Block (CMU) Calculator
Concrete masonry unit (CMU) walls are estimated by net face area — gross wall area minus door and window openings. This block calculator applies the standard 8×8×16-in block rate of 1.125 blocks per square foot, adds your waste allowance, and rounds up to whole blocks. It also estimates 80-lb mortar bags and, when you choose it, the concrete grout needed to fill the block cores — partial or full.
Total horizontal run of the wall.
Finished height from footing to top of block.
Total area of doors, windows, and vents to deduct. 0 if none.
Grout fill adds strength; required by code for some walls.
Covers cuts and breakage. 7% is typical for block.
Blocks needed
289
- 80-lb mortar bagsRounded up — one bag lays ≈30 blocks (rule of thumb).
- 10
- Net wall area
- 240ft²
- Grout fillSelect a fill option above to estimate grout.
- 0yd³
How it was calculated
- Gross area = 30 ft × 8 ft = 240.0 ft²
- Net area = 240.0 − 0.0 openings = 240.0 ft²
- Blocks = ceil(240.0 × 1.125 × 1.07) = 289
- Mortar bags = ceil(289 ÷ 30) = 10
- Grout fill = 0 (hollow wall)
How it works
The block count comes from net face area: gross wall area minus openings, multiplied by 1.125 blocks per square foot. That rate is the geometry of an 8×8×16-inch block plus a ⅜-inch mortar joint — each laid unit covers about 0.889 ft², so one square foot needs 1.125 units.
Waste multiplies the count and the result rounds up to whole blocks. Block work needs a smaller allowance than brick — around 7% — because cuts are fewer and a miscut block can often be flipped and reused.
Mortar uses the rule-of-thumb yield of about 30 standard blocks per 80-lb bag of masonry mortar. Dividing and rounding up gives the bag count with a built-in cushion for deep joints.
Grout fill is separate: a fully grouted 8-inch CMU core takes roughly 0.35 ft³ of concrete per block. Choosing “partial” halves that, matching the practice of grouting every other core or only reinforced cells.
Worked examples
Garage wall with a window
A 30 ft × 8 ft wall with a 20 ft² window = 220 ft² net. 220 × 1.125 × 1.07 = 264.825, so 265 blocks rounded up. Mortar: ceil(265 ÷ 30) = 9 eighty-pound bags.
Fully grouted garden wall
A 20 ft × 10 ft solid wall = 200 ft². 200 × 1.125 × 1.07 = 240.75, so 241 blocks. Full grout fill: 241 × 0.35 ft³ = 84.35 ft³ = 3.12 yd³ of concrete grout.
How to measure
- Measure each straight wall run separately and add them — corners get counted on both runs if you measure the outside face.
- Measure height from the top of the footing, not from grade; buried courses still count.
- Measure every opening (doors, windows, vents) as a rectangle and total them before entering the openings field.
- For stepped or sloped tops, measure the average height across the run and note it is approximate.
- Round footing and wall dimensions the same way your block supplier sells — full and half blocks matter for bond patterns.
- Order grout from a ready-mix supplier for anything over about half a yard; bag-mixing dozens of bags by hand is slow and inconsistent.
Assumptions & limitations
Planning estimate
- The 1.125 blocks/ft² rate is an industry rule of thumb for standard 8×8×16-in CMU with ⅜-in joints; other block sizes need their own rates.
- Mortar yield (one 80-lb bag ≈ 30 blocks) and grout fill (≈ 0.35 ft³ per block) are rule-of-thumb values; joint depth and block style shift them.
- Openings are deducted as rectangles — arched or irregular openings are approximate.
- This is a material-planning estimate, not structural design; reinforced, grouted, and retaining walls require professional engineering and code compliance.
Frequently asked questions
Multiply net wall area (length × height minus openings) by 1.125, add about 7% waste, and round up. A 10 ft × 8 ft wall is 80 ft² → 90 blocks before waste, 97 with 7%. That rate assumes standard 8×8×16-inch blocks with ⅜-inch mortar joints.
“Cinder block” is the old colloquial name — modern units are concrete masonry units (CMUs), made with aggregate rather than cinders. The calculator treats them as the same 8×8×16-inch unit at 1.125 blocks per square foot. True cinder blocks are rare today, so order CMUs and use the same math.
Roughly one 80-lb bag of masonry mortar per 30 standard blocks — a rule of thumb that assumes standard ⅜-inch joints. A 200-block wall needs about 7 bags; the calculator rounds up. Deep joints, cold weather, and inexperienced laying all consume more mortar.
It depends on the wall. Hollow block is fine for non-structural garden walls; structural, retaining, and tall freestanding walls typically need grouted cores with rebar per code. Full grout takes about 0.35 ft³ of concrete per block — select the fill option above to see the yardage for your wall.
Yes — always deduct door and window openings from the face area first, or you will over-order significantly. A 3×7 ft door is 21 ft², which is nearly 24 blocks you do not need to buy. Enter the total opening area in the calculator’s openings field.
Typically 90 to 108 standard 8×8×16 blocks per pallet, depending on the manufacturer. That is handy for ordering: a 265-block job is about three pallets. Confirm the exact pallet count with your supplier before finalizing the order, since it varies by region.