How Much Concrete for a Shed Slab (8x8 to 16x20)

A 10x12 shed slab poured 4 inches thick takes 1.63 cubic yards of concrete with 10 percent waste built in: 74 bags of 80 lb mix, or a 1.75 yard ready-mix order at about $244 in material. An 8x8 slab is 0.87 cubic yards (40 bags); a 16x20 is 4.35 cubic yards, which is a truck, not a stack of bags. The tables below cover every common shed footprint at 4 and 6 inches.

Slab / FootingRound Column / Post

Length (ft)

Width (ft)

Thickness (in)

Waste buffer (%)

Bag size

40 lb50 lb60 lb80 lb

Price per cubic yard ($ (optional))

Volume

40.0 cu ft

Ready-mix (incl. waste)

1.63 cu yd

80 lb bags needed

74

Estimated cost

—

Bags needed by bag size

Bag size	Bags needed
40 lb	147
50 lb	118
60 lb	98
80 lb	74

Includes a 10% waste buffer. Ready-mix concrete is typically sold in quarter- or half-yard increments. Round up when ordering.

Slab or gravel pad: settle this before pricing concrete

Most kit and prefab sheds arrive with their own floor, a frame of pressure-treated skids or joists with plywood over it. That floor needs something flat, level and well drained to sit on, and 4 to 6 inches of compacted crushed stone does the job for a fraction of the cost of concrete, in an afternoon, with no curing wait. Rain that blows under the door drains straight through it. If that is your shed, read how much gravel for a shed base and size the stone with the gravel calculator. You may not need this page at all.

A slab is the right answer when the slab is the floor. Sheds sold without one, most metal sheds, anything you will roll a mower, a motorcycle or a loaded engine hoist across, and any shed that becomes a workshop with a bench and machines all want concrete. So does a shed in a wind zone where the building department wants it anchored to something heavier than itself, and a shed you want mice kept out of. Concrete also takes anchor bolts, which make the building and its foundation one piece.

What a slab costs beyond money: it is permanent. The location has to be right the first time, because you will not be moving it when the garden plan changes. A gravel pad can be raked out and seeded over.

Match the slab to the footprint, then decide on a lip

Measure the shed base, not the size printed on the box. A nominal 10x12 kit is often an inch or two under in each direction at the sill, and some makers quote the roof overhang rather than the walls. The dimension that matters is the outside of the bottom plate or the skid frame.

Two conventions work. Pour the slab exactly to the footprint, so the siding runs down past the slab edge and sheds water onto the ground with no ledge for rain to sit on. Or pour it 2 to 4 inches larger on every side, which gives a margin for a shed that is not quite square, room for sill sealer and a drip edge, and a lip to butt a ramp against. Either way, resist pouring it a foot bigger for safety. A wide apron holds water against the siding, and the bottom of a shed wall is the part that rots first.

The tables use nominal sizes. If you add a lip, enter the real slab dimensions in the calculator above rather than the shed size, because 4 inches on all four sides adds more area than it looks like.

Shed slab concrete at 4 inches

Four inches over a compacted base is the standard storage-shed and light-workshop slab. Every figure here includes the 10 percent waste allowance the site’s calculators apply, and the ready-mix column is rounded up to the quarter yard a plant will actually batch. The last column is material only at $150 a yard, a typical 2026 price; delivery and short-load fees come on top.

Shed size	Floor area	Cubic yards (10% waste in)	80 lb bags	60 lb bags	Ready-mix order	Material at $150/yd
8x8	64 sq ft	0.87	40	53	1 yd	$130
8x10	80 sq ft	1.09	49	66	1.25 yd	$163
8x12	96 sq ft	1.3	59	79	1.5 yd	$196
10x10	100 sq ft	1.36	62	82	1.5 yd	$204
10x12	120 sq ft	1.63	74	98	1.75 yd	$244
10x14	140 sq ft	1.9	86	115	2 yd	$285
12x12	144 sq ft	1.96	89	118	2 yd	$293
12x16	192 sq ft	2.61	118	157	2.75 yd	$391
12x20	240 sq ft	3.26	147	196	3.5 yd	$489
14x20	280 sq ft	3.8	172	229	4 yd	$570
16x20	320 sq ft	4.35	196	261	4.5 yd	$652

Read the bag columns as a labor estimate as much as a shopping list. Forty bags is a morning with a mixer. Ninety is a full day for two people who have done it before, and 196 is not a bag job.

Where a 6 inch slab earns its extra half yard

Going from 4 to 6 inches adds half again to the concrete, and on a slab this small that is the whole budget difference between a shed floor and a shop floor. The cases where it pays:

• A workshop with a mill, a lathe, a wood stove on a masonry hearth, or a cabinet saw that never moves.
• A compact tractor with implements, a car, or a trailer that gets parked inside.
• A slab the building department has asked for at 6 inches, which happens once the shed is large enough to need a permit in some jurisdictions. Ask before you dig.

Soft or expansive soil is not on that list. Thickness alone does little against ground that moves; a #4 rebar grid in a 4 inch slab, sized with the rebar calculator, does more for less concrete, and a thickened edge (below) does the rest.

