Concrete Block (CMU) Calculator

Why 0.889 sq ft per block is exact, not estimated

A standard CMU is specified at 15⅝ × 7⅝ in, and its nominal size — 16 × 8 in — is the unit plus one ⅜ in mortar joint, the convention the concrete masonry industry documents (CMHA, formerly NCMA, tech note CMU-TEC-001-23) precisely so walls tile in clean modules. That makes the per-block wall area definitional: (16 × 8) ÷ 144 = 8⁄9 ≈ 0.889 sq ft, or 112.5 blocks per 100 sq ft before waste. Nothing was measured and nothing was looked up in a table: the engine divides the two nominal dimensions and keeps the full-precision result; only the display is rounded. The worked wall: 200 sq ft ÷ the face = 225 blocks — nothing is rounded at this stage — then 5% breakage waste lifts it to 236.25, purchased as 237 blocks with 0.75 to spare. Mortar: 225 laid blocks ÷ 13 per 80 lb bag = 17.308, rounded up to 18 bags (breakage waste consumes no mortar, so bags are figured on the laid count).

Three figures on this page come from outside the arithmetic: the nominal face, the mortar yield, and the waste default. Where each one comes from, how it is labeled, and how a test pins it so it cannot drift is the subject of the guide Coverage figures and where they come from. The rest of this page explains what the block engine does with them.

The five stages, in order

Every estimator on the site runs the pipeline documented on the methodology page. For concrete block the stages look like this, with the worked wall carried through each one:

1. Geometry. Wall length × wall height, both in feet, is the gross wall area in square feet, and the openings area is subtracted from it. Metric entries are converted to feet and square feet before this step, so the engine always works in one set of units. The worked wall: 25 × 8 = 200 sq ft with nothing to subtract, so the net area is also 200 sq ft.
2. Coverage. The net area is divided by the face area of one nominal block, 0.889 sq ft: 200 ÷ the face = 225 blocks. Because the nominal face already contains the joint, no separate joint allowance is added — the count is simply the number of 16 × 8 in modules the area holds. In general the result is fractional; here it is whole because 25 ft is 18.75 block lengths and 8 ft is exactly 12 courses.
3. Waste. The breakage percentage is applied to the block quantity as blocks × (1 + waste ÷ 100): 225 × (1 + 5 ÷ 100) = 236.25. It is applied before any rounding, so the margin is counted exactly once. The default is the Brick Industry Association's "at least 5 percent" added to net brick quantities, which this site applies to block as its masonry convention — a trade convention with a primary source — and the field accepts any figure from zero to one hundred.
4. Round-up. The waste-adjusted quantity is rounded up to a whole block: 236.25 becomes 237, and the 0.75 of a block you did not need is reported as spare. This is the only rounding on the block count; the mortar branch below makes the same kind of round-up once more. Every intermediate value keeps full precision and is rounded only for display. A quantity that lands within a hair of a whole number is treated as whole, so an exact fit never buys a phantom block (worked below).
5. User price. If you type a price per block, the cost is that price times the rounded count. Leave the field blank and no cost line appears. The tool never supplies, averages, or guesses a price of its own — every cost figure on this site comes from the number you enter.

Mortar runs as a side branch off the second stage. The laid block count is divided by the manufacturer's 13 blocks per 80 lb bag and rounded up: 225 ÷ 13 = 17.308, so 18 bags. It is computed on the net count rather than the waste-adjusted one because a block that breaks on the pallet is never bedded in mortar. Neither the waste field nor the price field touches the mortar line; only the net wall area does.

What each input does

Units. Imperial mode takes feet for the two lengths and square feet for the openings. Metric mode takes metres and square metres and converts them with exact factors — 0.3048 m per foot for the lengths and 0.09290304 m² per square foot for the openings, both consequences of the international definition of the foot. The engine sees feet either way, so the imperial and metric arithmetic is identical; in metric mode the result box reports the net wall area in square metres and the block count is the same nominal-module count it always is.

Wall length. The total run of wall to be laid, in feet. For a wall that turns corners, add every segment and enter the sum: the engine treats whatever you enter as one straight rectangle and has no notion of corners. The corner and half units in your coursing layout replace stretchers roughly one for one in area, which is why the area method still lands close — the FAQ on specialty units says more.

Wall height. In feet — not inches, and not courses. An 8 ft wall is entered as 8; the same wall typed as 96 is the misreading discussed below. Height in inches divided by the 8 in nominal course height is the course count, a whole number when the height lands on the module and a fraction otherwise; the engine counts by area rather than by whole courses, so a partial course contributes its partial area and nothing more.

