How many bags of concrete do I need for a 10x10 slab?

A 10 ft × 10 ft slab at 4 in thick needs 1.23 cubic yards of concrete — 56 eighty-pound bags, 75 sixty-pound bags, or 112 forty-pound bags. Add the usual ten per cent for waste and those become 62, 82 and 123. That is well over a tonne of sacks to haul and mix by hand, which is why most people switch to delivered ready-mix somewhere around one cubic yard.

How many 80 lb bags of concrete make a yard?

Forty-five. One 80 lb bag yields about 0.6 cubic feet of mixed concrete and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags. The smaller sizes scale the same way: 60 sixty-pound bags or 90 forty-pound bags per cubic yard. All three come to 3,600 lb of dry mix per cubic yard, because the yields are set from the same recipe.

How much does a yard of concrete cost?

Delivered ready-mix runs roughly $140 to $200 per cubic yard across most US markets, with the mix design, the strength and your distance from the plant moving it inside that band. On top of the concrete itself expect a delivery charge, a short-load fee if you order under about 3 cubic yards, and surcharges for weekend or after-hours pours. Bagged concrete is far dearer per yard: 45 eighty-pound bags at $6.00 each is $270 for the same cubic yard.

How much concrete do I need for a 24x24 garage slab?

A 24 ft × 24 ft garage floor is 576 square feet, so it takes 7.11 cubic yards at 4 in thick, 8.89 cubic yards at 5 in, and 10.67 cubic yards at 6 in. Add ten per cent for waste and even the 4 in version is a serious delivery. Note that the 6 in pour tips past the 10 cubic yards a standard mixer carries, so it becomes a two-truck job that has to be booked back to back to avoid a cold joint.

Is it cheaper to buy bags of concrete or order ready-mix?

Per cubic yard ready-mix wins comfortably — roughly $160 against about $270 in sacks — but small orders carry a short-load fee that flattens the advantage. Taking a typical $60 surcharge, the two routes cross at roughly 0.55 of a cubic yard, about 25 eighty-pound bags. Below that, bags are cheaper and far more convenient. Above it, ready-mix is cheaper, faster, more consistent, and does not cost you a weekend at a mixer.

How thick should a concrete slab be?

Four inches is the residential standard for patios, sidewalks and shed bases. A driveway carrying cars wants 4 to 5 in over a properly compacted base; a garage floor wants 5 to 6 in; and anything taking an RV, a trailer or delivery trucks should be 6 to 8 in with rebar. Thickness buys more than reinforcement does on a slab-on-grade, so if the budget stretches to one upgrade, an extra inch of concrete usually beats an extra mat of steel.

How much does a cubic yard of concrete weigh?

About 4,000 lb — two tons — for normal-weight concrete, which is roughly 2,400 kg per cubic metre. That is the figure to check a trailer or a wheelbarrow run against, and it is why a mixer truck carrying 10 cubic yards is close to 20 tons of load. Bagged concrete works out lighter on paper, at 3,600 lb of dry mix per cubic yard, because the rest of the weight is the water you add on site.

Construction

Concrete Calculator

Concrete volume is length × width × thickness in feet, divided by 27 for cubic yards. Enter a slab, footing, round column, stair flight or curb run and get the order in cubic yards, cubic metres and cubic feet — with 40, 60 and 80 lb bag counts, a waste allowance, and the cost either way.

Measurements

Patios, driveways, garage and shed floors, sidewalks.

Length

Width

Thickness4 in for a patio, 5–6 in for a driveway or garage

How many identical slabs?

Ordering

Waste allowanceSpillage, uneven sub-grade, bowed forms

Ready-mix priceDelivered, per volume unit

$/ yd³

Bag size to priceAll three counts are shown; this one gets costed

Price per bagOne 80 lb sack

$/ bag

Concrete needed

1.23

10 ft × 10 ft × 4 in

m³

0.94

ft³

33.33

Order with 10% waste

1.36 yd³

Bags of pre-mix

80 lb

bags

60 lb

bags

40 lb

123

bags

Rounded up, waste included · one 80 lb bag yields 0.6 ft³

Estimated material cost

Ready-mix

$217

1.36 yd³ @ $160.00

80 lb bags

$372

62 bags @ $6.00

Ready-mix wins on materials by $155 — before delivery, mixer hire, and the hours of mixing.

