Concrete Block Calculator — Blocks, Mortar & Grout

Using these numbers

Block count already includes your waste allowance — order that many; don't add extra on top unless you have a specific reason to expect more breakage than usual for this job.
Mortar bags round up to the nearest whole bag — and, as explained in the methodology above, are calculated from your order count rather than the raw block count — that builds in a small allowance for mortar's own waste.
Core-fill grout is shown as a volume (cubic yards), not a bag count — and only appears for 8″ block. If you need a bag count for a specific grout product, divide the volume shown by that product's stated yield per bag.
None of these numbers include labor, delivery, sales tax, or a price — this tool estimates materials, not cost.
Single-wall and multi-wall project modes — single-wall mode estimates one wall; multi-wall project mode estimates a full rectangular enclosure (all four walls) at once, with the block overlap at the four corners already subtracted rather than double-counted — shown as its own figure above the block count.

Methodology

Every formula and constant this calculator uses is below, with its source, so you can check it yourself. Where a figure is derived rather than looked up directly, the arithmetic is shown, not just the answer.

Block count

A wall is built from a grid of block-plus-mortar-joint units — the module. The module's size, not the block's own size, is what determines how many blocks fill a given wall area, because the mortar joint takes up space too.

A standard CMU's actual (not nominal) face is 15.625 in long by 7.625 in high, the same for every nominal thickness — 4″, 6″, 8″, and 12″ block all share this face size; only the thickness behind it differs. The standard mortar joint is 3/8 in (0.375 in). Both figures come from Angelus Block's published CMU dimension reference, which cites ASTM C90, the governing US standard for concrete masonry units.

At the standard 3/8″ joint, block plus joint works out to exactly 16 in × 8 in — which is why block is sold and laid by "16-by-8" coursing even though the block itself measures 15⅝ × 7⅝. That isn't a coincidence: ASTM C90 defines a CMU's nominal size as its actual size plus one standard joint. This calculator computes the module from your actual entered joint thickness rather than assuming the 3/8″ default, so a non-standard joint correctly changes the block count too.

module length = actual block length + joint thickness
module height = actual block height + joint thickness
module area   = module length × module height
           = 16 in × 8 in = 128 in² = 0.8889 ft²  (at the standard 3/8″ joint)

net wall area = (wall length × wall height) − opening area
raw block count = net wall area ÷ module area
block count = ceiling( raw block count × (1 + waste %) )

Mortar

Mortar bags are calculated from the block count you'd actually order — the count after waste is added — not from the raw, pre-waste count.

Thirteen is how many standard 8×8×16 blocks one 80 lb bag of QUIKRETE Mortar Mix (Product No. 1102, meets ASTM C270 Type N) lays, per the manufacturer's own data sheet. The same data sheet states the same 13-block figure for its Type S product, Mason Mix. As a cross-check, a separate 3000 lb bulk product from the same manufacturer lays approximately 530 blocks — 14.1 blocks per 80 lb-equivalent — close enough to the retail figure to support it rather than contradict it.

Strictly, waste — breakage, offcuts, blocks damaged in delivery — never gets mortared into a wall, so a purist's formula would compute mortar from the raw, pre-waste block count. This calculator uses the order count instead, deliberately. Mortar has its own waste that this model doesn't otherwise capture — mix dropped from the trowel, a partial bag left at the end of a course, joints scraped and repointed — and using the order count folds in a small, reasonable allowance for that without adding a second waste input to configure. The result runs slightly conservative, and that's the intended direction: running short of mortar mid-wall is worse than having part of a bag left over.

mortar bags = ceiling( block count ÷ 13 )

Core-fill grout (8″ block only)

Core-fill grout is offered for 8″ block only, and only when you choose to fill cores — every core, or cores at your chosen reinforcement spacing.

0.233 ft³ per block is derived from QUIKRETE's Bulk Masonry Mortars & Corefill Masonry Grouts data sheet, which states that a 3000 lb bag of fine grout yields 0.95 yd³ (25.65 ft³) and fills the cores of approximately 110 standard 8″×8″×16″ blocks:

The same data sheet gives a coarse-grout product as 0.90 yd³ (24.30 ft³) covering approximately 105 blocks:

Two different grout products from the same manufacturer land within 0.8% of each other — strong agreement between independent figures — so this calculator uses 0.233 ft³ per block. The division is shown here, not just the answer, because a competing calculator was found publishing 0.014 ft³ per cell — about 0.028 ft³ per block, roughly 8 times too low against this derivation — while a different figure on the same page implied about 0.5 ft³ per block, meaning its own two numbers disagreed with each other by about 18 times. A reader who can check this arithmetic against the data sheet can catch a mistake like that; a reader who's only handed a final number can't.