Openings to subtract. The combined area of every door, window, and gap in the wall, in square feet (square metres in metric mode), default zero. It is an area, not a dimension: a 3 × 7 ft door is 21 sq ft, and several openings go in as one total. The engine rejects an openings figure as large as the wall itself. Because openings come off the area before waste is applied, the blocks that would have filled them are not bought — and the cutting around them is left to the waste percentage, which is one reason the default is described as a floor.

Waste. The default of 5% is the one waste figure on this site with a primary source — BIA Technical Note 10's "at least 5 percent" added to net brick quantities, which this site applies to block as its masonry convention — and it is still a convention, not a measurement, which is why the field is editable. Enter zero to see the pure geometry count. The guide Waste factors, explained covers how to choose a figure for your own layout and crew.

Price per block. Optional, and supplied only by you: the tool has no price table, and with the field empty no cost line appears. Enter a price and the cost is that price times the rounded block count. It multiplies the rounded count deliberately — a block you must buy but will not lay is paid for in the estimate, as it would be at the register. Mortar is not priced; the tool takes no mortar price.

The figures this page relies on, and their status

• Nominal face 16 × 8 in — a documented industry convention (CMHA tech note CMU-TEC-001-23): the specified unit plus one ⅜ in joint. Not adjustable; it is what the tool counts.
• Face area 0.889 sq ft — pure arithmetic on the two nominal dimensions, computed by the engine from those dimensions and kept at full precision, rounded only for display; equivalently 112.5 blocks per 100 sq ft.
• Mortar, 13 standard blocks per 80 lb bag — a manufacturer figure from the QUIKRETE Mortar Mix No. 1102 data sheet, stated "up to" and stated for "standard (8 in by 8 in by 16 in) blocks" only. Fixed in this tool, with no field to change it; the tool counts exactly that block, which is why it can use the figure at all.
• Waste default 5% — a trade convention with a primary source (BIA Technical Note 10, "at least 5 percent"); adjustable.
• Unit factors — 12 in per foot and 144 sq in per square foot, 0.3048 m per foot and 0.09290304 m² per square foot — definitions, all exact.

How the site sources, labels, and tests each of these is the guide's subject; this list only says which of them this page depends on and what kind of figure each one is.

Worked examples

Every figure below is computed at build time by the same engine that runs the form above — if the engine changed, these paragraphs would change with it, or the build would fail.

The house wall: 25 × 8 ft

Length 25, height 8, openings left blank, waste left at the default. Gross area 200 sq ft; net the same. Divided by the face: 225 blocks. Cross-check by coursing: 25 ft is 300 in ÷ 16 = 18.75 block lengths per course, 8 ft is 96 in ÷ 8 = 12 courses, and 18.75 × 12 = 225 — the same figure, because dividing by the face area and multiplying the two per-dimension counts are the same arithmetic. Waste takes it to 236.25; buy 237, 0.75 spare. Mortar: 17.308 bags rounds up to 18.

A wall with a door and a window: 30 × 8 ft

The door is 3 × 7 ft = 21 sq ft and the window 4 × 3 ft = 12 sq ft, both from the geometry engine, so the openings field gets 33. Gross area 240 sq ft; net 207 sq ft; divided by the face, 232.88 blocks — fractional now, because 207 sq ft is not a whole number of modules. Waste lifts it to 244.52; buy 245, 0.48 spare. Mortar on the 232.88 laid blocks: 17.913 → 18 bags. The openings removed 33 sq ft, which is 37.13 blocks' worth: the same wall with nothing subtracted counts 270 and buys 284.

A low garden wall: 12 × 2 ft

24 sq ft ÷ the face = 27 blocks — 9 per course over 3 courses. Waste takes it to 28.35; buy 29, 0.65 spare. Mortar: 2.077 bags rounds up to 3. On a wall this small the round-up is a large share of the answer — most of the last block and most of the last bag are spare on paper — and because the mortar yield is an "up to" figure, the last bag may be less spare in practice than it looks.

An exact fit: 16 × 4 ft at zero waste

64 sq ft ÷ the face = 72 blocks: 12 per course, 6 courses. With waste set to zero the adjusted quantity is still 72, so the purchase is 72 with 0 spare. In floating-point arithmetic a division like this can land a hair above the whole number; the package step treats a quantity within one part in a billion of a whole as exact, so no phantom block is added. Mortar: 5.538 → 6 bags.

The house wall at 0%, 5%, and 10% waste

The same 225 net blocks buy 225 at zero waste (0 spare), 237 at the default (0.75 spare), and 248 at 10% (0.5 spare). On a wall of this size each percentage point is 2.25 blocks. The mortar line reads 18, 18, and 18 bags respectively — unchanged, because it is figured on the laid count.