Under 3 yd³ most suppliers add a short-load fee of roughly $60 or more, on top of a one-yard minimum order.

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One cubic yard = 27 ft³. Bag yields are the manufacturers' nominal figures for standard pre-mixed concrete.

The basics

How concrete volume is worked out

Every concrete estimate is the same two moves: find the volume of the shape in cubic feet, then convert it into the unit your supplier sells in. Nothing about concrete makes the geometry special — what makes it error-prone is that one dimension is almost always quoted in inches while the others are in feet.

V = L × W × T

Volume first, in cubic feet

Multiply length L, width W and thickness T with all three in the same unit. A footing is a deep narrow slab, a column swaps the rectangle for π r², and a stair flight is a stack of rectangles — but the shape is always area times depth.

10 ft × 10 ft × 4 in → 10 × 10 × 0.3333 = 33.33 ft³

yd³ = ft³ ÷ 27

Then into the unit you buy in

A cubic yard is a cube three feet on every side, so it holds twenty-seven cubic feet. That single division is what turns a measurement into an order, and skipping it is the reason people call a plant asking for a truckload of patio.

33.33 ft³ ÷ 27 = 1.23 yd³ = 0.94 m³

Method

How to size a pour by hand

Four steps take you from a tape measure to a number you can read down the phone to a concrete plant.

1
Put every dimension in the same unit
Convert thicknesses and widths quoted in inches to feet by dividing by twelve. Metric measurements work identically — normalise to metres and divide by nothing, since a cubic metre is already the unit of sale.
4 in ÷ 12 = 0.3333 ft
2
Find the volume of the shape
Rectangles are L × W × T. Round columns are π r² h, using the radius, so halve the tube diameter first. Break anything irregular into rectangles and add them.
10 × 10 × 0.3333 = 33.33 ft³
3
Convert into cubic yards or cubic metres
Divide cubic feet by twenty-seven for cubic yards, or multiply by 0.0283 for cubic metres. Ready-mix is ordered in one or the other, never in cubic feet.
33.33 ÷ 27 = 1.23 yd³
4
Add waste, then round to whole bags or quarter yards
Add five to ten per cent for spillage, an uneven sub-grade and forms that bow under load. Bags always round up; ready-mix is usually ordered to the nearest quarter yard.
1.23 × 1.10 = 1.36 yd³ · 62 × 80 lb bags

Worked examples

The same method across five shapes

A slab, a strip footing, a set of piers, a stair flight and a curb run — every one solved the same way, and every figure below computed by the calculator above rather than typed in.

A 12 ft × 16 ft patio poured 4 in thick

Given L = 12 ft, W = 16 ft, T = 4 in

Thickness into feet: 4 ÷ 12 = 0.3333 ft

Length × width × thickness: 12 × 16 × 0.3333 = 64.00 ft³

Cubic feet into cubic yards: 64.00 ÷ 27 = 2.37 yd³

2.37 yd³

= 1.81 m³ = 64.00 ft³

Order with 10% waste: 2.61 yd³80 lb bags: 11860 lb bags: 15740 lb bags: 235

The thickness is the whole game on a slab: taking the same patio to 6 in adds half again to the order.

Quick chart

Concrete for common slab sizes

The answer to “how much concrete for a 20 × 20 slab” and a hundred variations of it. Every common slab size at the three thicknesses people actually pour, in cubic yards and in eighty-pound bags. Figures are exact — add five to ten per cent before you order.