Core fill isn't offered for 4″, 6″, or 12″ block in this version. The QUIKRETE data sheet this figure is derived from covers only the standard 8″ unit, and core geometry doesn't scale in proportion to block width — a thicker block's face-shell and web thickness aren't simply larger by the same ratio, so estimating the other sizes by scaling the 8″ figure would produce a number that looks reasonable and is wrong. This calculator states "not available" rather than guess.

Fine grout:   25.65 ft³ ÷ 110 blocks = 0.2332 ft³ per block
Coarse grout: 24.30 ft³ ÷ 105 blocks = 0.2314 ft³ per block
(0.8% apart — used value: 0.233 ft³ per block)

grouted blocks = block count × core-fill fraction
grout volume   = grouted blocks × 0.233 ft³

Why waste is an input, not a constant

Waste is a planning allowance, not a measured constant. It depends on your crew, how many corners and openings the wall has, and how much cutting the job requires — none of which this calculator can know from a wall's length and height alone. The default of 5% is a starting point for a straightforward wall with few cuts; more complex layouts warrant a higher figure, which is why it's an adjustable input rather than a fixed number.

This matters because the manufacturer coverage figures above — 13 blocks per bag of mortar, 0.233 ft³ of grout per block — are QUIKRETE's stated yields under normal conditions. The manufacturer's own documentation notes that its yields are approximate and don't allow for uneven subgrade, waste, or similar site conditions. The waste input is what accounts for that gap, and it's applied to the block count before mortar and grout are calculated from it — not added on afterward as an afterthought.

Mortar type assumption

The mortar figures above assume Type N mortar mix (750 psi minimum compressive strength), suitable for above-grade interior load-bearing walls and parapets. Per ASTM C270 Table X1.1, Type N is not the right choice for exterior load-bearing walls or below-grade work — see the FAQ for the full Type N vs. Type S breakdown. Bag coverage doesn't change with mortar type: QUIKRETE's data sheet lists the same 13-blocks-per-80-lb-bag figure for its Type S product, so the mortar bag count above holds regardless of which type your project needs.

Constants register

Constant	Value	Source	As of
CMU actual face size (all nominal widths)	15.625 in × 7.625 in	Angelus Block, "CMU Basics — Dimensions and Sizes" (citing ASTM C90)	2026-08-12
Standard mortar joint thickness	0.375 in (3/8 in)	Angelus Block, "CMU Basics — Dimensions and Sizes"; ASTM C90 nominal-size definition	2026-08-12
Mortar coverage, standard 8×8×16 block, Type N	13 blocks per 80 lb bag	QUIKRETE Mortar Mix Product No. 1102 data sheet (ASTM C270 Type N)	2026-08-12
Mortar mix yield	0.83 ft³ per 80 lb bag	QUIKRETE Mortar Mix Product No. 1102 data sheet	2026-08-12
Core-fill grout volume, 8″ CMU (both cores)	0.233 ft³ per block	Derived from QUIKRETE Bulk Masonry Mortars & Corefill Masonry Grouts data sheet — see derivation above	2026-08-12
Mortar Type N minimum compressive strength	750 psi	QUIKRETE bulk masonry data sheet, per ASTM C270	2026-08-12
Mortar Type S minimum compressive strength	1,800 psi	QUIKRETE bulk masonry data sheet, per ASTM C270	2026-08-12
Mortar Type M minimum compressive strength	2,500 psi	QUIKRETE bulk masonry data sheet, per ASTM C270	2026-08-12
Wall weight, average completed wall (72 combinations: 3 density classes × 4 thicknesses × 6 grouting categories)	see calculator — psf, keyed by density class, thickness, and grout spacing	Angelus Block, "Wall Weights and Areas," excerpted from 2015 Design of Reinforced Masonry Structures (CMACN)	2026-08-15
CMU actual width by nominal thickness — 4″/6″/8″/12″	3.625 in / 5.625 in / 7.625 in / 11.625 in	Angelus Block, "CMU Basics — Dimensions and Sizes" (citing ASTM C90)	2026-08-12
CMU actual width, 10″ nominal — DERIVED, not directly sourced	9.625 in	No manufacturer citation found for 10″; derived from the same ASTM C90 nominal = actual + 3/8 in relationship the other four widths independently confirm (10 − 0.375 = 9.625)	2026-08-12

Concrete block vs. cinder block — is there a difference?

