Concrete Block Calculator

A concrete block calculator returns the 8x8x16 blocks a wall needs, the number to buy with breakage, and the 80 lb mortar bags. It multiplies wall area, minus door and window openings, by 1.125 blocks per square foot. A 5% default waste allowance sets the purchase count, while mortar bags are based on the blocks actually laid.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Wall Area (openings subtracted)

160 sq ft

Includes a 5% waste allowance.

Shopping list

Concrete blocks (8x8x16)189 blocks180 in the wall, the rest covers breakage and cuts.
Mortar mix14 80 lb bags

Breakdown
Wall Area (openings subtracted)	160 sq ft
Blocks in the Wall	180
Blocks to Buy (with waste)	189
Mortar Mix Bags (80 lb)	14
Estimated Cost (Blocks)
Estimated Cost (Mortar)

How to Calculate Concrete Block

A standard concrete masonry unit (CMU) has a nominal face of 8 in. by 16 in. That works out to 112.5 blocks per 100 sq ft of wall (1 ÷ (8 × 16 ÷ 144) = 1.125 blocks per sq ft), per NCMA TEK 02-01B. The nominal size already bakes in the standard 3/8 in. mortar joint. The block itself is actually 7-5/8 in. by 15-5/8 in., specified 3/8 in. under nominal on each module so courses land on an even 8 in. grid. That means no separate joint-width input is needed. Wall area is Length × Height, with any door or window area subtracted before the block count is figured, since blocks aren't laid there. A waste allowance is added on top of that design count, because broken or cut blocks still need to be purchased but don't end up in the finished wall. The mortar bag count is based on the design count itself, since mortar is only consumed by blocks actually laid. Mortar mix yield is 13 standard 8x8x16 blocks per 80 lb bag, from QUIKRETE's Mortar Mix (No. 1102) data sheet.

Example: 20 ft long, 8 ft tall wall with no openings, at 5% waste

A 20 ft long, 8 ft tall wall with no openings, at 5% waste: 1. Gross wall area: 20 × 8 = 160 sq ft 2. Net wall area (no openings to subtract): 160 sq ft 3. Raw block count: 160 × 1.125 = 180 blocks 4. Design block count (rounded up): 180 blocks 5. Blocks to buy with 5% waste: 180 × 1.05 = 189 blocks 6. Mortar bags: 180 ÷ 13 = 13.85, rounded up to 14 bags (80 lb QUIKRETE Mortar Mix) If this wall had a 3 ft × 7 ft door opening (21 sq ft), net wall area would drop to 139 sq ft, cutting the design count to 157 blocks and the mortar to 13 bags.

Concrete Block: Compare Scenarios

Scenario	Wall Area (openings subtracted)	Blocks in the Wall	Blocks to Buy (with waste)	Mortar Mix Bags (80 lb)
Small (10x8 ft wall)	80 sq ft	90	95	7
Medium (20x8 ft wall)	160 sq ft	180	189	14
Large (50x8 ft wall with a door and window)	360 sq ft	405	426	32

What Is Concrete Block?

A concrete block calculator converts a wall's dimensions into the number of standard 8x8x16 concrete masonry units (CMU) and bags of mortar mix needed to build it. It exists because block count doesn't come from a simple area conversion most people would guess correctly. The nominal 8x16 face size already includes the mortar joint. Mortar consumption depends on how many blocks are actually laid, not how many you bought to cover breakage.

Concrete Block: Pros and Cons

Pros

Uses the actual nominal CMU face dimension (8x16 in., NCMA TEK 02-01B) instead of an approximate blocks-per-square-foot rule of thumb
Separates the "blocks in the wall" figure from "blocks to buy" so waste/breakage overage does not silently inflate the mortar estimate too
Lets openings (doors, windows) be subtracted before the block count is figured, rather than requiring a manual area correction afterward

Cons

Assumes standard 8x8x16 units in running bond. A different block width, a stack bond pattern, or a mix of unit shapes (corner, bond beam, lintel blocks) changes the count
Does not include grout fill for reinforced cores. See the Concrete Block Fill calculator once core spacing and grout type are known
Mortar yield is a QUIKRETE-specific figure. A different mortar product or a thicker-than-standard joint will cover fewer blocks per bag

Concrete Block: What a Pro Checks

Remember the nominal 8x16 in. face already bakes in the standard 3/8 in. mortar joint, per NCMA TEK 02-01B. There's no separate joint-width adjustment to make on top of the block count this calculator gives you.
Subtract door and window openings before you order, not after. Blocks aren't laid in an opening, and skipping this step is the easiest way to end up with more blocks than the wall actually needs.
Budget a few extra standard blocks for lintel or sash units around any opening. This calculator doesn't break those out separately, since they depend on your specific opening size and supplier's stock.

Frequently Asked Questions

How many concrete blocks are there per square foot of wall?

1.125 blocks per square foot, or 112.5 per 100 sq ft, for a standard 8x8x16 CMU. This comes directly from the block's nominal 8 in. by 16 in. face (128 sq in., or 0.889 sq ft) per NCMA TEK 02-01B. The nominal size already accounts for the standard 3/8 in. mortar joint.

How many bags of mortar do I need per block?

About 1 bag per 13 standard 8x8x16 blocks, based on QUIKRETE's Mortar Mix (No. 1102) data sheet, which states one 80 lb bag lays up to 13 blocks. A different mortar product may publish a different yield. Check its data sheet if you're using something else.

How much waste should I add for concrete block?

This calculator defaults to 5% for breakage and cuts around corners and openings. There's no single industry-wide standard percentage for block waste the way there is for ready-mix concrete ordering, so treat this as a practical planning assumption. Use more for a wall with many corners or a lot of cutting, and less for a long, simple, unbroken run.

Does this calculator include grout for filling the block cores?

No. This calculator covers the blocks and the mortar joints between them. If your wall is reinforced and the cores need to be grouted solid or at reinforced cells, that's a separate volume calculation. Use the Concrete Block Fill calculator once you know which cores are being filled.

Does it matter if my wall has doors or windows?

Yes. Enter their combined area in the "Doors & Windows" field, and it's subtracted from the wall area before the block count is calculated, since no blocks are laid in an opening. Lintel or sash blocks around the opening itself aren't broken out separately. Budget a small number of extra standard blocks, or check with your supplier for the specific units your opening needs.

What block size does this calculator assume?

Standard 8 in. by 8 in. by 16 in. nominal CMU, the most common size for residential and light commercial walls. If you're using a different width (4, 6, 10, or 12 in.), the block count per square foot of wall face is unchanged. Width only affects the wall's thickness and the concrete or grout volume if the cores are filled, not the face count calculated here.

Sources

Concrete & Masonry