Cinder Block Calculator – Count Blocks Before You Buy

How waste changes the order

Same wall, different waste allowances. The table updates live with your wall size.

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How it works

Details that change the count

Three walls, three orders, one method

The calculator counts one wall at a time, by one method, and the three preset walls on the page are the best introduction to that method, because they span the sizes most readers actually build. The garden wall, 20 feet by 5 feet, is the default: a gross area of 100.00 square feet, no openings, an exact count of 112.50 blocks, 113 blocks to lay, and 124 blocks to buy with the usual 10 percent waste allowance, which is 11 extra blocks. The garage wall, 24 feet by 8 feet, is larger in both dimensions: a gross area of 192.00 square feet, an exact count of 216.00 blocks, 216 blocks to lay, and 238 blocks to buy, which is 22 extra. The shed wall, 10 feet by 8 feet, is smaller: a gross area of 80.00 square feet, an exact count of 90.00 blocks, 90 blocks to lay, and 100 blocks to buy, which is 10 extra. Every figure in this section was read from the calculator’s own displayed results for the stated inputs.

Notice what is exact and what is rounded in those three cases, because the distinction is the whole method. The exact counts, 112.50, 216.00, and 90.00, are net area divided by the block face area of 0.8889 square feet, which is the 16 by 8 inch nominal face. The garage and shed walls divide exactly, so their exact counts are whole numbers and their lay counts equal their exact counts, while the garden wall’s exact count of 112.50 rounds up to 113 to lay, because half a block cannot be laid as a whole block and a short wall is worse than one spare block. The buy counts are then computed from the exact counts, not from the rounded lay counts, which is the rule the page states and the next section proves: 112.50 times 1.10 is 123.75, rounded up to 124, while 216.00 times 1.10 is 237.60, rounded up to 238, and 90.00 times 1.10, which the engine’s unrounded arithmetic carries a fraction above 99.00, rounded up to 100. The extra blocks, 11, 22, and 10, are measured against blocks to lay, so the buy count minus the lay count is the waste allowance in blocks, and the percentage is applied to the exact count, not to the rounded one.

The last row of that table shows the openings step, which is the step most often skipped and therefore the step that most often causes overbuying. A 21 square foot opening in the garden wall, about one standard door, reduces the net area from 100.00 to 79.00 square feet, because the gross area minus the openings is the net area, and the exact count falls from 112.50 to 88.88, as displayed, giving 89 blocks to lay and 98 blocks to buy, which is 9 extra. Skipping the opening would have ordered 124 blocks for a wall that needs 98, an overbuy of 26 blocks on a small wall. Add up every opening in the wall, doors, windows, and any other opening, and enter the total in square feet, not as a count of openings, because the calculator subtracts area, and a count of openings without their sizes cannot be subtracted. A wall with two openings of different sizes needs both areas added together before entry, just as the board-and-batten page’s openings box needs areas, not counts.

Multiple walls are counted the same way, one at a time, and the results added together by hand, because the calculator counts a single rectangular wall, not a building. A garden with two walls, one 20 by 5 feet and one 10 by 8 feet, needs 124 plus 100, which is 224 blocks to buy at 10 percent waste, if both walls are straight runs with no shared corners. Walls that share a corner need care, because the corner blocks belong to one wall’s count or the other’s, not both, and counting the shared corner twice overbuys by the corner’s block count. The base guide’s advice for corners applies here: raise the waste allowance to 15 percent for walls with many corners, steps, or angled cuts, where more blocks end up cut or broken, rather than counting corners as separate walls. Run the calculator once per wall, note each wall’s buy count, adjust for shared corners by hand, and add the walls together, with the waste allowance set for the most complex wall in the group if the walls share an order.

Waste: why the buy count comes from the exact count, not the rounded one

The waste rule on this page is stated in one sentence in the result panel, and it is worth understanding exactly, because it changes the order by a block on the most common wall size. The buy quantity is worked out from the unrounded exact count, not from the rounded block count. For the garden wall, 112.50 times 1.10 is 123.75, rounded up to 124. Rounding first and then adding waste would give 113 times 1.10, which is 124.30, rounded up to 125, one block too many. That one block is the difference between a method and a habit: the method applies waste to the true requirement and rounds once at the end, while the habit rounds twice, once to lay and once to buy, and each rounding adds up to a block. On a single wall the difference is one block; across several walls ordered together, the habit’s extra blocks add up, and they are blocks paid for but never laid.

The waste table on the page shows the same rule across four allowances for the garden wall: 0 percent gives 113 blocks to buy and 0 extra, 5 percent gives 119 and 6 extra, 10 percent gives 124 and 11 extra, and 15 percent gives 130 and 17 extra. Read that table as a trade-off, not as a menu: 0 percent is exact laying with no margin for a single broken or miscut block, which is a gamble on a straight wall and a certainty of a shortfall on a complex one. Five percent is a small margin for a short, straight, single-height wall with few cuts. Ten percent is the usual default for cuts and breakage, and it is the right starting point for most garden walls. Fifteen percent is for walls with many corners, steps, or angled cuts, where more blocks end up cut or broken, as the base guide says. Extra blocks are measured against blocks to lay, so the 0 percent row always shows 0 extra, and the extra count, not the percentage, is what arrives on the pallet beyond the laying count.

