Concrete Block Calculator
Estimate CMU or cinder blocks for one or multiple walls, deduct exact openings, and keep block overage separate from the mortar you will actually use.
Use separate wall sections when dimensions or openings change.
Block overage and mortar handling loss are separate. Mortar fields update with the purchase method you select.
CMHA reference About one 70 lb masonry-cement bag per 30 hollow units with face-shell bedding, plus about one ton of masonry sand per eight cement bags. This is not a preblended mortar-bag yield.
CMHA reference 8.5 ft³ of mortar per 100 ft² of net wall under 3/8 in joints and face-shell bedding. The published reference already includes 10% mortar waste.
Add supplier pricing or pallet quantity
Use your current supplier quote. The calculator does not assume a national block price or pallet size.
How to Use
- Add the wall sections - Enter each wall length and height. Keep separate walls as separate rows so openings and totals stay easy to audit.
- Deduct exact openings - Add the measured width, height, and quantity for doors, windows, or other full openings on the wall where they occur.
- Choose the nominal CMU module - Standard 4, 6, 8, 10, and 12 inch CMU all use an 8 × 16 inch nominal face module for ordinary modular estimating.
- Set block and mortar allowances separately - Block overage adds spare units for cuts or breakage. Mortar overage applies only to the mortar used to lay the installed blocks.
- Choose how you buy mortar - Use a named preblended product yield, enter your own bag yield, use the CMHA masonry-cement-and-sand rule of thumb, or view the CMHA wall-area mortar volume reference.
A standard nominal 8 × 8 × 16 inch CMU uses an 8 × 16 inch modular face, so a wall takes about 1.125 units per square foot before openings and overage - about 113 whole blocks for 100 square feet. Enter the actual wall and opening dimensions because the purchase order also depends on spare blocks and the mortar product or batching method you use.
How to Use the Concrete Block Calculator
Start with the finished face of each wall. Enter its length and height, then add any doors, windows, or other full openings with their measured dimensions. If the project has several wall runs, keep them as separate wall sections rather than averaging their sizes.
Choose the nominal CMU size that matches the units you plan to buy. Standard 4, 6, 8, 10, and 12 inch CMU typically share the same 8 × 16 inch nominal face module, so changing thickness identifies the wall unit but does not change ordinary face coverage.
Block overage and mortar overage are deliberately separate. Spare blocks that remain on a pallet do not consume mortar. The calculator therefore finds the blocks actually needed for the net wall first, rounds that installed quantity to whole units, adds the selected block overage for the order, and calculates mortar from the installed blocks or the independent CMHA wall-area reference.
Measure the Wall and Openings
Nominal vs Actual CMU Dimensions
Why Mortar Calculators Give Different Answers
There is no useful universal meaning for “one bag of mortar.” A bag of preblended mortar mix already contains the dry ingredients needed for a product-specific yield, while a bag of masonry cement is one component of mortar that is batched with separate sand. Treating those as interchangeable bags can move an order by a large amount even when the block count is correct.
For preblended mortar, this calculator uses the selected product yield in blocks per bag. Sakrete currently lists about 12 standard 8-inch blocks per 80-pound bag for its Type S and Type N mortar mixes. QUIKRETE publishes up to 13 standard 8 × 8 × 16 blocks per 80-pound bag for Mason Mix Type S. Those are named-product yields, not universal constants.
For site-mixed mortar, the CMHA estimating guide uses a different rule of thumb: one 70-pound bag of masonry cement generally lays about 30 hollow units with face-shell bedding, and about one ton of masonry sand is used for every eight bags of masonry cement. CMHA also says to round smaller sand quantities up for waste, and to add one-half ton when the calculated sand quantity exceeds three tons. The calculator applies that sand-planning rule separately from the cement-bag count.
Concrete Block and Mortar Formulas
The calculator keeps geometry, block overage, and mortar purchasing in separate steps:
Opening area = Sum of opening width × opening height × quantity
Net wall area = Gross wall area − opening area
Block module area = Nominal face length × nominal face height
Theoretical blocks = Net wall area ÷ nominal block module area
Blocks actually laid = Theoretical blocks rounded up to a whole unit
Blocks to order = Blocks actually laid × (1 + block overage), rounded up
Preblended mortar bags = Blocks actually laid ÷ product yield × (1 + mortar overage), rounded up
CMHA reference mortar volume = Net wall area × 8.5 ft³ ÷ 100 ft² under the published CMHA assumptions
The 8.5 ft³ CMHA reference already includes 10% mortar waste under the stated 3/8-inch-joint and face-shell-bedding assumptions, so the calculator does not add another 10% to that reference volume.
