Figure out how many concrete blocks you need before you buy a single one. This concrete block calculator handles full projects with multiple walls, door and window cutouts, reinforced cells, and a full cost estimate.
The basics
What This Calculator Does
This free concrete block calculator (CMU calculator) works out how many concrete masonry units you need, plus mortar, grout, and rebar where they apply. Add one wall or several, subtract door and window openings, choose single or double wythe, and turn on reinforcement if your wall needs it. You’ll get a block count, a cost breakdown, and a diagram of how the blocks lay out.
Blocks stack in a running-bond pattern with a mortar joint between each one. Any openings you add are subtracted from the block count before it’s calculated.Step by step
How to Use This Concrete Block Calculator
Step-1Set Units & Wall TypeChoose feet or meters, then pick single wythe for a standard wall or double wythe if you’re building a cavity wall with two leaves.
Step-2Pick a Block SizeGo with the standard 16″×8″ CMU, a half-height or jumbo block, or enter your own custom dimensions.
Step-3Add Your Wall SegmentsEnter the length, height, and corner count for each wall. Building an L-shaped structure or several walls? Add as many segments as you need and the totals will combine automatically.
Step-4Deduct Doors & WindowsAdd any openings to a segment and their area comes off the block count for you. No manual math required.
Step-5Turn On Reinforcement (If Needed)For load-bearing or code-required walls, check “Reinforced” and set your rebar spacing. You’ll get an estimated grout and rebar quantity alongside the blocks.
Step-6Add Pricing (Optional)Drop in a price per block, mortar bag, grout bag, and labor cost to see a full project cost estimate, not just a block count.
Step-7Calculate, Save & CompareHit Calculate to see your full breakdown and a wall diagram. Save the result to compare against other walls later, or print it straight to PDF for your job file.
The basics
Understanding Block Sizes and Mortar Joints
The most common CMU in the United States is the 8×8×16 block — 8 inches tall, 8 inches wide, and 16 inches long. Laid with a 3/8-inch mortar joint, each block covers about 0.89 sq ft of wall face, which works out to roughly 1.125 blocks per square foot. The calculator uses this same math, then adjusts it for your actual wall size, any openings, corners, and whether you’re building single or double wythe.
Block Size (L×H)
Face Area (with 3/8″ joint)
Blocks per sq ft
Common Use
16″ × 8″ (standard)
≈ 0.89 sq ft
≈ 1.125
Exterior walls, foundations, retaining walls
16″ × 4″ (half-high)
≈ 0.46 sq ft
≈ 2.18
Decorative walls, sill courses
16″ × 12″ (jumbo)
≈ 1.35 sq ft
≈ 0.74
Fast construction, heavy load-bearing
Good to know
Single vs. Double Wythe: What’s the Difference?
A single wythe wall is one layer of block thick — the standard for garden walls, sheds, and most interior partitions. A double wythe (or cavity) wall uses two parallel leaves of block with a gap between them, often for extra strength, insulation, or a finished look on both sides. Because it’s essentially two walls in one, double wythe roughly doubles your block count — the calculator handles that multiplication for you once you flip the toggle.
Good to know
Openings, Corners & Waste: Why the Real Count Is Different
A plain length-times-height calculation always overshoots. Doors and windows remove blocks you’d otherwise count. Corners need extra interlocking blocks at each course to tie the wall together. And no matter how careful the crew is, some blocks crack or get cut wrong. That’s why this calculator subtracts your openings first, adds a small allowance per corner, and then applies your waste factor — usually 5% for a simple straight wall, closer to 10% for anything with lots of corners or openings — on top of the adjusted total.
For reinforced walls
Rebar and Grout, in Plain Terms
Some walls — retaining walls, load-bearing walls, anything your local code calls out — need vertical steel inside the block cores, filled solid with grout. Rebar spacing is just how far apart those vertical bars sit, typically 24″ to 48″ on center. Tighter spacing means more bars and more grout, but a stronger wall. Turn on “Reinforced” and set your spacing, and you’ll get an estimated rebar length and grout volume alongside your block count. Treat these as planning numbers — your engineer or local code will set the actual spacing requirement.
Common questions
Frequently Asked Questions
How many concrete blocks per square foot?
A standard 16×8 inch CMU with a 3/8-inch mortar joint covers about 0.89 square feet, so you need roughly 1.125 blocks per square foot — around 113 blocks per 100 square feet before waste. Enter your actual wall dimensions above for an exact number, openings and all.
Can I calculate more than one wall at once?
Yes. Add as many wall segments as your project needs — a front wall and a side wall, or each leg of an L-shaped structure — and the calculator totals the blocks, mortar, and cost across all of them together.
How does the calculator handle doors and windows?
Add each opening’s width, height, and quantity to a wall segment. That area is subtracted from the wall before the block count is worked out, so you’re not ordering blocks for space you won’t be filling.
Do I need rebar and grout for my wall?
It depends on the wall. Retaining walls, load-bearing walls, and anything over a certain height usually need vertical rebar and grout-filled cores by code. A garden wall or low decorative wall usually doesn’t. If you’re not sure, check with your local building department or a structural engineer before you order materials.
What’s the difference between single and double wythe?
Single wythe is one layer of block — the standard for most walls. Double wythe uses two layers with a cavity between them, roughly doubling the block count. Use the wall type setting to switch between them.
How much mortar and grout do I need?
Mortar runs about 0.9 cubic feet per 100 blocks, or roughly 2 bags of 60-pound pre-mixed mortar. Grout, if your wall is reinforced, is estimated from your rebar spacing and wall height — the calculator converts both into bag counts for you automatically.
Should I include a waste factor?
Yes, always. A 5–10% waste factor covers cuts around openings and corners plus ordinary breakage. Use 5% for a simple straight wall and closer to 10% for anything with more openings or corners.
Can I save or print my results?
Yes. Save a calculation to come back to it later or compare it against another wall, and use the print button to export a clean, ready-to-share PDF of your results.
⚠️ Material Disclaimer: This concrete block estimator provides approximate quantities for planning and budgeting purposes only. It does not account for footings or overall structural design, and rebar/grout figures are planning estimates, not engineering specifications. Always verify quantities with your supplier before purchasing, and consult a licensed structural engineer for load-bearing, retaining, or reinforced wall design.