Four sides entered as one run: 80 × 6 ft

An enclosure with 4 sides of 20 ft goes in as a single 80 ft length. 480 sq ft ÷ the face = 540 blocks — 60 per course, 9 courses, counted as if the wall were straight. Waste takes it to 567, which happens to be a whole number, so the purchase is 567 with 0 spare: the waste allowance and the rounding remainder are separate ideas, and here one is present and the other is not. Mortar: 41.538 → 42 bags.

Metric: 6 × 2.4 m with 2 m² of openings

In metric mode the form converts before the engine runs: 6 m ÷ 0.3048 = 19.69 ft, 2.4 m ÷ 0.3048 = 7.87 ft, and 2 m² ÷ 0.09290304 = 21.53 sq ft. Gross area 155 sq ft (14.4 m²); net 133.47 sq ft (12.4 m²). Divided by the face, 150.16 blocks; waste lifts it to 157.66; buy 158, 0.34 spare. Mortar: 11.551 → 12 bags. The block face is not converted — the count is in nominal 16 × 8 in modules whatever unit the wall was measured in.

The house wall with a price

Type 1.37 into the price field — a number typed for the example, not a price of anything — and one line is added to the result: 237 × $1.37 = $324.69. Nothing else moves. The cost multiplies the rounded count, so the 0.75 of a spare block is paid for, as it would be at the register; the 18 mortar bags are counted but not priced.

Reading the output

The first line is the purchase count — blocks after waste and the round-up, the number to give the yard. The second shows the net wall area (square feet, or square metres in metric mode) divided by the face area to give the exact block count; the face is displayed rounded to 0.89, but the division used the full-precision value, which is why 200 sq ft reads 225 blocks rather than the figure you would get by dividing by the rounded display value. The third line is the waste-adjusted quantity, still fractional; the fourth is the mortar bag count at the manufacturer's 13 blocks per bag. A cost line appears only when you have entered a price. The copy button captures the block count, the net area, and the mortar bags in one line, ready to paste into a materials list.

Common misreadings

Height typed in inches. The house example: the 25 × 8 ft wall entered with a height of 96 becomes 2,400 sq ft, 2,700 blocks, 2,835 to buy and 208 mortar bags — 12 times the order. Both dimension fields take feet; if the count looks like a warehouse, check the height field first.

An opening entered as a dimension. Typing 7 for a 3 × 7 ft door subtracts 7 sq ft instead of the door's 21. The field wants width × height, summed over every opening.

Counting from the specified face. Dividing the wall by the 15⅝ × 7⅝ in specified unit, or adding a joint allowance on top of the nominal count, counts the joint twice and overbuys. Nominal dimensions exist so that this is never done; the joint is already inside the 0.889 sq ft.

Reading the exact count as the purchase count. 225 is what the geometry needs; 237 is what to buy. The waste and the round-up sit between the two lines on purpose.

Applying waste twice. The breakage allowance is already inside the purchase count. Adding "a few extra for safety" on top is a second margin — reasonable if that is what you want, but know that it is a second one.

Reading the mortar bags as waste-inclusive. They are not: the 18 bags on the house wall carry no waste, and the yield behind them is stated "up to". The FAQ says to treat the figure as a floor.

Centimetres in the metre fields. Metric mode takes metres for the lengths and square metres for the openings. A height typed in centimetres inflates both the area and every count that follows; the field labels carry the unit for that reason.

Where the model stops

This is a purchasing count, and the site estimates material quantities only. It does not determine whether a block wall is adequate for any purpose, whether it complies with a building code, what footing, reinforcement, grouting, or core fill a wall needs, or whether a permit applies — those questions belong to a licensed engineer or your building department, exactly as the note under the form says.

Within its own job, the model simplifies. It counts stretcher-equivalent area, not specialty units — corners, halves at openings, lintels, and bond beams are left to your coursing layout. It assumes the standard ⅜ in joint and the 16 × 8 in nominal face; a different joint or a different unit height changes the face area and therefore the count. The block's width, its depth into the wall, does not enter the arithmetic at all, and the mortar yield is stated for "standard (8 in by 8 in by 16 in) blocks" only. Waste is one flat percentage on the block quantity; the mortar figure carries no waste and is an "up to" figure the tool treats as exact. Grout, reinforcement, the cutting pattern around openings beyond the waste percentage, pallet quantities, delivery minimums, and regional product variation are outside the model, and so is any price it did not receive from you.

The rest of the masonry aisle

For clay brick the same wall-area method runs on the brick calculator with BIA's per-size coverage; for flatwork underfoot, the paver calculator counts units from your paver's own dimensions. Filling block cores or pouring a footing? Size the concrete with the slab calculator or bag calculator.

The concept pages behind this tool are Coverage figures and where they come from and Waste factors, explained; the full list is on the guides index, and the shared pipeline is on the methodology page.

Frequently asked questions

Everything computes locally in your browser. Sources for the mortar yield and the nominal-dimension convention are on the methodology page.