Yes, though in casual use the two terms are often used interchangeably — and for this calculator's math, that's fine. The formula is identical either way, which is why one page and one engine correctly serve both search terms.

Concrete block (CMU) is the correct modern industry term. Today's concrete block is made with a sand-and-gravel aggregate mix, which gives it the strength to serve as a genuinely structural material — load-bearing walls, foundations, retaining walls. When a mason, a building inspector, or a supplier's spec sheet says "block," this is what they mean. CMU stands for concrete masonry unit.

Cinder block, strictly speaking, is a specific and largely historical product. Cinder-aggregate block was made with coal cinders — the ash left over from burning coal — in place of sand and gravel. It's lighter and noticeably weaker than modern concrete block, and it's rarely manufactured today; most of the coal-fired plants that produced the cinders it needs have themselves closed. The term outlived the product: "cinder block" stuck around in everyday speech as a generic name for any gray concrete masonry unit, much the way "Kleenex" gets used for facial tissue generally.

What you'll actually find at a supplier today, whichever name you use, is standard concrete block (CMU) sand-and-gravel aggregate, sold in the nominal 4″, 6″, 8″, and 12″ widths this calculator supports. Ask a supplier for "cinder block" and you'll be sold concrete block; genuine cinder-aggregate units aren't a normal stock item at most building-supply yards anymore.

Does the distinction matter for your project? For estimating quantity — how many blocks, how much mortar, how much grout — no. The calculation depends only on the block's face size and your wall's dimensions, which is why this one calculator correctly handles both search terms. It can matter for structural decisions: if you're building anything load-bearing — a foundation wall, a retaining wall, anything carrying real load — you're working with modern concrete block regardless of which name you searched, and it should be specified and built as such. For lighter-duty projects — a raised garden bed, a fire-pit surround, informal yard structures — the historical "cinder block" framing (lighter-duty, non-structural) is closer to how people actually use the term today, even though the block itself is the same modern CMU product.

Block Wall Cost Calculator — takes the block count above and prices it against a dated, sourced band, with mortar and grout shown as quantities it will not put a price on.