Concrete required for common slab sizes at 4, 5 and 6 inches thick, in cubic yards and 80 lb bags
Slab	Area	4 in thick		5 in thick		6 in thick
Slab	Area	yd³	80 lb bags	yd³	80 lb bags	yd³	80 lb bags
4 × 4 ftAC pad / small landing	16 ft²	0.20	9	0.25	12	0.30	14
6 × 6 ftHot-tub or shed pad	36 ft²	0.44	20	0.56	25	0.67	30
8 × 8 ftSmall shed base	64 ft²	0.79	36	0.99	45	1.19	54
10 × 10 ftPatio or shed floor	100 ft²	1.23	56	1.54	70	1.85	84
10 × 12 ftSmall patio	120 ft²	1.48	67	1.85	84	2.22	100
12 × 12 ftStandard patio	144 ft²	1.78	80	2.22	100	2.67	120
12 × 16 ftLarge patio	192 ft²	2.37	107	2.96	134	3.56	160
12 × 20 ftSingle-car driveway apron	240 ft²	2.96	134	3.70	167	4.44	200
16 × 20 ftSingle garage	320 ft²	3.95	178	4.94	223	5.93	267
20 × 20 ftTwo-car garage	400 ft²	4.94	223	6.17	278	7.41	334
20 × 24 ftTwo-car garage, deep	480 ft²	5.93	267	7.41	334	8.89	400
24 × 24 ftLarge two-car garage	576 ft²	7.11	320	8.89	400	10.67	480
24 × 30 ftThree-car garage / shop	720 ft²	8.89	400	11.11	500	13.33	600
30 × 40 ftBarn or workshop slab	1,200 ft²	14.81	667	18.52	834	22.22	1,000
40 × 60 ftPole barn / small warehouse	2,400 ft²	29.63	1,334	37.04	1,667	44.44	2,000

Bag counts are rounded up and assume the nominal 0.6 ft³ yield of an 80 lb sack. Past roughly one cubic yard, order ready-mix instead.

Reference

What one bag of concrete actually makes

Pre-mixed concrete is sold in three sack sizes, and every bag count on this page comes from the nominal yields printed on them. Coverage is how much slab a single bag fills at a given thickness — the number that tells you whether bagging the job is realistic.

Bag	Yield	Bags per yd³	Bags per m³	Dry mix per yd³	Covers at 4 in	Covers at 6 in
80 lb	0.6 ft³	45	59	3,600 lb	1.80 ft²	1.20 ft²
60 lb	0.45 ft³	60	78	3,600 lb	1.35 ft²	0.90 ft²
40 lb	0.3 ft³	90	118	3,600 lb	0.90 ft²	0.60 ft²

One 12 in sonotube pier 4 ft deep takes 3.14 ft³ — about 6 eighty-pound bags per pier.

The decision

Ready-mix or bags?

The question every small pour turns on. Bags are cheap to start and brutal to scale; ready-mix is cheap per yard and carries a fixed cost floor. The crossover is lower than most people expect, and it moves with the short-load fee your local plant charges.

under half a yard

Bags win the small jobs

Setting posts, patching a step, pouring a few pier footings: you mix what you need, when you need it, with no truck to meet and no minimum order. The dry sacks also keep, so anything left over is not wasted.

Crossover at about 0.55 yd³ — roughly 25 eighty-pound bags

past one yard

Ready-mix wins everything bigger

One consistent batch, placed in one continuous pour, with none of the mixing. The plant also controls the mix design and slump, which is the part a hand-mixed pour never quite matches across dozens of sacks.

$160/yd³ delivered against about $270/yd³ in 80 lb sacks

Materials cost of bagged concrete against delivered ready-mix at increasing order sizes
Order	Cubic feet	80 lb bags	Bagged at $6.00	Ready-mix at $160/yd³	Reality check
0.25 yd³	6.75	12	$72	$100	Short-load fee applies
0.50 yd³	13.50	23	$138	$140	Short-load fee applies
1.00 yd³	27.00	45	$270	$220	Short-load fee applies
2.00 yd³	54.00	90	$540	$380	Short-load fee applies
5.00 yd³	135.0	225	$1,350	$800	One truck, no surcharge
10.00 yd³	270.0	450	$2,700	$1,600	One truck, no surcharge

Ready-mix column includes a $60 short-load fee below 3 yd³. Materials only — no delivery, mixer hire, or your Saturday.

