Concrete Block Calculator

How to use this block calculator

1. Enter the wall as length and height, not as square footage. Both get you a block count. Only one of them can tell you whether the wall lands on the block grid, which is the answer you actually need before you order anything.
2. Pick the block width. The face is the same across the range, so the width does not change the block count. It changes the core fill, which is where the money is.
3. Take out the openings. A standard entry door is 36 by 80 inches, which is 20 square feet of wall you are not laying. Enter them in inches, the way they are sold.
4. Choose the mortar you are actually buying. A bag of pre-mixed mortar and a bag of masonry cement are not the same purchase, and the calculator gives you a different shopping list for each.
5. Add core fill and joint reinforcement if the wall calls for them. Both are ordered separately and both are routinely forgotten until the day they are needed.
6. Take it with you. Copy the result as text, share the link (it carries every input), or download the material list as a PDF for the counter at the yard.

Why this concrete block calculator is different

I put the same wall into the five calculators ranking for this search. Every one of them answered the question. The gaps they share are all in the same place.

• None of them asks whether the wall is buildable. Masonry is a module: a standard block occupies a nominal 16 inches of length and 8 inches of height once you count its joints. A wall 12 ft 4 in long is 9.25 blocks per course, and one 4 ft 3 in high is 6.375 courses. Both of those mean cutting block on every single course, and both are trivially avoidable if you find out before you dig the footing instead of after. That check is the first thing a mason does and the last thing any of these pages does.
• None of them subtracts a door. Five calculators, and every one wants a plain rectangle. Put a 36 by 80 in door in a 20 by 8 ft wall and the block count drops from 180 to 158, which is most of a pallet you were about to pay for and store.
• "Bags of mortar" is three different answers. Two of the five give you a single undifferentiated bag count. A bag of pre-blended mortar is a finished product you add water to; a bag of masonry cement is roughly a third of a mortar and needs about three cubic feet of sand carried home with it. This calculator prices both routes and tells you the sand figure, because on a wall this size the sand is a trailer load, not an afterthought.
• Not one of them cites a source. No manufacturer data sheet, no association technical note, nothing. That matters because their numbers disagree: block mortar on the pages I checked ran from 7 bags per 100 blocks to 12 blocks per bag, roughly a 30 per cent spread with nothing to adjudicate it. Every yield here links to the document it came from, and where the published source contradicts itself we say so rather than quietly picking a side.
• Half blocks are invisible everywhere. A running-bond wall with square ends closes every second course with a half unit. On the reference wall below that is 12 of them, and your supplier stocks them as a separate line item.
• Three of the five make you press Calculate. This one recomputes as you type, keeps the running total on screen on a phone, and puts every input in the URL so the link you send your builder opens on your wall, not a blank form.
• Nothing here is gated. No email, no account, no ads. The embed is a copy-paste iframe with no tracking attached, which none of the five offer at all.

How it works (the math behind the numbers)

The block count is one division. Everything interesting is in what you divide, and in the two questions that division cannot answer.

net_area   = (L x H) - every opening
face_area  = nominal_length_in x nominal_height_in / 144
blocks     = ceil(net_area / face_area x (1 + waste))

courses    = wall_height_in / 8         <- must be a whole number
per_course = wall_length_in / 16        <- must be a whole number

mortar     = ceil(blocks / 13)          per 80 lb bag of pre-mix
grout      = net_area x rate_per_sqft   rate from the wall thickness
wire_lf    = net_area x 12 / spacing_in

A standard block is nominal 8 by 16 inches on the face. ASTM C90 sets the actual unit 3/8 in under that in each direction, so a 15 5/8 by 7 5/8 in block plus its 3/8 in joints occupies exactly 8 by 16 in of wall. That is 112.5 blocks per 100 square feet, or 113 once you round up to whole units, and it is the same 1.125 blocks per square foot whether the block is 4 in wide or 12.

Work the reference wall. Twenty feet long, eight feet high, is 160 square feet and 180 blocks laid as 12 courses of 15. Add the standard 5 per cent for breakage and cuts and you order 189. That is 15 bags of 80 lb pre-mixed mortar at the manufacturer's stated yield of 13 blocks per bag, or 17 bags of masonry cement plus 51 cubic feet of sand (1.89 cubic yards) if you are mixing your own. Both routes are about the same number of bags, and only one of them needs a sand pile.