Common CMU Sizes and the 8 × 16 Module
Common 4-, 6-, 8-, 10-, and 12-inch CMU differ in wall thickness but typically retain the same nominal 8 × 16 inch face module used for wall-count estimating:
| Nominal CMU size | Typical specified size | Nominal face module | Block count effect |
|---|---|---|---|
| 4 × 8 × 16 in | 3⅝ × 7⅝ × 15⅝ in | 8 × 16 in | Same face coverage |
| 6 × 8 × 16 in | 5⅝ × 7⅝ × 15⅝ in | 8 × 16 in | Same face coverage |
| 8 × 8 × 16 in | 7⅝ × 7⅝ × 15⅝ in | 8 × 16 in | Same face coverage |
| 10 × 8 × 16 in | 9⅝ × 7⅝ × 15⅝ in | 8 × 16 in | Same face coverage |
| 12 × 8 × 16 in | 11⅝ × 7⅝ × 15⅝ in | 8 × 16 in | Same face coverage |
Use the thickness that matches the unit you are buying. The calculator does not choose wall thickness for structural performance.
Worked Concrete Block Examples
A 20 × 8 ft wall has 160 ft² of face area. Standard 8 × 16 inch modules require 180 blocks to cover that area. With 5% block overage, the purchase order is 189 blocks. If the selected preblended mortar yield is 12 blocks per 80-pound bag and the mortar overage is 5%, the mortar order is 16 bags. The CMHA wall-area reference is 13.6 ft³ of mortar under its published assumptions.
For a 24 × 10 ft wall with one 3 × 7 ft door and one 3 × 4 ft window, the gross area is 240 ft² and the openings remove 33 ft², leaving 207 ft². That is 232.875 theoretical standard modules, rounded to 233 blocks actually laid. With 10% block overage the block order is 257, while the 12-block-per-bag mortar calculation still starts from the 233 installed blocks rather than the 257 ordered blocks.
What This Calculator Does Not Size
This is a material-planning calculator, not a masonry structural-design tool. It does not determine reinforcement, grout or core fill, bond beams, lintels, control joints, foundation dimensions, bearing, wind or seismic requirements, or whether a wall is permitted for the intended use.
Those requirements depend on the masonry unit, wall configuration, loads, plans, and governing code. Use the project drawings and local requirements for structural decisions, then use this calculator for the wall-face block and mortar purchasing quantities that fit the supported assumptions.
CMU and Mortar Source Notes
- CMHA explains that specified concrete masonry unit dimensions are typically 3/8 inch less than nominal dimensions so units work with 3/8-inch modular mortar joints.CMHA - CMU-TEC-001 Concrete Masonry Unit Basics
- CMHA TEK 03-11 uses about 113 standard units per 100 ft² and about 8.5 ft³ of mortar per 100 ft² under 3/8-inch joints, face-shell bedding, and a 10% mortar-waste allowance.CMHA - TEK 03-11 Concrete Masonry Basement Wall Construction
- CMHA TEK 04-02A gives a site-mixed estimating rule of thumb of about 30 hollow units per 70-pound bag of masonry cement with face-shell bedding and about one ton of masonry sand per eight bags of masonry cement. It also says to round smaller sand quantities up for waste and to add one-half ton when more than three tons of sand is used.CMHA - TEK 04-02A Estimating Concrete Masonry Materials
- Sakrete currently lists 12 standard 8-inch blocks per 80-pound bag for both its Type S and Type N mortar mix product pages.Sakrete - Mortar Mix Type S
- QUIKRETE Mason Mix Type S lists up to 13 standard 8 × 8 × 16 concrete blocks per 80-pound bag. Product yields are approximate and should be checked against the current package or data sheet.QUIKRETE - Mason Mix Type S Data Sheet
Next Steps
Block and mortar quantities are only one part of a masonry project:
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Frequently Asked Questions
How many standard concrete blocks are in one square foot?
A standard nominal 8 × 8 × 16 inch CMU uses an 8 × 16 inch modular face, which equals 0.8889 square feet. That works out to about 1.125 blocks per square foot before openings or block overage.
How many concrete blocks do I need for 100 square feet?
The modular calculation is 112.5 standard 8 × 16 face modules for 100 square feet, so you need 113 whole blocks before adding any project-specific overage. CMHA also uses about 113 standard units per 100 square feet as an estimating rule of thumb.
How many bags of mortar do I need for concrete block?
Use the yield for the mortar product you will actually buy. Current 80-pound preblended products in this calculator range from about 12 to 13 standard 8-inch blocks per bag, while CMHA site-mixed masonry-cement guidance uses a different batching basis. That is why the calculator does not use one universal mortar-bag constant.
Should I deduct windows and doors from a block wall estimate?
Yes when they are complete openings. Enter the actual opening width, height, and quantity on the wall where they occur. The calculator deducts those measured areas rather than assuming an average door or window size.
Why are actual concrete block dimensions smaller than the nominal size?
CMHA notes that specified CMU dimensions are typically 3/8 inch less than nominal dimensions so the unit plus a 3/8-inch mortar joint fits the modular dimension. Wall-count estimating therefore uses the nominal module.
Is CMU the same as cinder block?
CMU means concrete masonry unit and is the standard technical term for modern concrete block units. “Cinder block” is still widely used in everyday searches, but modern units are generally manufactured as concrete masonry units rather than traditional cinder-based block.