Vocabulary

Concrete, cement, mortar and grout

Four words that get used interchangeably and mean four different products. Getting them wrong is not just pedantry: ordering cement when you meant concrete, or bedding a structural post in mortar, produces work that fails.

Cement: The grey powder — usually Portland cement. It is the glue, not the material. On its own it is never poured; asking for a yard of cement when you mean concrete is the classic builders'-merchant mix-up.
Concrete: Cement, water, sand and coarse aggregate (gravel or crushed stone). The aggregate carries the load and the cement paste binds it, which is why concrete is strong in compression and needs steel to be strong in tension.
Mortar: Cement, lime, sand and water, with no coarse aggregate. It is a bedding and bonding material for brick and block, deliberately weaker than the units it joins so cracks land in the joints, not the masonry.
Grout: A high-slump cement mix poured into voids — block cores, post bases, under baseplates. It flows where concrete would bridge and leave a hollow, and it is not a substitute for structural concrete.
Ready-mix: Concrete batched at a plant and delivered wet in a rotating drum. You buy it by the cubic yard or cubic metre, you get a consistent mix design, and the clock starts the moment water hits the cement.
Bagged or pre-mix concrete: A dry blend of cement, sand and stone in a sack; you add water. Convenient and forgiving on small pours, but it costs far more per cubic yard and every bag is another one to lift and mix.
Slump: How far a cone of fresh concrete sags — the standard measure of workability. A wetter, higher-slump mix is easier to place and weaker once cured, so adding water on site to make life easier costs you strength.

Mix design

Which strength of concrete to order

Ready-mix is specified by its compressive strength at twenty-eight days, in pounds per square inch or in megapascals. Volume tells the plant how much to send; strength tells them what to send. Ordering high is cheap insurance on a driveway and pointless on a fence post.

Strength	Metric	Typical use
2,500 psi	17 MPa	Non-structural fill, blinding, light footpaths with no vehicles
3,000 psi	20 MPa	The residential default: patios, sidewalks, shed bases, most footings
3,500 psi	25 MPa	Foundation walls, driveways in mild climates, basement slabs
4,000 psi	28 MPa	Driveways, garage and workshop floors, anything exposed to freeze-thaw
4,500 psi	31 MPa	Heavy vehicle parking, RV pads, suspended slabs
5,000 psi	35 MPa	Industrial floors, structural columns and beams, high-load paving

Most bagged pre-mix cures to around 4,000 psi, which is why bags are perfectly adequate structurally — the objection to them is labour and cost per yard, not strength. A rough hand-mix recipe of one part cement, two parts sand and three parts gravel lands near 3,000 psi, but a batched mix is far more consistent.

Sizing

How thick, and how much steel

Thickness drives the order more than anything else on this page: an inch across a large slab is a whole extra truck. These are the residential conventions — always defer to your local code and to an engineer for anything structural.

Job	Thickness	Usual reinforcement
Sidewalk or footpath	4 in	Fibre mix, or 6×6 welded wire mesh
Patio	4 in	Mesh optional; control joints matter more
Shed or garden-building base	4 in	6×6 mesh, thickened edge
Residential driveway (cars)	4–5 in	#3 rebar at 18 in on centre, or mesh
Garage floor	4–6 in	#3–#4 rebar at 16–18 in on centre
RV pad, truck or trailer parking	6–8 in	#4 rebar at 12–18 in on centre
Continuous strip footing	8–12 in deep, 16–20 in wide	2–3 runs of #4 rebar, continuous
Pad footing under a post	10–12 in deep	#4 mat, or none for light decks

Steel does not stop a slab-on-grade cracking; it holds the crack closed once it happens. Reinforcement also has to sit in the middle third of the slab to do anything at all, which means chairs or mesh supports — steel laid on the sub-grade and never lifted is the most common wasted expense on a residential pour.

On the day

Placing and curing the pour

Ordering the right volume is half the job. The other half happens in the four hours after the truck arrives, and it decides whether the slab you paid for lasts thirty years or three.

compact the base first

The sub-grade sets the volume

A rutted or soft base swallows extra concrete and settles unevenly afterwards. Grade it, compact it, and check the depth in several places — the average depth is what you should be ordering against, not the nominal one.

joints every 24–36×

Control joints decide where it cracks

Concrete shrinks as it cures and will crack somewhere. Cutting or tooling joints at a spacing of twenty-four to thirty-six times the slab thickness in inches decides where. Cut them within a day of the pour.

