Concrete Block Calculator
Building a block wall, fence, or foundation stem? Enter the wall length and height and pick your block size to estimate how many standard CMU blocks you need, how many 80 lb bags of mortar, and — if you are reinforcing — the optional core-fill volume. All figures include your waste factor.
Blocks required
198blocks
Mortar bags (80 lb)
8
Wall area
160.00 ft²
Estimate includes your waste factor. Figures are estimates, not a quote.
How to use this calculator
- 1Enter the total wall length and height in feet (multiply by Quantity for several identical walls).
- 2Choose your block size. Standard 8×8×16 and 12×8×16 are hollow and can be core-filled; 4×8×16 is treated as solid.
- 3Add a waste factor of about 10% for cuts and breakage.
- 4Turn on core fill if you are grouting cells, and set the fill percentage (100% fills every cell).
Formula & rounding
Blocks = wall area (length × height) × 1.125 blocks per square foot for a 16×8 in nominal face, rounded up after waste. Mortar ≈ 1 bag per 25 blocks. Core fill ≈ 0.32 cf per 8 in block (0.50 cf for 12 in). Counts always round up; core fill is shown in cubic yards first, bags second.
20 ft × 8 ft wall, 8 in block, fully core-filled
- Wall area = 20 × 8 = 160 square feet
- Blocks = 160 × 1.125 × 1.10 (waste) = 198 blocks
- Mortar = 198 ÷ 25, rounded up = 8 bags
- Core fill = 198 × 0.32 = 63.36 cubic feet = 2.35 cubic yards
= 198 blocks, 8 mortar bags, 2.35 yd³ core fill
Standard CMU sizes & coverage
The three block sizes this calculator supports. Coverage is per nominal 16 × 8 inch face (a 3/8 inch mortar joint), so all three share the same 1.125 blocks per square foot — the size changes the wall thickness and the core-fill volume, not the face coverage.
| Nominal size | Actual size | Blocks / sq ft | Core fill / block |
|---|---|---|---|
| 4 × 8 × 16 in | 3⅝ × 7⅝ × 15⅝ in | 1.125 | solid — no fill |
| 8 × 8 × 16 in | 7⅝ × 7⅝ × 15⅝ in | 1.125 | ≈ 0.32 cu ft |
| 12 × 8 × 16 in | 11⅝ × 7⅝ × 15⅝ in | 1.125 | ≈ 0.50 cu ft |
Core fill applies to hollow 8 in and 12 in block only. Figures are planning estimates — confirm exact dimensions and yields with your block supplier.
Frequently asked questions
How many concrete blocks do I need?
Multiply the wall area (length × height) by 1.125 blocks per square foot for standard 16×8 in face block, then add a waste factor. A 160 sq ft wall needs about 198 blocks with 10% waste.
How many bags of mortar per block?
An 80 lb bag of mortar lays roughly 25 standard blocks, so plan about 3–4 bags per 100 blocks. Thicker joints or rough conditions use more. The estimate rounds the bag count up.
How much concrete to fill block cells?
An 8 inch hollow block holds about 0.32 cubic feet of fill per block; a 12 inch block about 0.50. Fully filling 198 eight-inch blocks takes about 2.35 cubic yards. Core fill is usually poured with ready-mix or grout, not bags.
Does this subtract doors and windows?
Not in this version — it estimates a solid wall, which keeps you from under-ordering. Subtract openings yourself if they are large, or simply keep the extra blocks as spares.
What mortar joint thickness does this assume?
A standard 3/8 inch joint, which is why the estimate uses 1.125 blocks per square foot for a nominal 16 × 8 inch face. Running-bond walls with wider joints use slightly fewer blocks but more mortar; thin-set or surface-bonded walls differ again.
How many 80 lb bags of mortar for 100 blocks?
About 3 to 4 bags, since one 80 lb bag of mortar lays roughly 25 standard blocks. Wide joints, partial blocks, rough weather, and inexperienced crews all push usage higher, so the bag count is rounded up rather than down.
What is the difference between a concrete block and a CMU?
They are the same thing in everyday use: CMU stands for "concrete masonry unit", the formal name for the standard hollow concrete block. Sizes are given nominally (8 × 8 × 16 in) but the actual block is 3/8 inch smaller on each face to leave room for the mortar joint.
Assumptions & sources
- Blocks per sq ft
- 1.125 for a 16 × 8 in nominal CMU face including a 3/8 in mortar joint (= 144 ÷ 128).
- Mortar yield
- ≈25 standard blocks per 80 lb mortar bag (typical range 25–33; rounded up).
- Core-fill volume
- ≈0.32 cu ft per 8 in block, ≈0.50 cu ft per 12 in block; 4 in block is treated as solid.
- Waste factor
- Default 10% for cuts and breakage; counts round up. Door/window openings are not deducted.
- Not included
- Rebar, footings, lintels, and itemized cost — block height and reinforcement are code-governed; consult a professional.
See the methodology & sources for how these values, formulas, and rounding are chosen.
Helpful concrete guides
- Concrete bag sizes & yields — helps compare bag yields when core fill or small grout batches are easier than ready-mix.
- How to calculate concrete — shows the cubic-foot and cubic-yard conversions used for optional core-fill volume.
This is an estimate, not a quote. Concrete quantities, bag yields, block coverage, and prices vary with product, brand, mix, region, supplier, tax, delivery, and on-site conditions. Always confirm with your supplier and round up for safety. For structural or code-related work, consult a qualified professional or your local building authority.