Shed size	Cubic yards (10% waste in)	80 lb bags	60 lb bags	Ready-mix order	Material at $150/yd
8x8	1.3	59	79	1.5 yd	$196
8x10	1.63	74	98	1.75 yd	$244
8x12	1.96	89	118	2 yd	$293
10x10	2.04	92	123	2.25 yd	$306
10x12	2.44	110	147	2.5 yd	$367
10x14	2.85	129	172	3 yd	$428
12x12	2.93	132	176	3 yd	$440
12x16	3.91	177	235	4 yd	$587
12x20	4.89	220	294	5 yd	$733
14x20	5.7	257	343	5.75 yd	$856
16x20	6.52	294	392	6.75 yd	$978

Notice that an 8x8 at 6 inches is the same 1.3 yards as an 8x12 at 4. If the shed is small and the floor has to be strong, the thicker pour is still bag territory.

Bags or a truck: the crossover sits right around one cubic yard

An 80 lb bag yields 0.6 cubic feet of placed concrete, so 45 of them make a cubic yard; a 60 lb bag yields 0.45 cubic feet, 60 to the yard. The bags per cubic yard guide has the full table including the 40 and 50 lb sizes, and the concrete calculator switches between them.

Three sizes in the 4 inch table sit under or near a yard: the 8x8 at 0.87, the 8x10 at 1.09 and the 8x12 at 1.3. Those are the bag jobs. A ready-mix plant will deliver a single yard, but on an order that small the short-load fee, typically somewhere between $75 and $200 in 2026 depending on the plant, can cost as much as the concrete, and the truck still needs a route to the back garden. Bags at a typical 2026 price of $5 to $8 each for 80 lb come to a similar total for an 8x8, and you can carry them through the side gate.

From 10x10 to 12x12, between 1.36 and 1.96 yards, the money is close to a wash once the short-load fee is counted, and the labor is not. Seventy-four bags for a 10x12 is 74 trips to the mixer, each one a measured bucket of water and a timed mix, and the far corner of the slab is beginning to set while you are still placing the near one. A pour from bags has to be planned as one continuous placement with a mixer running the whole time, two people minimum, starting at the far side and keeping a wet edge, or it ends with a cold joint across the floor. From 12x16 up, at 118 bags and more, order the truck. The concrete slab cost guide compares the two routes in dollars for every size.

One middle option worth a phone call: tow-behind concrete trailers from rental yards, which typically carry about a yard of mixed concrete. That is enough for the 8x8 in one trip and the 8x10 in two.

Thickened edges once the shed passes about 200 square feet

A plain 4 inch slab floats on its gravel. Once a shed gets to roughly 200 square feet, which is also the permit threshold in many places, the edge of the slab starts carrying real wall and roof load, and the usual answer is a turned-down edge: the slab thickens along its perimeter into a footing, commonly around 12 inches wide and 8 to 12 inches deeper than the slab, with two continuous bars of #4 rebar in it. In frost regions the building department may want that footing taken down to frost depth for a larger shed, which is a different foundation entirely. Check with your local building department first, rather than dig a trench you then have to deepen.

The tables do not include that edge, and it is not small. Run it as a second slab in the calculator: length equal to the perimeter (64 feet for a 12x20), width equal to the footing width, thickness equal to the extra depth below the slab. On a 12x20 the edge alone comes to more than a cubic yard, enough to move the job from a bag pour to a truck. Size the edge bars with the rebar size chart and count them with the rebar calculator.

What goes under the slab and what gets set into it

Four inches of compacted gravel

Strip the topsoil, compact the subgrade, then place 4 inches of crushed stone with fines (road base, crusher run, 3/4 inch minus) and compact it in two lifts. Order the stone as loose volume, which is more than the compacted depth; the gravel calculator handles that, and the material waste factors guide sets out why compaction and waste are separate allowances. A shed slab on 4 inches of gravel over compacted soil needs nothing else under it in most of the country.

Vapor barrier

A sheet of 6 mil polyethylene, 10 mil if you can get it, laid over the gravel with the joints lapped a foot. Without it, ground moisture comes up through the slab and everything stored on the floor gets damp from underneath. For a shed that holds a mower and garden tools you can skip it; for a workshop with tools that rust, do not.

Anchor bolts

Set 1/2 inch J-bolts into the wet concrete at the edges. A common pattern is one within a foot of each corner and the rest no more than about 6 feet apart along each wall, projecting enough to pass through the sill plate and take a washer and nut. Your shed plan or the building department sets the spacing, and in wind zones it is usually tighter. Mark the bolt positions on the forms before the pour, because a bolt that lands under a door opening or on a stud location is a nuisance forever. The sill plate and wall framing that go onto those bolts are what the framing calculator counts.

A slight fall for water

A shed floor that is dead level holds every drop that comes in. Set the forms so the slab falls about 1/8 inch per foot toward the door, which on a 12 foot deep shed is 1.5 inches from back wall to threshold, and the rain that blows in runs out again. The surrounding grade needs to fall away from the slab on all sides as well.

Control joints when a side runs past 10 to 12 feet

Concrete shrinks as it cures, and a slab longer than about 10 to 12 feet in any direction will crack somewhere; a control joint, tooled while the concrete is wet or sawn the next morning to a quarter of the slab depth, decides where. The 8x8 through 10x10 slabs need none. A 10x12 or 12x12 can go without, with a single joint across the middle as cheap insurance. A 12x20 wants one joint across the 20 foot side at 10 feet. A 14x20 or 16x20 wants one joint each way, which makes four panels. Under a shed floor the joint is invisible, so there is no reason to skip it.

Give the slab a week before the shed goes up on it and the full 28 days before anything heavy is parked inside. Keep the sill plate off bare concrete with a foam sill sealer or treated lumber, and the floor you poured will outlast the shed on top of it.