Core fill is the figure that moves most. The published grout table is stated in cubic yards per 100 blocks and also in blocks filled per cubic yard, and those two columns are two views of one number. Where they disagree this calculator follows the blocks-per-cubic-yard column, which reconciles across the whole table; the printed cubic-yard column does not, in two cells of the 10 in row. That decision is documented in the data file rather than buried, because a tool whose stated cross-check contradicts its own answer by six per cent is worse than one that shows its working.

Core fill: cubic yards of grout per 100 blocks

By wall thickness and how many cores you are filling. These are the figures the calculator uses, rendered from the same data file the build gate checks.

An 8 in wall with every cell filled takes 1.00 cubic yard per 100 blocks, which is the easiest number in masonry to remember: one cubic yard fills 100 blocks. A 12 in wall filled every 24 in takes 0.68. Four inch block has no row because its cores are too small to fill, and the calculator will not offer you one.

Three worked examples

A 20 by 8 ft garden wall in 8 in block

160 square feet, 180 blocks, 12 courses of 15. Both numbers are whole, which is what you want to see: this wall was drawn on the module and every course closes with a full unit. Order 189 with waste and 15 bags of mortar. Of those blocks, 12 are halves, two per every second course at the two square ends. Mixing that much mortar by hand is the part people underestimate, and a paddle in a cordless drill turns a bag in about a minute.

The same wall with a door in it

Put a 36 by 80 in door through it and you lose 20 square feet, so the wall is 140 square feet and 158 blocks, or 166 with waste. That is 22 blocks you were about to buy, stack, and then move twice. No calculator on page one of this search will take a door out for you, which is a strange place for the whole field to have stopped.

A 12 ft 4 in by 4 ft 3 in wall, which is the real lesson

It is 52.4 square feet and 59 blocks, and both of those figures are useless. The wall is 9.25 blocks long and 6.375 courses high, so every course needs a cut unit at one end and the top course needs ripping down its length, which is the one cut a block saw is worst at. Pull the length back to 12 ft or push it out to 13 ft 4 in, and the height to 4 ft or 4 ft 8 in, and the whole job is whole blocks. Three inches of drawing changes a weekend. Cutting a wall's worth of block also means silica dust everywhere, which is a shop vac job rather than a broom job.

What changes the count, and what does not

• Block width does not change the block count. A 12 in block has the same 8 by 16 in face as a 6 in one, so a wall of either takes 1.125 blocks per square foot. What the width changes is the core fill, which roughly doubles between 6 in and 12 in block, and the weight of every unit you lift.
• Joint thickness does. The 1.125 figure assumes 3/8 in joints, which is what the nominal dimensions are built around. Lay half inch joints and you get slightly fewer blocks and noticeably more mortar; the block count moves by about one per cent and the mortar by nearer fifteen.
• Waste is 5 per cent, not zero and not fifteen. Five is the figure block suppliers print next to their own calculators. Go higher on a wall with a lot of cuts, corners or openings, because that is where breakage actually happens, and lower is only sensible if you are happy to make a second trip.
• Face-shell bedding is what the mortar yields assume. The published figure of 13 blocks per 80 lb bag is for mortar on the face shells only, which is standard practice. Butter the cross webs as well and you will use meaningfully more than the bag says.
• Corners eat blocks differently from ends. A square end needs half units on alternate courses; a corner does not, because the two walls run past each other and the bond takes care of itself. Set square ends to zero if the wall dies into existing structure at both ends.
• Grout is not mortar and the bags are not interchangeable. Core fill grout is a fluid, high-slump mix designed to flow into cells; mortar is stiff by design. Over roughly a cubic yard, ordering ready-mix grout by the yard beats mixing eighty-odd bags of it on site by a wide margin.
• Reinforcement spacing is an engineering decision. This calculator will tell you how much wire or grout a given spacing takes. It will not tell you what spacing your wall needs. Any structural, basement or retaining wall gets that number from the code official or the engineer, not from a web page.
• Dry-lay the first course before you mix anything. This is the physical version of the module check. Lay the bottom course out on the footing with joints spaced and you will see a wall that is a quarter block too long while it is still free to fix. A decent work light is what makes a course that has crept out of line visible at course four rather than course twelve.

Frequently asked questions

How do I calculate how many concrete blocks I need?

Multiply the wall length by its height for the gross area, subtract every door and window, then divide by the face area of one block. A standard 8 by 16 in block covers 0.889 square feet of wall including its joints, so multiply the net square footage by 1.125. Add 5 per cent for breakage and cuts and round up. For 100 square feet of wall that is 112.5 blocks, so 113 units before waste.