What waste covers, and what it does not, is the second half of the waste decision, and the base guide lists both sides plainly. Waste covers cut blocks, broken blocks, and small measuring errors. It does not cover piers, pilasters, bond beams, caps, lintels, mortar, or rebar, which are separate items. A reader who raises the waste allowance to cover a pier is misusing the allowance: the pier needs its own count, computed from its own dimensions, while waste covers only the breakage and cutting on the wall itself. Similarly, mortar is not a block, rebar is not a block, and caps and lintels are not blocks, so none of them belong in the block count at any waste percentage. Order those items separately, from their own measurements, and let the block count count blocks, with waste for breakage and cutting only. A block order that secretly includes non-block items is an order that cannot be audited, and an audit that cannot be performed is how a small wall ends up with a large surplus.

Block size is the assumption underneath every count on this page, and it deserves a paragraph because it is the assumption most likely to differ on a real order. A standard block is sold as 16 by 8 inches, but the block itself is slightly smaller, about 15 and 5/8 by 7 and 5/8 inches, and the usual 3/8 inch mortar joint fills the gap, so one block plus its joint covers the full 16 by 8 inch face. This calculator counts on the nominal 16 by 8 inch face, which is why the face area is 0.8889 square feet. If your blocks are a different size, the page’s own advice applies: divide your net area by your own block face area instead. That division is the whole method in one line, and it generalises the calculator to any block: net area divided by face area, rounded up to lay, then waste applied to the exact count and rounded up to buy. A supplier who stocks a non-standard block will confirm its face area, and that figure, not the nominal 16 by 8, is the one to use for that order.

Ordering, and the walls this calculator does not design

Ordering is the last step, and it has two checks that cost nothing and prevent the two most common order errors. First, confirm the final order with your supplier, as the page’s own estimate note says, because suppliers stock blocks in their own quantities and may round an order to pallets or packs, which changes the delivered count from the calculated buy count. A calculated buy count of 124 blocks delivered as full packs may arrive as more or fewer than 124, and the difference should be understood before delivery, not discovered on site. Second, confirm the block size with the supplier, because the count assumes the nominal 16 by 8 inch face, and a different stock size changes every count on the page. Those two confirmations, size and pack quantity, are the whole difference between a calculated count and a delivered order, and both are a phone call, not a calculation.

The boundary of this calculator is structural, and it is stated in the base FAQ and in the estimate note, so it bears repeating here without softening. This page counts blocks only, as a material estimate. Structural walls and retaining walls must follow local building codes, and tall or load-bearing walls need an engineer. A block count for a retaining wall is not a retaining wall design: it does not check overturning, drainage, footing size, reinforcement, or any other structural requirement, and a wall built from a count alone, without a design, is unsafe however accurate the count. Use this page to order blocks for a garden wall, a shed wall, or a non-load-bearing partition within the limits your local rules allow, and use an engineer and the local code for anything taller, load-bearing, or retaining. The count is the easy part of a structural wall; the design is the part that keeps it standing, and no calculator on this page performs it.

A final checklist, used in order, prevents the errors this guide has described. Measure the wall length and height in feet, at more than one point if the wall is uneven, and use the measurements the wall actually has. Add up every opening in square feet and enter the total, not a count. Confirm the block face area matches your supplier’s stock, nominal 16 by 8 inches or otherwise. Choose the waste allowance for the wall’s complexity, 10 percent for a straight run, 15 percent for corners, steps, or angled cuts. Read the breakdown: gross area, minus openings, net area, exact blocks, blocks to lay, waste allowance, blocks to buy, and confirm each line against your own measurements. Confirm the order quantity and the block size with the supplier. That is the whole job before the first block is laid, and it takes less time than one return trip to the yard, which is what guessing block counts leads to, as the page’s own introduction says.

Worked check: from wall size to buy count, by hand

The whole method fits in four lines, and checking it by hand is the fastest way to trust the page. Take the garden wall with its 21 square foot opening. Gross area is 20 times 5, which is 100.00 square feet. Net area is 100.00 minus 21.00, which is 79.00 square feet. Exact blocks are 79.00 divided by 0.8889, which the page displays as 88.88. Blocks to lay are 88.88 rounded up, which is 89. Blocks to buy at 10 percent waste are 88.88 times 1.10, which is about 97.77, rounded up to 98, as displayed, with 9 extra blocks against the 89 to lay. Each step uses only the previous step’s figure, and the only rounding happens twice: once up to lay, and once up to buy, with the buy rounding applied to the exact count, not to the lay count. If your hand check disagrees, the disagreement will be at one of those steps, and the step names the error: a gross area error is a measurement error, a net area error is an openings error, an exact-count error is a face-area error, and a buy-count error is a waste-rule error.

