Concrete Block Calculator

The formula

Coverage and yield assumptions

A standard 8 × 8 × 16 block measures 7-5/8 × 7-5/8 × 15-5/8 in actual; the 3/8 in joint brings it to the 8 × 16 in nominal face, or 1.125 blocks per square foot. Half blocks, corner blocks and bond beam units are counted the same way; order a mix based on the number of corners and openings.

Sizes and types

Nominal 8 in wide units are the workhorse for foundations and retaining walls; 4, 6, 10 and 12 in widths exist for partitions and taller walls. Stretcher blocks have flanged ends, corner blocks have one flat end, and bond beam blocks have a channel for horizontal rebar. The Concrete Masonry & Hardscapes Association's TEK notes describe the common shapes and their uses.

Mortar and grout

Mortar holds the units together; grout fills reinforced cores. A rough rule is about one 80 lb bag of type S or N mortar mix for every 25–30 blocks laid with full face shell bedding, but check the product label. Grouted cores need far more material: an 8 in block wall with every core filled takes roughly 0.25–0.3 cu ft of grout per block.

Footings and reinforcement

Block walls need a continuous concrete footing, typically twice the wall width and below the frost line. Vertical rebar is set in the footing and grouted into cores at the spacing your plans or code require. Walls retaining soil, or taller than about 4 ft, normally need engineering and a permit.

Layout tips

• Dry-lay the first course to check spacing at corners and openings.
• Plan wall lengths and openings in 8 in increments to avoid cutting.
• Strike joints when thumbprint hard for a dense, weather-tight finish.
• Wear a respirator when cutting block to limit silica dust.

Common mistakes to avoid

• Planning wall lengths that are not multiples of 8 in, forcing many cuts.
• Skipping a proper footing below the frost line.
• Leaving out vertical reinforcement where code requires it.
• Laying blocks that are wet, which weakens the mortar bond.
• Building a soil-retaining block wall without drainage.

Waste factor

5% covers broken units and cuts on a simple straight wall. Use 10% for walls with many corners, pilasters or openings, or when blocks are moved several times on site.

Imperial and metric

Switch the toggle above the inputs to enter ft ↔ m, in ↔ cm. The page converts metric entries to imperial before calculating, using NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm), and shows key results in both systems.

Worked examples

Garden wall 40 × 4 ft

Inputs: wall length 40 ft; wall height 4 ft; block length (nominal) 16 in; block height (nominal) 8 in; waste factor 5%.

Result: You need 189 concrete blocks.

• Units to buy (rounded up): 189 blocks
• Wall area: 160 sq ft (14.9 m²)
• Blocks per sq ft: 1.125

Garage foundation 96 × 3 ft

Inputs: wall length 96 ft; wall height 3 ft; block length (nominal) 16 in; block height (nominal) 8 in; waste factor 5%; cost per unit $2.4.

Result: You need 341 concrete blocks.

• Units to buy (rounded up): 341 blocks
• Wall area: 288 sq ft (26.8 m²)
• Blocks per sq ft: 1.125
• Estimated material cost: $818.40

Half-high 4 in units

Inputs: wall length 20 ft; wall height 3 ft; block length (nominal) 16 in; block height (nominal) 4 in; waste factor 10%.

Result: You need 149 concrete blocks.

• Units to buy (rounded up): 149 blocks
• Wall area: 60 sq ft (5.6 m²)
• Blocks per sq ft: 2.25

Quick reference for common sizes

The table repeats the first worked example at smaller and larger sizes, keeping every other setting the same, so you can see how the concrete block calculator result scales.

Limitations

The count is for a solid rectangular wall. Subtract large openings yourself and add lintels and specialty units separately. The calculator does not size footings, rebar or grout.