How many concrete blocks are in 100 square feet?

113 standard blocks, from 112.5 exactly. That figure holds for any wall thickness, because a 6 in, 8 in and 12 in block all present the same 8 by 16 in face to the wall. With a 5 per cent waste allowance, order 119.

How many bags of mortar do I need for 100 blocks?

8 bags of 80 lb pre-mix of pre-mixed mortar, working from the manufacturer's stated yield of 13 standard blocks per bag, which QUIKRETE publishes identically for its Type S and Type N products. If you are mixing masonry cement and sand instead, the trade figure is 8 to 9 bags of masonry cement per 100 blocks, with about three cubic feet of sand per bag. The bag counts look similar and the two shopping lists are not.

How many blocks high can I go per course, and how many courses in 8 feet?

A course is one block high, and the nominal height is 8 in including the bed joint, so an 8 ft wall is exactly 12 courses and a 4 ft wall is 6. Any wall height that is not a multiple of 8 in leaves a partial top course. Lengthwise the module is 16 in, so wall lengths in multiples of 16 in (1 ft 4 in, 2 ft 8 in, 4 ft, and so on) close with whole units at both ends.

How much does it cost to fill concrete block with concrete?

By volume rather than by block. An 8 in wall with every cell filled takes 1.00 cubic yard per 100 blocks, so one cubic yard of grout fills about 100 blocks. Filling every cell of the 180-block reference wall takes 1.8 cubic yards. In 80 lb bags at 0.60 cubic feet each that is 81 bags, which is the point where ready-mix delivered by the yard becomes both cheaper and very much easier.

What is the difference between a cinder block and a concrete block?

Cinder block is the older product, made with coal cinders or fly ash as the aggregate, and it is lighter and weaker. What is sold today at any yard in the United States is concrete block, a concrete masonry unit made to ASTM C90. The two names are used interchangeably in conversation and the dimensions are the same, so this calculator covers both. If you are working with genuinely old cinder block in an existing wall, treat its load capacity as an unknown.

Do I need to fill concrete block cores with grout?

Most freestanding garden walls and non-structural partitions do not. Basement walls, retaining walls, anything carrying a floor or roof load, and anything in a high-wind or seismic area generally do, at a spacing the code or an engineer sets. Filling cells roughly doubles a wall's weight and its cost, so it is not a free upgrade you throw in for safety. Get the requirement from the drawings and then come back here for the volume.

Related tools

• Brick Calculator — the same job for modular brick, where the unit count per square foot is six times higher and the mortar matters more.
• Retaining Wall Calculator — segmental retaining wall block, which is a different product with a different setback and base course.
• Yards of Concrete Calculator — for the footing under all of this, which has to be poured and cured before a single block goes down.
• Sonotube Calculator — round pier footings, if the wall is sitting on piers rather than a strip footing.
• All free tools by Jake — the full set of calculators and reference charts.

Methodology and sources

The block factor is derived from the nominal module that ASTM C90 defines and cross-checks against the 1.125 blocks per square foot figure the masonry associations publish. Mortar yields are manufacturer figures from the product data sheets, not rules of thumb. The core fill table is the published grout calculation table for concrete block construction, used with the blocks-per-cubic-yard column as canonical, and the two cells where the source contradicts itself are named and explained in the data file rather than silently smoothed over. Every figure quoted in this article is pinned to an assertion that runs on every build, so the numbers above cannot drift away from the calculator below them.

• ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units (nominal vs actual dimensions)
• Concrete Masonry & Hardscapes Association (CMHA/NCMA) — concrete masonry technical resources
• QUIKRETE Mason Mix No. 1136 data sheet — 80 lb bag lays up to 13 standard 8x8x16 blocks
• QUIKRETE Mortar Mix No. 1102 data sheet — same stated block yield, Type N
• Grout Calculations for Concrete Block Construction (cu yd per 100 block, blocks per cu yd)
• QUIKRETE Concrete Mix No. 1101 data sheet — 80 lb bag yields 0.60 cu ft
• Cemex Block Calculator — the 5% waste convention and the joint-reinforcement output
• Inch Calculator, Block Mortar Calculator — 1.125 blocks per sq ft cross-check

This calculator is reviewed annually for source currency. About Jake · Last reviewed September 25, 2026.

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