Cinder & Concrete Block Calculator
This cinder block calculator estimates how many concrete blocks you need for a wall, along with the bags of mortar to lay them, the concrete to fill the block cores, and the material cost. Enter the wall width and height, pick a block size, allow for waste, and deduct any door and window openings — the calculator returns the block count with a dimensioned wall diagram and a lay-out cut list. It works for both "concrete block" and "cinder block": the two names describe the same modular masonry unit today, so the results apply either way (see the glossary for how the terms differ historically). Every figure comes from the wall area and the block dimensions you enter, so each number can be checked by hand.
Blocks needed
198
Total blocks to purchase (includes waste)
- Mortar bags
- 17
- Core fill
- —
- Wall area
- 160 sq ft
- Blocks per ft²
- 1.125
80 lb pre-mix bags
Enable core fill to estimate grout
Dimension diagram
| Item | Qty | Note |
|---|---|---|
| Full blocks | 174 | 12 courses × 15 per course |
| Half blocks (running-bond ends) | 6 | One per alternate course |
Layout positions — illustrative only; buy the block count shown above (includes waste).
About this cinder block calculator
Use this calculator whenever you need to estimate the blocks, mortar and core-fill concrete for a masonry wall — a garden wall, a foundation stem wall, a retaining wall or a shed base. Enter the wall size and block dimensions and every figure is worked out from the area you enter, so it stays checkable by hand. For other layout jobs, see all construction calculators.
How to use the cinder block calculator
- 1
Measure the wall width and height
Measure the finished wall face in feet — its total width and its total height. The calculator multiplies them to get the gross wall area, which is the starting point for the block count. Use the same units for both.
- 2
Pick the block size
Choose the block from the size list. US blocks share a nominal 16-inch-long by 8-inch-high face, so the blocks-per-square-foot figure is the same across widths; the width (4, 6, 8, 10 or 12 inches) sets the wall thickness and the core-fill volume. The standard unit is 8 × 8 × 16 inches.
- 3
Set the waste percentage
Add a waste allowance to cover cuts, breakage and off-cuts — 5% to 10% is typical for a straightforward wall. The calculator increases the block count by this percentage and rounds up to whole blocks so you buy enough.
- 4
Deduct door and window openings
Enter the width and height of any doors or windows. Their area is subtracted from the gross wall area, so you are only charged blocks for the net masonry you actually build, not the openings.
- 5
Toggle core fill and enter a price
Turn on core fill if you are grouting the hollow cores solid (common for reinforced or structural walls) to get the concrete volume needed. Enter a price per block to add a material cost estimate. Both are optional.
- 6
Read the blocks, mortar and fill
The calculator reports the number of blocks to buy, the bags of mortar to lay them, and — if core fill is on — the concrete volume for the cores, plus the dimensioned diagram and cut list.
Concrete block size reference
Look up how many blocks each nominal size covers per square foot and how much concrete fills 100 block cores, so you can match a block size to your wall and estimate core fill at a glance.
| Nominal size (W × H × L) | Blocks per ft² | Core-fill (yd³ / 100 blocks) |
|---|---|---|
| 8 × 8 × 16 in | 1.125 | 1.00 |
| 4 × 8 × 16 in | 1.125 | — |
| 6 × 8 × 16 in | 1.125 | 0.83 |
| 10 × 8 × 16 in | 1.125 | 1.25 |
| 12 × 8 × 16 in | 1.125 | 1.54 |
Frequently asked questions
How many cinder blocks do I need per square foot?
A standard block with a 16-inch-long by 8-inch-high face covers 128 square inches, so you need 144 ÷ 128 = 1.125 blocks per square foot — that is 1⅛ blocks, or about 112.5 blocks per 100 square feet of wall. Multiply your net wall area (after deducting openings) by 1.125, then add a waste allowance and round up.
How much mortar do I need for a block wall?
This calculator estimates roughly one bag of mortar for every 12 blocks, based on standard 80 lb bags of pre-blended mortar mix (source: Inch Calculator). Divide your total block count by 12 and round up to get the number of 80 lb bags. Actual coverage varies with joint thickness and site conditions, so buy a little extra.
What is the difference between a concrete block and a cinder block?
In modern usage the terms are synonyms — both name a hollow modular masonry unit, and searching either returns the same product. Historically a "cinder block" was cast with coal cinders (fly ash) as a lightweight aggregate, while a "concrete block" uses sand and gravel and is heavier and stronger. Today almost all units sold are concrete blocks, so this calculator treats them the same.
How do I account for doors and windows?
Measure each opening and enter its width and height. The calculator adds up the opening areas and subtracts them from the gross wall area, so the block count is based on the net masonry area only. This avoids buying blocks for space that will be a door or window.
How much concrete do I need to fill block cores?
Core fill is the concrete or grout that fills the hollow cores of the blocks solid — it is a core-fill estimate only, not a slab or footing estimate. The volume depends on the block width: as a rule of thumb one cubic yard fills about 120 blocks for 6-inch, 100 for 8-inch, 80 for 10-inch and 65 for 12-inch wide blocks. Turn on core fill and the calculator returns the volume for your block count and width.
How much waste should I add?
A waste allowance of 5% to 10% covers cut blocks at corners and openings, breakage during handling, and the occasional reject. Use the lower end for a simple rectangular wall and the higher end for walls with many openings, corners or curves. The calculator applies your percentage and rounds up to whole blocks.
What size is a standard cinder block?
The standard US block has a nominal size of 8 × 8 × 16 inches (width × height × length). The actual block measures about ⅜ inch smaller in each direction — roughly 7⅝ × 7⅝ × 15⅝ inches — so that with a ⅜-inch mortar joint the finished unit lays out on the nominal 8-inch module.
Can this calculator estimate mortar and cost too?
Yes. Along with the block count it estimates the bags of mortar (about one 80 lb bag per 12 blocks) and, when you enter a price per block, a material cost. Turn on core fill to also get the concrete volume for grouting the cores. All of it appears with the dimensioned diagram and cut list.
Cinder block terms glossary
- Nominal vs actual size
- The nominal size (e.g. 8 × 8 × 16 in) includes the mortar joint and is used for layout; the actual block is about ⅜ in smaller in each dimension (about 7⅝ × 7⅝ × 15⅝ in) so it fits the nominal module once laid.
- Course
- A single horizontal row of blocks in a wall. Wall height in blocks equals the wall height divided by the 8-inch nominal course height.
- Running bond
- The standard block layout where each course is offset by half a block from the course below, so the vertical joints do not line up. It spreads load and is the pattern the diagram shows.
- Core fill (grout)
- Concrete or grout poured into the hollow cores of the blocks to make the wall solid, often around reinforcing steel. It adds strength and mass and is estimated separately from the blocks themselves.
- Face shell
- The outer solid walls of a hollow block on the exposed faces. Mortar is usually applied to the face shells when laying block (face-shell bedding).
- Waste factor
- The extra percentage of blocks added to cover cuts, breakage and rejects, typically 5% to 10%, so you do not run short mid-job.