The number nobody agrees on
Search for how much concrete fills a block and the first page of results
offers 0.014, 0.067 and 0.27 cubic feet. That is a spread of twenty to
one for the same question, and not one of those pages says whether it
means a core or a block, or which width of block it had in mind.
There is no authority to look this up in, so this page does not pretend
there is. It derives it.
What the standard actually fixes
ASTM C90 does not publish a fill volume. What it publishes, via
the NCMA table cited below, is two minimum
dimensions: the face shell thickness, which is 1 inch at 6 inch nominal
width and 1-1/4 inch at 8 inch and above, and a single web thickness of
3/4 inch regardless of width. The core is what those leave behind, and
that is geometry rather than opinion.
core width = (15.625 − 3 webs × 0.75) ÷ 2 = 6.688 in
core depth = actual width − 2 × face shell
core volume = width × depth × 7.625 ÷ 1728
An 8 in block: 7.625 − 2.5 = 5.125 in deep
6.688 × 5.125 × 7.625 ÷ 1728
= 0.151 ft³ a core, 0.303 a block
Width moves it more than you would think
The face shell minimum stops growing at 8 inch nominal. Every inch of
width above that goes straight into the core, so the fill per block
climbs far faster than the block does.
Fill by block width, from this calculator
Core depth is the actual width less two face shells. The last two columns are the calculator's own output for those jobs.
A 12 inch block takes two and a half times the fill of a 6 inch one.
Ordering for the wrong width is the largest single mistake available on
this page, which is why the width sits at the top of the form.
A worked wall
A 20 by 8 foot wall in 8 inch block is 15 blocks a
course and 12 courses — 180 blocks,
360 cores. Solid grouted that is
54.4 cubic feet, or
2.02 cubic yards. At a filled
core every 32 inches it is a quarter of that.
Which way the error runs
A minimum shell means the largest core a conforming block can
have. Real units are cast thicker than the standard demands and their
webs taper, so they hold less than this figure. The order therefore comes
out over rather than short — and that is the direction you want, because
running dry part way up a grouted core leaves a cold joint inside a
structural wall that nobody can see afterwards to know about.
What it costs
The quantity above is ours and it is cited. The rate is yours, and that
is not evasion — it is the part that actually moves. Fill is sold by the
cubic yard delivered, and the price shifts by region, by month, and by
whether the plant is sending a truck out for two yards or twenty. A
dollar figure printed here would be wrong somewhere and out of date
everywhere.
So: take the volume this page gives you to the
bags calculator, put your own
yard price and bag price in, and it will tell you which way round is
cheaper for your job. Above roughly a cubic yard it is a truck either
way, and getting it into the cores — pumped, or craned in a bucket — is
usually the bigger line on the invoice than the concrete. The guide to
bags or ready-mix works
through where that crossover sits.
Common mistakes
- Using an 8 inch figure for 12 inch block. The 12 takes 78% more fill, so you arrive with barely half of what the wall wants.
- Reading a per-core figure as per-block. Out by half. A standard block has two cores.
- Spacing by blocks rather than by inches. "32 in o.c." is a filled core every 32 inches of wall, not every second block.
- Forgetting the bond beam. It is not in this figure and it is not small.
The rest of the job
The concrete block calculator
counts the blocks and the mortar for the wall this fill goes into, and
hands the dimensions straight over. For the footing under it, the
slab calculator and the
bags calculator cover the pour,
and the retaining wall calculator
handles a segmental wall where the blocks are dry-laid instead.
Common questions
How much concrete does it take to fill one block?
For a standard 8 inch block, 0.303 cubic feet — two cores of about 0.151 each. A 6 inch block takes 0.214, a 10 inch 0.421 and a 12 inch 0.539. Those are not figures read off somebody's chart: they are the block's outside dimensions less the two face shells and the webs between them, which is the only part of this that any standard actually fixes.
Why do the numbers online disagree so much?
Because they are measuring different things and none of them says which. Search for a per-block fill figure and the first page offers 0.014, 0.067 and 0.27 cubic feet — a spread of twenty to one. Some are per core, some per block, some for a different block width, and some appear to be mortar rather than fill. There is no authority to defer to, so this calculator does not defer to one. It derives the core from ASTM C90, published via NCMA TEK 1-1F, and shows its working on the ticket.
Does block width really change it that much?
More than people expect. The face shell minimum does not grow with the block — it is 1 inch at 6 inch nominal and 1-1/4 inch at 8 inch and above, and it stops there. So every extra inch of block width goes straight into the core. A 12 inch block holds two and a half times the fill of a 6 inch one. Getting the width wrong is the single biggest error you can make on this page, which is why it is the first field.
Should I fill every core or only some?
That is your engineer's decision and your local code's, and this calculator will not guess it. A reinforcement schedule reads like "#4 at 32 in o.c.", which means a filled core every 32 inches of wall, full height — not every third block. Enter that spacing and you get the partial fill. Leave it blank and you get a solid-grouted wall, which is every core.
