Block wallsBlocks, mortar and core fill

Concrete Block Calculator

This concrete block calculator tells you how many blocks to buy for a wall. Enter the wall size and any door or window openings. The result also estimates the 80 lb bags of mortar and, if you choose a core spacing, the grout to fill the cores of standard 8 × 8 × 16 in blocks. People also call them cinder blocks.

Quick answer: a 20 × 8 ft wall with one 3 × 7 ft door is 139 ft² net, which takes 156.38 standard 8×8×16 blocks, or 165 with 5% waste, and 13 bags of 80 lb mortar.

Blocks are the net wall area divided by the face area of one block. A standard 8 × 16 in face is 8/9 ft², so a wall takes 1.125 blocks per square foot (NCMA). Add 5% for waste and breakage, then round up. Mortar bags come from a product label or an NCMA rate, and core-fill grout from an NCMA table.

Example: a 20 × 8 ft wall with one 3 × 7 ft door is 160 − 21 = 139 ft². That is 139 × 1.125 = 156.375 blocks, or 165 with 5% waste. Mortar is 165 ÷ 13 per bag = 13 bags by the QUIKRETE label, or 10 bags at the NCMA rate of 16 per bag.

Method

How this calculator works

The block count is geometry from the face of the block. Waste is an allowance, and mortar and grout come from published rates for the standard 8 × 8 × 16 in block. Your mortar label and your wall design come first.

Step 1 · Math

Net wall area

The wall area is its length times its height. Doors and windows are subtracted, so you only count the wall you will lay. For a stepped wall or several walls, add the areas with the square footage calculator and enter the total.

Net area (ft²) = Length × Height − Openings × Area per opening

Step 2 · Source: NCMA TEK 04-02A

Blocks per square foot

NCMA divides the wall area by the area one unit shows in the wall. A standard unit shows 8 × 16 in, which is 128 in² or 8/9 ft², so it takes 1.125 blocks per square foot, or 112.5 per 100 ft². Half-high (4 × 16 in) and half-length (8 × 8 in) faces are 64 in², which is 2.25 per square foot by the same arithmetic. NCMA warns that counts vary with skill, non-modular layouts and numerous returns and corners.

Blocks per ft² = 144 ÷ Face area (in²)Net blocks = Net area × Blocks per ft²

Step 3 · Source: NCMA allowance

Waste

NCMA's counts include about 5% for waste and breakage, so 5% is the default. You can change it. A wall with many corners, returns or cut blocks needs more.

Blocks to buy = ceil(Net blocks × (1 + Waste ÷ 100))

Step 4 · Source: two mortar rates

Mortar bags

The default is the QUIKRETE Mortar Mix No. 1102 label: each 80 lb bag lays up to 13 standard 8 × 8 × 16 in blocks. It is the more conservative figure. NCMA Table 2 is more generous, at 16 conventional units per preblended 80 lb bag, and includes nominal waste for face-shell bedding. The calculator orders from the label and shows the NCMA bag count as a note. Both apply to the standard block only.

Mortar bags = ceil(Blocks to buy ÷ 13)NCMA check = ceil(Net blocks ÷ 16)

Step 5 · Source: NCMA Table 3

Core-fill grout

If you fill cores with grout, NCMA Table 3 gives the cubic feet per 100 ft² of wall for each wall thickness and spacing. The figures are for two-core units, include 3% waste and assume every core is filled at that spacing. The order is rounded up to 0.05 yd³, and suppliers set their own minimums. Which cores are filled, and with what steel, is decided by the wall design.

Grout (ft³) = Net area ÷ 100 × Table 3 valueGrout (yd³) = Grout (ft³) ÷ 27

Block sizes the calculator offers

Block size	Face, ft²	Blocks per ft²	Mortar and grout figures
8 × 8 × 16 in (standard)	0.8889	1.125	Yes
4 × 8 × 16 in (half-high)	0.4444	2.25	None sourced, so none given
8 × 8 × 8 in (half-length)	0.4444	2.25	None sourced, so none given

NCMA Table 3: grout for filled cores (the data behind step 5)

Wall thickness	Every 8 in	Every 16 in	Every 24 in	Every 32 in	Every 40 in	Every 48 in
6 in wall	25.6	12.8	8.6	6.4	5.2	4.3
8 in wall	36.1	18.1	12.1	9.1	7.3	6.1
12 in wall	58.9	29.5	19.7	14.8	11.8	9.9

Two-core units, 3% waste included. Read across for the spacing of the filled cores.

What each field does

Field	What to enter
How do you know the wall area?	Pick Length × height for one rectangular wall, or Total area if you already know the area. For a stepped wall or several walls, add the areas yourself and use Total area.
Wall length and Wall height	The face of the wall you will lay, in feet and inches in the US view or meters and centimeters in the metric view.
Wall area	Used when you pick Total area. Square feet in the US view or square meters in the metric view.
Openings and Area of one opening	The number of doors and windows, and the area of one. They are subtracted from the wall. For openings of different sizes, enter 1 opening and the total area of all of them. A 3 ft × 7 ft door is 21 ft².
Block size	Standard 8 × 8 × 16 in is the default. Half-high (4 × 8 × 16 in) and half-length (8 × 8 × 8 in) are counted from their faces, but mortar and grout are given for the standard block only.
Waste allowance	The default is 5%, which NCMA includes for waste and breakage. A value under 5% shows a warning.
Wall thickness (grout only)	Choose 6, 8 or 12 in. It is used only when you fill cores with grout and does not change the block count.
Grout-filled cores	No grout by default. Otherwise choose how far apart the filled cores are, from every 8 in to every 48 in. The calculator fills every core at that spacing. It does not design the reinforcement.

