Measure wall faces
Measure the length and height of each rectangular wall face. If several walls are identical, enter one wall and use its quantity.
Gross wall area = length x height x quantity
Concrete & masonry
Concrete Block Calculator
Calculate how many concrete blocks or CMUs you need from a known coverage rate or detailed wall, opening, block, and mortar- joint dimensions. Add your own allowance, convert the result to pallets or packages, and estimate optional material cost.
Calculation method
How to calculate how many concrete blocks you need
Concrete block quantity is based on the net wall face area and the repeating face module occupied by each concrete masonry unit (CMU) plus its mortar joints. Use Quick Coverage when your manufacturer or supplier gives a verified blocks-per-area value. Use Detailed Walls when you know the specified block face dimensions and intended joint sizes.
Subtract openings
Deduct rectangular doors, windows, and other areas that will not receive concrete block.
Net wall area = gross wall area - openings
Build the block module
Detailed mode adds the head joint to the specified block length and the bed joint to the specified block height.
Module = (block length + head joint) x (block height + bed joint)
Add allowance and round
Apply the extra-material percentage you choose, round up to whole blocks, then round to whole pallets or packages only when a package quantity is entered.
Planned blocks = base blocks x (1 + allowance %)
Concrete block formula
Detailed Walls treats the wall as a repeating rectangular module. The calculator first determines the module area, then calculates how many modules fit into the net wall face area.
Module lengthspecified block length + head joint
Module heightspecified block height + bed joint
Module areamodule length x module height
Base block quantitynet wall area / module area
Do not enter nominal block dimensions and then add the mortar joint again. For modular CMU, nominal dimensions already represent the block-plus-joint module. Detailed mode expects the specified face dimensions of the unit itself, then adds the entered joints once.
Worked example: standard 8 x 16 module
Suppose a wall is 20 ft x 10 ft with one3 ft x 4 ft opening. The net wall area is:
20 x 10 - 3 x 4 = 188 ft2
A typical inch-pound CMU with an 8-in-high x 16-in-long nominal face module has specified face dimensions of7.625 in high x 15.625 in long. With0.375 in head and bed joints, the module is:
(15.625 + 0.375) x (7.625 + 0.375) = 16 x 8 = 128 in2
One square foot contains 144 square inches, so the theoretical coverage is 144 / 128 = 1.125 blocks per ft2. The base quantity is therefore:
188 x 1.125 = 211.5 blocks
With a 10% allowance, the exact planned quantity is 232.65 blocks, so the whole-block requirement rounds up to 233 blocks.
If the selected product ships 90 blocks per pallet, the calculator rounds purchasing to 3 pallets = 270 blocks, leaving an estimated purchasing surplus of37 blocks.
Quick Coverage vs. Detailed Walls
Use Quick Coverage when...
- Your supplier publishes blocks per ft2 or blocks per m2.
- The exact product uses a proprietary or unusual module.
- You want to match a manufacturer takeoff table.
- The system is dry-stack, adhered, or otherwise nonstandard.
Use Detailed Walls when...
- You know the specified block face dimensions.
- You know the intended head and bed joint thicknesses.
- You need to subtract several wall openings.
- You want to see the implied module and coverage rate.
Nominal, specified, and actual CMU dimensions
Concrete masonry dimensions can be confusing because nominal size and unit size are not the same thing. Nominal dimensions describe the modular space occupied by the unit plus its mortar joint. Specified dimensions describe the dimensions used for manufacture and calculation; an individual unit's actual measured size can vary within permitted tolerances.
For a common nominal 8 x 8 x 16 in CMU, the specified dimensions are typically 7.625 x 7.625 x 15.625 in. The wall thickness does not affect the simple face-count calculation, so Detailed Walls uses the specified face length and height.
Width still matters for structural design, mortar, grout, reinforcement, weight, and other material quantities. Those are separate calculations and are not inferred here.
Inch-pound CMU and hard-metric CMU are not identical
A typical inch-pound 8 x 16 in face module uses specified dimensions about 7.625 x 15.625 in with 0.375-in joints. A hard-metric 200 x 400 mm face module commonly uses a 190 x 390 mm specified face with 10 mm joints. These systems are close, but they are not mathematically identical.