FAQ

How do I calculate how many concrete blocks I need?: Divide your wall's area (length × height, minus any openings) by the area one block-and-mortar-joint module covers, then add a waste allowance and round up to a whole block. For a standard 8″ block at the standard 3/8″ mortar joint, one module covers 0.889 sq ft, so the raw count is wall area ÷ 0.889. This calculator does that math live as you enter your wall's dimensions — see the methodology section above for the full formula and its sources.
Is there a difference between a cinder block and a concrete block?: Yes — see "Concrete block vs. cinder block" below for the full explanation. In short: concrete block (CMU) is the modern, structural product made with sand-and-gravel aggregate; cinder block is an older term for a lighter, weaker unit originally made with coal-cinder aggregate, now largely historical, though the name persists in everyday use for concrete block. The quantity math is identical either way, which is why this calculator covers both terms.
How many cement blocks are required for 1000 square feet?: About 1,125 blocks before waste — 1,000 sq ft ÷ 0.889 sq ft per block. Add your waste allowance on top; this calculator does that automatically for whatever wall size you enter.
How many blocks are in 1 sq ft?: 1.125 blocks — one square foot divided by the 0.889 sq ft standard block-and-joint module, before any waste allowance.
How much do 1000 concrete blocks cost?: This calculator estimates materials — block count, mortar, and grout — not price. The block wall cost calculator on this site does price them: 1000 standard 8″ blocks come to roughly $1,250–$2,500 against a dated, sourced national band, and it shows that band's source and date rather than asking you to take the number on trust. Block prices move with region, supplier and quantity, so treat any national figure as a planning range and confirm with a local yard.
How many blocks do I need for a 12x20 shed?: Switch to multi-wall project mode above and enter 20 ft for length, 12 ft for depth, and your shed's wall height. It estimates all four walls together as one rectangular enclosure, subtracting the block overlap at the four corners — more accurate than estimating each wall separately and adding the four counts by hand, which over-counts the corners.
How much block fill per 100 blocks?: At the 8″ figure this calculator uses — 0.233 ft³ of grout per fully filled block — 100 fully grouted blocks need about 23.3 ft³, or roughly 0.86 cubic yards, before any waste. This figure is specific to 8″ block; core fill isn't available for other thicknesses yet.
Is it cheaper to pour concrete or use cinder blocks?: This calculator doesn't compare cost between building methods — that depends on labor, formwork, local material pricing, and the specific project, all outside what a materials calculator can estimate. As a general pattern, block construction typically needs less specialized formwork than poured concrete but more labor to lay individual units. A local contractor's quote is the reliable way to compare the two for your project.
How much will 2 yards of concrete cover at 4 inches thick?: That's a poured-concrete slab question rather than a block-wall one, so it's outside what this calculator covers — but the arithmetic is simple enough to answer directly: 2 cubic yards is 54 cubic feet, and at a 4 in (1/3 ft) thickness that covers 54 ÷ (1/3) = 162 sq ft.
How many bags of cement will lay 1000 blocks?: About 77 bags (1,000 ÷ 13, rounded up to a whole bag), using the 13-blocks-per-80-lb-bag figure from QUIKRETE's Mortar Mix data sheet — see the methodology section. For 100 blocks, that's about 8 bags (100 ÷ 13 = 7.7, rounded up). This calculator does the same rounding for whatever block count your wall works out to.
How many sq ft does a cinder block cover?: One standard block-and-joint module covers 0.889 sq ft of wall — the same figure whether the units are called cinder block or concrete block, since the calculation depends only on the block's face size, not its aggregate.
How many bags of concrete does it take to fill a cinder block?: One 8″ block's cores hold about 0.233 ft³ of grout. An 80 lb bag of fine grout mix yields about 0.62 ft³, so one bag fills roughly 2 to 3 blocks (0.62 ÷ 0.233 ≈ 2.7). This calculator reports grout as a volume in cubic yards rather than a bag count, since bag yield varies more by product than the fill volume itself does.
Is it cheaper to mix your own concrete or buy bags?: This calculator's figures are based on a pre-blended bagged mortar mix (QUIKRETE Mortar Mix, Type N), the simpler and more consistent option for most DIY and small-contractor jobs — you're paying for convenience and consistent proportions rather than mixing cement, sand, and lime separately. Site-mixing raw materials can be cheaper per unit volume at larger scale but requires accurate proportioning and more labor. This calculator doesn't estimate cost either way.
Can I use Quikrete for cinder blocks?: Yes — QUIKRETE's mortar and grout products aren't specific to one type of block. The coverage figures this calculator uses (13 blocks per 80 lb bag of mortar mix, 0.233 ft³ of grout per 8″ block) come from QUIKRETE's own data sheets and apply the same way whether the units are labeled concrete block or cinder block, since both are laid and grouted the same way.
How long will a cinder block last?: There's no sourced lifespan figure for this, so none is given here. In general, modern concrete block (CMU) is a long-lived structural material when properly built and drained. Older cinder block — made with coal-cinder aggregate rather than sand and gravel — is more porous and generally less durable, particularly with freeze-thaw or moisture exposure, which is part of why it's largely been replaced in new construction. For a specific structure's expected service life, a local structural engineer or mason is a better source than a general estimate.
How many blocks are required for a 12x12 feet room?: Switch to multi-wall project mode above and enter 12 ft for both length and depth, plus your chosen wall height. It estimates all four walls of the room together as one rectangular enclosure, with the block overlap at each corner already subtracted from the total — the exact count still depends on wall height, which is why that's an input rather than assumed.
How much does an 8 foot by 12 foot cinder block wall cost?: This calculator's default view already shows this exact wall size — 12 ft long by 8 ft high is the worked example above — but as materials, not cost: the block, mortar, and (at 8″ thickness) grout quantities it needs. For the money, the block wall cost calculator on this site loads the same 12 × 8 wall and prices it at roughly $143–$285 for the blocks, with mortar and grout counted but deliberately left unpriced.
How many blocks will 1 yard of concrete fill?: Concrete blocks are precast units, not something poured and filled from ready-mix concrete, so this isn't quite the right way to frame block estimating. If the question is really about filling the hollow cores of existing block with grout, one cubic yard goes a long way: at this calculator's sourced figure of 0.233 ft³ of grout per 8″ block, a cubic yard (27 ft³) fills roughly 116 blocks (27 ÷ 0.233 ≈ 115.9). If the question is about a poured concrete slab or footing instead, that's a different calculation than this tool covers.
Should I use Type N or Type S mortar?: This calculator's mortar figures are based on Type N mortar mix (750 psi minimum compressive strength), suitable for above-grade interior load-bearing walls and parapets — per ASTM C270 Table X1.1, it isn't the right choice for exterior load-bearing walls or below-grade work. Type S (1,800 psi) is the standard choice for those; Type M (2,500 psi) is reserved for the most demanding structural or below-grade uses. If your wall needs Type S or M, the bag-count figures here carry over: QUIKRETE's data sheet lists the same 13-blocks-per-80-lb-bag coverage for its Type S product — check your specific product's data sheet to confirm.