A 4 in slab wants joints roughly every 8 to 12 ft

keep it damp for a week

Curing is water, not drying

Concrete gains strength through a chemical reaction that needs moisture. Letting a fresh slab dry out in sun or wind can cost a large share of its final strength, so cover it, mist it or use a curing compound for the first several days.

one continuous placement

Do not let it set in stages

Concrete placed against a batch that has already stiffened forms a cold joint — a weak plane through the slab. Order the whole volume for one pour, and if it takes two trucks, book them back to back.

Common mistakes

Where concrete estimates go wrong

Six errors account for almost every short order and every wasted half-truck. Five of them are arithmetic, and the sixth is optimism about the sub-grade.

inches ÷ 12

Leaving the thickness in inches

Every dimension has to be in the same unit before you multiply. A thickness typed as four rather than a third of a foot inflates the answer twelvefold, and it is by far the most common arithmetic slip on a pour.

ft³ ÷ 27

Forgetting to divide by twenty-seven

The multiplication gives cubic feet, but concrete is sold by the cubic yard. Skipping the conversion is how people order a truckload for a patio, or a wheelbarrow for a garage.

order 5–10% over

Ordering the exact number

Sub-grade is never perfectly flat, forms bow outward under the weight, and some of the load stays in the drum. Ordering to the exact figure reliably means running short at the far corner.

measure the dips

Assuming a flat sub-grade

A base with ruts and soft spots swallows noticeably more concrete than the nominal thickness suggests. Grade and compact first, then measure the depth in several places and use the average.

45 sacks per yard

Bagging a pour that wants a truck

Bags are the right answer for a post or a small pad. Past roughly a cubic yard the sack count, the mixing time and the price per yard all turn against you at once.

one continuous pour

Splitting a slab across two days

Concrete placed against concrete that has already set forms a cold joint, which is a weak plane and a crack waiting to happen. Order enough for the whole pour and place it in one go.

Worth knowing

Rules worth remembering

Six facts that turn a concrete order from a guess into a decision, none of which are printed on the bag.

27 ft³ in a cubic yard

The only conversion that matters

Three feet cubed. Every concrete estimate in imperial units ends with this division, and almost every wildly wrong one skipped it.

an inch is a lot

Thickness scales the whole order

Volume is linear in depth, so adding an inch to a large slab adds a quarter to the order. It is the single biggest lever on both the cost and the delivery.

A 24 × 24 garage: 7.11 yd³ at 4 in, 10.67 yd³ at 6 in

measure across, not around

Columns use the diameter

The formula wants the radius, so a tube measured around its circumference instead of across its face inflates the answer by more than three times. Halve the diameter and nothing else.

a truck holds about ten yards

Capacity shapes the plan

Order size interacts with delivery in both directions: too small and you pay a short-load fee, too large and you have to sequence trucks so the pour never stops.

Under 3 yd³ expect a surcharge; over 10 yd³ expect two trucks

always round bags up

You cannot buy part of a sack

Bag counts are ceilings, never averages. Rounding down to the nearest sack is how a pour ends one bag short with the mixer still running.

order five to ten per cent over

Waste is not optional

Spillage, a sub-grade that is never perfectly flat, forms that bow outward, and concrete left in the drum all take their share. Coming back for a second small load costs far more than the surplus would have.

FAQ

Common questions

Multiply length by width by thickness with every dimension expressed in feet, then divide the answer by 27 to get cubic yards, because a cubic yard is 3 ft × 3 ft × 3 ft. A 10 ft × 10 ft slab poured 4 in thick works out as 10 × 10 × 0.333 = 33.33 cubic feet, which is 1.23 cubic yards or 0.94 cubic metres. The thickness is the step that trips people up: it is quoted in inches on site, so it has to be divided by 12 before it joins the multiplication.

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