Worked example: a 20 × 8 ft wall with a door, 8×8×16 blocks

Arithmetic checked

The 5% waste default, mortar from the label, and an optional core fill every 16 in in an 8 in wall. The block count is the calculator's own example result. A 3 ft × 7 ft door is 21 ft².

Step	Math	Result
Wall area	`20 × 8`	160 ft²
Openings	`1 door × 3 × 7`	21 ft²
Net wall area	`160 − 21`	139 ft²
Blocks without waste	`139 × 1.125`	156.375 blocks
Add 5% waste	`156.375 × 1.05`	164.19 blocks
Round up	`ceil(164.19)`	165 blocks
Mortar bags, QUIKRETE label	`165 ÷ 13`, rounded up	13 bags of 80 lb
Mortar bags, NCMA generic rate	`156.375 ÷ 16`, rounded up	10 bags of 80 lb
Optional grout, 8 in wall, every 16 in	`139 ÷ 100 × 18.1`	25.16 ft³
Grout in cubic yards	`25.16 ÷ 27`	0.932 yd³
Grout to order, rounded up to 0.05 yd³	`ceil(0.932 ÷ 0.05) × 0.05`	0.95 yd³
You would buy		165 blocks and 13 bags of mortar mix (10 by the NCMA rate)

Disclaimer

Results are planning estimates based on the dimensions and assumptions you enter. They are not engineering, structural or building-code advice. Product yields, densities and coverage vary by manufacturer, batch and site conditions. Check product labels, your supplier and your local building department before you order materials or start work. Read the full calculation disclaimer.

Accuracy

What affects your estimate

The count follows from the face of the block. Mortar and grout depend on the product and on how the wall is designed.

Block size

Half-high blocks show half the face, so the same 139 ft² wall takes 312.75 net and 329 with 5% waste, against 165 standard blocks. The calculator gives no mortar or grout for them because no sourced figure was found. Ask your supplier.

Which mortar rate you use

The QUIKRETE label gives 13 bags for the example wall and the NCMA rate gives 10. They are different products and assumptions, so use the label on the mortar you buy.

Wall dimensions and the 8 × 16 in module

A wall laid to the module comes out even: 8 ft high is 12 courses, and 20 ft long is 15 blocks per course. A 10 ft wall is 7.5 blocks long, so each course needs a cut block. Cuts raise the waste.

Core fill spacing

For an 8 in wall, NCMA's grout runs from 36.1 ft³ per 100 ft² with a filled core every 8 in down to 6.1 ft³ with one every 48 in. The spacing is a design decision, so enter the one you were given.

Quantity estimate, not a structural design

No reinforcement, footing, lintel or structural design is calculated. Wall height, steel, grout and mortar type come from the plans, your engineer and local code. Retaining walls are engineered, and retaining wall units come in other sizes, so use the manufacturer's coverage figure for them. Mortar and grout are cement-based, so wear gloves and eye protection.

Common mistakes

Forgetting to subtract the openings
A 3 ft × 7 ft door is 21 ft² you will not fill with block. Enter the doors and windows, or the count will be too high.
Mixing up the two mortar rates
The label rate is 13 blocks per bag and the NCMA rate is 16. Using 16 on a bag whose label says 13 leaves you short. The calculator orders from the label.
Using the 8 × 8 × 16 mortar and grout figures for other blocks
Those rates are for the standard block. For half-high or half-length units the calculator shows only the block count.
Reading the grout figure as a reinforcement or footing quantity
The grout is core fill at a spacing you were given. Steel and footings are not calculated. For footing concrete, use the concrete calculator with the size from your plans.

Questions people ask

How many concrete blocks do I need for a wall?

Subtract the openings from the wall area, multiply by 1.125 for a standard 8×8×16 block, add 5% for waste and round up. A 139 ft² wall takes 156.38 blocks net and 165 to buy. The guide on how many concrete blocks you need goes through courses and a quick table.

How many concrete blocks per square foot?

1.125 for a standard 8×8×16 block, because its face is 8/9 ft². That is 112.5 blocks per 100 ft², or 119 with the 5% allowance NCMA includes.

How many blocks do I need for a 10 ft by 8 ft wall?

The area is 80 ft², so 80 × 1.125 = 90 blocks net and 95 with 5% waste. Each course is 7.5 blocks long, so the last block in every course is cut in half.

How many bags of mortar for 100 blocks?

At the NCMA rate of 16 blocks per 80 lb bag, 100 ÷ 16 = 6.25, so 7 bags. At the QUIKRETE label rate of up to 13 blocks per bag, 100 ÷ 13 = 7.69, so 8 bags. The guide on how much mortar you need covers block and brick together.

How do I work out the grout to fill block cores?

Choose the wall thickness and the spacing of the filled cores, then the calculator takes the NCMA Table 3 figure per 100 ft². For an 8 in wall with a filled core every 16 in, 139 ÷ 100 × 18.1 = 25.16 ft³, or 0.93 yd³. Ask your supplier whether the cores are filled with grout or a concrete mix.

Sources

Where the numbers come from

NCMA, TEK 04-02A, Estimating Concrete Masonry Materials units per square foot, the 5% waste allowance, mortar per bag (Table 2) and grout per 100 ft² (Table 3), as hosted by CMHA, accessed October 16, 2026.
QUIKRETE, Mortar Mix No. 1102 data sheet up to 13 standard 8×8×16 in blocks per 80 lb bag (one product; other brands differ), accessed October 16, 2026.
NIST Handbook 44 (2026), Appendix C 27 cubic feet per cubic yard, accessed October 16, 2026.

Last reviewed October 16, 2026