The calculator converts the measurements you entered when you switch Imperial and Metric. It does not silently replace an inch-pound product with a hard-metric product. If your project uses hard-metric CMU, enter the dimensions for that actual product.
How should openings be measured?
Enter the rectangular wall area that will not contain block. Repeated openings with the same dimensions can use the quantity field. If an opening is irregular, divide it into simple rectangles when that gives a reasonable takeoff.
The calculator subtracts opening area only. It does not automatically add special jamb, sill, lintel, bond-beam, corner, return, or cut units around an opening.
Allowance, cuts, and special units
There is no universal allowance percentage for every concrete block project. Waste can depend on wall layout, cuts, breakage, handling, bond pattern, corners, returns, special units, site conditions, and how much reserve material you want available.
Use the allowance field as a planning input rather than a rule. If project drawings, the contractor, manufacturer, or supplier provides a takeoff allowance, use that project- specific value.
A percentage allowance does not tell you how many corner, half, sash, bond-beam, lintel, open-end, or other specialty units to order. Those should be taken off from the actual wall layout and plans.
Block pallets, packages, and cost
Pallet quantities vary with unit size, unit weight, manufacturer, plant, and distributor. Enter the actual number of units sold in the pallet or package you are pricing.
If you enter a package size, the calculator rounds the whole- block requirement up to complete packages and reports the purchased quantity and surplus. A price per block is applied to the purchased block quantity; a price per package is applied to the rounded package count.
Concrete block calculator FAQ
How many 8 x 8 x 16 blocks are in one square foot?
A standard 8-in-high x 16-in-long nominal face module occupies 128 in2, giving a theoretical coverage of 1.125 blocks per ft2. That is 112.5 theoretical units per 100 ft2, which rounds to 113 whole units before any added allowance. Confirm the dimensions and module for the exact product you are using.
Should I enter nominal or specified block dimensions?
Detailed Walls expects the specified unit face length and height, then adds the head and bed joints you enter. Do not enter nominal module dimensions and add the joint again, because that would count the joint twice.
Does block width affect the number of blocks in a wall?
Not in this simple face-area count when units have the same face length and height. Width affects many other quantities and design properties, but the number of face modules is determined by face length, face height, joints, and net wall area.
Does the calculator subtract doors and windows?
Yes. Detailed Walls subtracts the rectangular openings you enter before calculating the block quantity.
Does it calculate corner and bond-beam blocks?
No. It estimates total face-module quantity. Specialty unit types depend on wall layout and project details and should be taken off separately.
Does it calculate mortar, grout, or rebar?
No. Those depend on additional geometry, unit type, reinforcement, grouting pattern, and project requirements. Use a dedicated material calculator or the project specifications instead of inferring them from block count alone.
Can I use zero-width mortar joints?
Detailed mode accepts zero for a head or bed joint so it can represent a geometric dry-stack module. For a proprietary dry-stack or adhered system, however, a verified manufacturer coverage rate is usually the safer input.
How much extra concrete block should I buy?
There is no single correct percentage. Choose an allowance that reflects your layout, cuts, breakage, handling, special units, and project guidance.
Can I use Metric units?
Yes. The calculator supports meters, square meters, and millimeters and converts measurements already entered when you change systems.
Are hard-metric blocks the same as converted U.S. blocks?
Not exactly. A hard-metric 200 x 400 mm module and a converted 8 x 16 in module are close but not identical. Enter the dimensions of the actual product being used.
Can I save or share my estimate?
Yes. After a valid calculation, the page stores the current calculator state in the URL. Copy or share that URL to reopen the same project inputs and result later.
Assumptions and limitations
This calculator is a material-quantity estimator. Quick Coverage assumes the coverage value you enter applies to the exact CMU product and wall system. Detailed Walls assumes the entered specified face dimensions and joints form a repeating rectangular module over the net wall area.
The estimate does not create a coursing plan, optimize cuts, determine bond pattern, identify specialty units, size lintels, calculate reinforcement, calculate core grout, estimate mortar, evaluate structural capacity, or check code requirements.
Wall openings are treated as rectangular area deductions and are not checked for overlap. Pallet/package quantities and pricing are only as accurate as the supplier values entered.
Before purchasing, confirm wall dimensions, actual CMU size, nominal/specified dimension terminology, joint dimensions, specialty-unit requirements, pallet quantities, and the project drawings or specifications that apply to the work.