Block Wall Calculator: How Many Blocks Do You Need?

This block wall calculator works out how many concrete blocks you need for a wall, plus the mortar and volume of the wall itself. You enter the wall length and height, the block size, and the mortar joint thickness, and it returns the number of blocks (rounded up to whole blocks) and the wall volume in cubic feet and cubic yards. It is a planning tool for a single wall face; it does not account for waste, openings, or footing concrete.

Formula

The calculator uses the standard block count formula for a single wythe (one layer thick) wall:

Number of blocks = (Wall length in inches ÷ (Block length + Joint width)) × (Wall height in inches ÷ (Block height + Joint width))

Where:

• Wall length (l) – the horizontal run of the wall, in feet.
• Wall height (h) – the vertical face of the wall, in feet.
• Block length (bl) – the nominal face length of one block, in inches (commonly 16 in).
• Block height (bh) – the nominal face height of one block, in inches (commonly 8 in).
• Mortar joint (j) – the thickness of the vertical and horizontal mortar joint, in inches (typically 3/8 in).

The product is rounded up to a whole number because you cannot order a fraction of a block. The wall volume is simply length × height × block depth (the depth is assumed equal to the block length in this calculator, which is correct for a standard 8-inch block laid flat; for other depths, adjust manually).

Worked Example

Using the default values: wall length 20 ft, wall height 6 ft, block 16 in × 8 in, joint 3/8 in.

1. Convert wall length to inches: 20 ft × 12 = 240 in.
2. Add joint to block length: 16 + 0.375 = 16.375 in.
3. Divide: 240 ÷ 16.375 = 14.66 blocks per course (horizontal).
4. Convert wall height to inches: 6 ft × 12 = 72 in.
5. Add joint to block height: 8 + 0.375 = 8.375 in.
6. Divide: 72 ÷ 8.375 = 8.60 courses (vertical).
7. Multiply: 14.66 × 8.60 = 126.1 blocks. Round up to 127 blocks.

The calculator gives 127 blocks, matching the example. The wall volume (assuming 8-inch depth) is 20 × 6 × (16/12) = 160 ft³, or 5.93 yd³.

Why Block Count Matters

Ordering too few blocks stops the job halfway; ordering too many wastes money and leaves a pile of heavy rubble. The count also drives the mortar estimate: a typical 8×16 block with a 3/8-inch joint uses about 0.02 to 0.03 cubic feet of mortar per block, but that varies with waste and technique. Knowing the block count lets you estimate mortar bags (each 80-lb bag yields roughly 0.6 ft³ of mortar) and helps you plan delivery logistics, since blocks are heavy – a standard 8×8×16 block weighs about 30-35 lb.

How to Use the Calculator Correctly

• Measure the wall length along the outside face, not the centerline, for a straight wall.
• Measure the wall height from the top of the footing to the top of the last course, not including a cap or parapet.
• Use the nominal block size (the face size including the mortar joint) – most concrete blocks are 16 in long, 8 in high, but check your supplier.
• Set the joint thickness to your actual trowel size; 3/8 in is standard, but some masons use 1/2 in for rough blocks.
• Round up the result to the next whole block; the calculator already does this.

Common Mistakes and Limitations

• Unit mix-ups: Entering wall dimensions in inches instead of feet, or block size in feet instead of inches, throws the count off by a factor of 12. Double-check the unit labels.
• Ignoring waste: The calculator gives a theoretical count. Real projects need 5-10% extra for cuts, breakage, and odd corners. Add that yourself – the calculator does not.
• Openings: Doors and windows reduce the block count. Subtract the area of openings (in square feet) from the wall area before dividing by the block face area (including joint).
• Mortar joint variation: A thicker joint uses fewer blocks but more mortar. The formula assumes uniform joints.
• Wall thickness: This calculator assumes a single wythe (one block thick). For a 12-inch thick wall (two wythes), double the block count. For a cavity wall, count each wythe separately.
• Footing and cap: The footing concrete and cap blocks are not included. Those are separate materials.

Block Size and Type Options

Concrete blocks come in a few standard face sizes. The most common is 16 in long × 8 in high (nominal), which is what the calculator defaults to. Other common sizes include:

• 8×4×16 (half-height, often used for decorative or low walls)
• 8×8×16 (standard)
• 8×12×16 (taller, for retaining walls)
• 12×8×16 (wider, for structural walls)

Block depth (the third dimension) affects the wall volume and the footing width, but not the face count. The calculator’s volume uses the block length as depth, which is correct for standard 8-inch blocks; if you use a 6-inch or 12-inch block, adjust the depth in your own volume calculation.

Cost and Ordering Considerations

Block prices vary by region, type (lightweight, heavyweight, split-face, etc.), and quantity. You will typically order by the pallet; a pallet holds about 90-120 standard blocks depending on weight. To estimate cost, multiply the block count by your local per-block price, then add 5-10% for waste. Mortar and rebar are separate costs. For a large wall, it is wise to get a quote from a supplier based on your exact block count and type – they can also advise on delivery and unloading.

Frequently Asked Questions

How many blocks do I need for a 20×6 foot wall?

Using standard 16×8 blocks with a 3/8-inch joint, you need 127 blocks (as shown in the example). Add 5-10% for waste, so order about 133-140 blocks.

Does the calculator include the footing?

No. The calculator only counts the blocks for the wall face. The concrete footing (usually 2-3 times the wall width) and any cap blocks are separate. You need to calculate those volumes separately.

What if my wall is not a perfect rectangle?

Break the wall into rectangular sections, calculate each section separately, then add the block counts. For corners, you may need special corner blocks or you can cut standard blocks; the waste allowance covers that.

How much mortar do I need per block?

A standard 8×16 block with a 3/8-inch joint uses about 0.02-0.03 cubic feet of mortar per block. An 80-lb bag of mortar mix yields roughly 0.6 cubic feet, so one bag covers about 20-30 blocks. This varies with waste and joint thickness.

Can I use this calculator for retaining walls?

Yes, for the block face count, but retaining walls often use interlocking blocks (like segmental retaining wall units) that have different sizes and no mortar. Use the block dimensions for your specific unit. Also, retaining walls require proper engineering for drainage and soil pressure – this calculator only gives the block count.

Why does the calculator round up?

Because you cannot buy a fraction of a block. Rounding up ensures you have enough to complete the wall; the extra blocks are used for cuts or stored for future repairs. The calculator rounds up to the nearest whole block after computing the exact fractional count.

Related

• Bed Calculations Alpha Beta: What They Mean
• Bed Calculator Radiation: What It Means
• Bed Calculator Lung: What It Is & How to Use It

in l | Wall length | ft | 20 | 0 | 1000
in h | Wall height | ft | 6 | 0 | 100
in bl | Block length | in | 16 | 4 | 24
in bh | Block height | in | 8 | 2 | 16
in j | Mortar joint | in | 0.375 | 0 | 1
out n | Blocks needed | blocks | ceil(round((l * 12 / (bl + j)) * (h * 12 / (bh + j)), 6)) | 0
out ft3 | Wall volume | ft³ | l * h * (bl / 12) | 1
out yd3 | Wall volume | yd³ | (l * h * (bl / 12)) / 27 | 2