The face area itself is worth verifying once, because every count on the page depends on it. Sixteen inches by 8 inches is 128 square inches, and 128 divided by 144 square inches per square foot is 0.8889 square feet, as displayed. That is the nominal face, block plus joint, not the block alone, which is slightly smaller, and the joint makes up the difference, as the base guide explains. A reader who divides by the block-alone area instead, about 15 and 5/8 by 7 and 5/8 inches, will get a smaller face area and a larger block count, overbuying by the joint area on every block. Use the nominal face for the count, confirm it with your supplier if the stock differs, and let the mortar joint do what it is designed to do: fill the gap between the block-alone size and the nominal size, so that the laid wall measures in whole nominal faces.

Corners, steps, and walls that are not rectangles

Real walls are rarely single rectangles, and the calculator’s single-rectangle method needs a small amount of hand work to fit them. A wall with a step, where the height changes partway along, is two rectangles, not one: measure each part’s length and height, compute each part’s area, and add the areas before entering, or run the calculator once per part and add the buy counts, which is safer, because waste and rounding apply per part when the parts are ordered together only if the parts share the same waste allowance. A wall with a corner that turns is two walls, not one, and the corner blocks must be counted once, not once per wall, so run each wall separately, decide which wall’s count owns the corner, and subtract the corner’s blocks from the other wall’s count by hand. A wall with an angled top, such as a gable end, is a rectangle plus a triangle, and the triangle’s area, half its base times its height, must be added to the rectangle’s area before the count, with the angled cuts counted in the waste allowance, which should rise toward 15 percent for such a wall, since angled cuts waste more blocks than straight cuts.

Openings in non-rectangular walls follow the same addition rule as openings in rectangular walls: add up every opening’s area and subtract the total from the gross area, whatever the wall’s shape. An arched opening is the one case where the area needs care, because its area is a rectangle plus a semicircle, not a rectangle alone, and using the rectangle alone overstates the opening and understates the block count. Measure the rectangular part and the semicircular part separately, add them, and enter the total with the other openings. A circular opening, such as a vent, is pi times the radius squared, and a small vent’s area is small enough that omitting it changes the count by less than a block, but the habit of omitting small openings is how counts drift: omit every small opening and the drift adds up, so include them all, however small, and let the rounding handle the fractions.

Partial walls, such as a wall built to half height as a garden divider, use the same method with the built height, not the full height to any imaginary line above it. Enter the height of the blocks actually laid, and the calculator returns the count for that height, just as the board-and-batten page’s wainscot rule enters the section height actually clad. A half-height wall that steps down in stages is a set of rectangles again, one per stage, and the same per-part addition applies. The method never changes: gross area from length and height, minus openings, divided by face area, rounded up to lay, waste on the exact count, rounded up to buy. Only the measurement changes, from one rectangle to several, and the calculator’s job is the arithmetic after the measurement, not the measurement itself.

When a wall’s shape defeats even the multi-rectangle method, because it curves or its height varies continuously, the honest approach is to bound the count rather than to invent it. Compute the count for the smallest rectangle that contains the wall, and for the largest rectangle contained within it, and the true count lies between the two, closer to the contained rectangle if the wall is mostly full, closer to the containing rectangle if it is mostly open. Order toward the upper bound with the usual waste allowance, and confirm with the supplier, because a curved wall’s blocks are cut to fit the curve, and cutting waste on a curve exceeds the straight-wall allowance. A bounded estimate stated as a range is more honest than a single figure pretended to be exact, and the supplier, who sees the wall’s shape, can narrow the range from experience that no calculator on this page has.

Piers, corners, and where the wall method stops

The wall method counts a flat rectangle. Real walls have corners, ends and piers, and each of those changes the count in a way the rectangle cannot see. A corner does not need an extra block per course if the blocks interlock in alternate courses, but it does need half blocks or cut blocks to keep the bond, and a pier, a thickened column of blocks at intervals along a long wall or beside an opening, is an addition on top of the wall count, not part of it. Count piers separately: the blocks in each pier, times the number of piers, added to the wall total before waste is applied.

The same separate-count rule applies to the courses below ground. A wall that starts below grade needs its buried courses counted at the same per-square-foot rate, and those courses are the ones most often forgotten, because they are invisible in the finished wall. Measure the full height from the footing to the top of the wall, not from the finished ground level, and check the result against the supplier, who will ask about piers, corners and below-grade courses before quoting. The companion guide on block counts works the corner and pier adjustments into its examples.

Mortar, and the order beyond the blocks

The block count is only the first line of the order. Mortar is the second: a rough planning figure is about 3 bags of pre-mixed mortar per 100 standard blocks, which covers the bed and head joints of a running-bond wall at the usual joint thickness. Thicker joints, a second wythe, or a wall laid by a less practised hand all use more, so treat 3 bags as the planning figure and confirm with the supplier against the actual joint and wall thickness.

Filled cores add a third line. Every core that is grouted needs concrete grout and, in a structural or retaining wall, the rebar that the grout surrounds, and the weight guide sets out how much a filled block weighs, because the delivery and handling plan changes with it. Order the blocks, the mortar and the grout as one delivery where the supplier allows it: split deliveries cost more per block, and a wall that runs out of mortar halfway up a lift cannot wait for a second truck without a cold joint in the middle of it.

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