks stay 8×8×16 in (about 200 × 200 × 400 mm nominal).
  • Rebar and grout are not calculated. See the note under the shopping list.

Worked example

A 20 ft long, 4 ft high wall, no openings, 5% waste

  1. Wall area: 20 × 4 = 80 sq ft
  2. Blocks: 80 × 1.125 = 90.0
  3. Add 5% waste: 90.0 × 1.05 = 94.5, round up to 95 blocks
  4. Mortar: 95 ÷ 12 = 7.92, round up to 8 bags of 80 lb mix

Common wall sizes

Blocks and mortar bags for common wall sizes with 5 percent waste
WallSq ftBlocksMortar bags (80 lb)
10 × 4 ft40484
20 × 4 ft80958
20 × 6 ft12014212
40 × 6 ft24028424

No openings, 5% waste, 1.125 blocks per sq ft, 12 blocks per 80 lb bag.

Questions people ask

How many blocks per square foot?

1.125 for a standard 8×8×16 block, from 144 sq in ÷ (8 × 16) sq in. That is about 12.1 per square meter. Add 5 to 10% for waste.

How many blocks for a 20 ft wall?

It depends on the height. Each 8 in course of a 20 ft wall is 15 blocks. A 4 ft high wall (6 courses) needs 95 blocks with 5% waste, and an 8 ft high wall needs 189.

How much mortar per block?

One 80 lb bag of mortar mix lays about 12 blocks, or about 0.083 of a bag (6.7 lb) per block, per QUIKRETE's Mortar Mix data sheet. Thick joints, wasted mortar and filled cores use more.

What is the actual size of an 8×8×16 block?

7-5/8 × 7-5/8 × 15-5/8 in. The "8×8×16" name is nominal: it counts the 3/8 in mortar joint, so the block plus its joint fits an 8 × 16 in space. That is why you plan wall sizes in 8 in and 16 in steps.

Can I build a retaining wall with cinder blocks?

Low, short garden walls are sometimes built that way, but a wall that holds back soil is structural. It needs a footing, drainage and often rebar and grout in the cores, and many places require a permit or an engineer above 3 to 4 ft. Check your local code. For purpose-made wall blocks, caps and gravel, use the retaining wall calculator.