Is the figure high or low?
Deliberately high, and it is worth knowing which. ASTM C90 sets minimum shell and web thicknesses, and a minimum shell means the largest core a conforming block can have. Real units are cast thicker than the standard demands and their webs taper, so they hold less than this. The error therefore runs towards ordering over — which is the direction that matters, because running dry part way up a grouted core leaves a cold joint inside a wall that is meant to be structural, and you cannot see it afterwards to know it is there.
What does block fill cost?
The quantity is above and it is ours to stand behind; the rate is yours, because it is the part that moves. Fill is priced per cubic yard delivered, and that price varies by region, by month, and by whether the plant is sending a truck for two yards or twenty — a figure printed here would be wrong somewhere and out of date everywhere. Put your own yard price into the bags calculator with the volume this page gives you and you have the whole answer. Above roughly a yard it is a truck job anyway, and pumping or craning it into the cores is usually the larger line on the invoice.
Does it include bond beams and lintels?
No, and the ticket says so. A bond beam course, a lintel over an opening and any cleanout blocks all take fill on top of what is here, and their volumes depend on details this page does not ask for. A wall with them takes more than this figure — treat it as the core fill and add the beams separately.
Sources
- Concrete mix density
- 140 lb/ft3
- 80 lb bag yield
- 0.6 ft³
- 60 lb bag yield
- 0.45 ft³
- 50 lb bag yield
- 0.375 ft³
- 40 lb bag yield
- 0.3 ft³
- 90 lb bag yield
- 0.675 ft³
- 30 kg bag yield
- 0.49440533379999996 ft³
- 20 kg bag yield
- 0.35314666699999997 ft³
- Default waste allowance
- 10%
- QUIKRETE Concrete Mix No. 1101 data sheet — published yields of 0.30, 0.375, 0.45, 0.60 and 0.675 ft3 for the 40, 50, 60, 80 and 90 lb bags, the 90 lb noted as regional availability
- QUIKRETE Concrete Mix No. 1101 data sheet — unit weight approximately 140 lb/ft3 per ASTM C138
- Sakrete Concrete Mix product data sheet, King Packaged Materials / Sika Canada — a 30 kg (66 lb) bag yields approximately 0.014 m3 (1/2 ft3)
- Blue Circle Multi-Purpose Concrete technical information, Tarmac — one 20 kg bag typically produces around 0.01 m3 of fresh concrete, supplied throughout the United Kingdom in 20 kg bags
- NRMCA Concrete in Practice CIP 36, Structural Lightweight Concrete — "Structural lightweight concrete has an in-place density (unit weight) on the order of 90 to 115 lb/ft3 (1440 to 1840 kg/m3) compared to normalweight concrete with a density in the range of 140 to 150 lb/ft3 (2240 to 2400 kg/m3)."
Bag yield is approximate and varies with aggregate and water content. Density 140 lb/ft3 is the ASTM C138 unit weight of mixed bagged concrete, about 3,780 lb/yd3, and is what every calculation here uses. Ready-mix delivered by truck is denser, about 150 lb/ft3 or 4,050 lb/yd3; that figure is stored separately as readyMixDensity and appears only where a page discusses ordering ready-mix. The two are not interchangeable. The 90 lb bag is regionally stocked rather than nationally, so it is flagged and listed last. Bag sizes are per market: the 40 to 90 lb bags are what North American merchants stock, and the 20 and 30 kg bags are what the UK, Canadian and Australian ones do. Switching the calculator to metric switches the shopping list with it, because a metric answer priced in pounds of product is not an answer anyone can buy. The 150 lb/ft3 ready-mix figure was carried for a long time as a point value with nothing behind it. NRMCA CIP 36 publishes it as the TOP of a range — normalweight concrete is 140 to 150 lb/ft3, and structural lightweight is 90 to 115 — so densityRanges now holds both and the weight calculator reports a span rather than a single number it cannot justify. Where one figure has to be picked, the top of the range is the safe end: over-stating a weight risks specifying too much truck, and under-stating it risks overloading one.
- Default waste allowance
- 5%
Nominal sizes already include the mortar joint, which is why a nominal 8 x 16 in face is 0.888 ft2 and one divided by that is exactly 1.125 blocks per square foot. Mortar is the published Mason Mix yield of 13 blocks per 80 lb bag, replacing an earlier 3 bags of 70 lb per hundred blocks that was roughly two and a half times short. Core volume for grout is computed from the published minimum face shell and web thicknesses rather than from a per-block grout figure. Minimum shells and webs give the largest possible core, so the grout order errs high — real units have thicker shells and tapered webs, and coming up short mid-lift means a cold joint in a grouted core. Block widths from 6 to 12 in carry their own face shell minimum, which is what makes core volume vary by so much more than the width alone suggests: a 12 in block holds nearly two and a half times the fill of a 6 in one, because the extra width goes into the core while the shells stay the same.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures
