How Much Concrete Do I Need for a Shed Slab? (8×10 to 16×20 Charts)

Quick Answer: Concrete for Common Shed Sizes

For a standard 4-inch thick shed slab, here are the most common sizes: An 8×10 slab needs 0.99 cubic yards (45 bags of 80lb concrete). A 10×12 slab needs 1.48 cubic yards (67 bags). A 12×16 slab needs 2.37 cubic yards (108 bags). A 16×20 slab needs 3.95 cubic yards (180 bags).

These numbers assume a flat, level pour with no waste. In practice, always order 10% extra to account for uneven ground, spillage, and slight thickness variations. For anything over 2 cubic yards, ordering ready-mix delivery is almost always cheaper and faster than mixing bags by hand.

4-Inch vs 6-Inch Slab: Which Thickness Do You Need?

A 4-inch slab (100mm) is sufficient for lightweight garden sheds, small storage buildings, and tool sheds where you won't be storing anything extremely heavy. This is the standard residential shed slab thickness and meets code requirements in most areas.

A 6-inch slab (150mm) is recommended when storing heavy equipment like riding mowers, workshop machinery, vehicles, or when the shed will be used as a garage. The 6-inch slab uses 50% more concrete but provides significantly more load-bearing capacity.

For a 10×12 shed, the difference is substantial: a 4-inch slab needs 1.48 cubic yards while a 6-inch slab needs 2.22 cubic yards. That's roughly $150–$250 more in concrete cost, but it's much cheaper than cracking a too-thin slab and starting over.

Complete Concrete Chart: Every Shed Size

8×8 shed (4" thick): 0.79 cu yd / 36 bags — 8×8 shed (6" thick): 1.19 cu yd / 54 bags. 8×10 shed (4" thick): 0.99 cu yd / 45 bags — 8×10 shed (6" thick): 1.48 cu yd / 67 bags. 8×12 shed (4" thick): 1.19 cu yd / 54 bags — 8×12 shed (6" thick): 1.78 cu yd / 81 bags.

10×10 shed (4" thick): 1.23 cu yd / 56 bags — 10×10 shed (6" thick): 1.85 cu yd / 84 bags. 10×12 shed (4" thick): 1.48 cu yd / 67 bags — 10×12 shed (6" thick): 2.22 cu yd / 101 bags. 10×16 shed (4" thick): 1.98 cu yd / 90 bags — 10×16 shed (6" thick): 2.96 cu yd / 135 bags.

12×12 shed (4" thick): 1.78 cu yd / 81 bags — 12×12 shed (6" thick): 2.67 cu yd / 121 bags. 12×16 shed (4" thick): 2.37 cu yd / 108 bags — 12×16 shed (6" thick): 3.56 cu yd / 162 bags. 12×20 shed (4" thick): 2.96 cu yd / 135 bags — 12×20 shed (6" thick): 4.44 cu yd / 202 bags.

14×20 shed (4" thick): 3.46 cu yd / 157 bags — 14×20 shed (6" thick): 5.19 cu yd / 236 bags. 16×20 shed (4" thick): 3.95 cu yd / 180 bags — 16×20 shed (6" thick): 5.93 cu yd / 270 bags. 16×24 shed (4" thick): 4.74 cu yd / 216 bags — 16×24 shed (6" thick): 7.11 cu yd / 323 bags.

Bags vs Ready-Mix: Cost Comparison

An 80lb bag of concrete mix costs $5–$7 at hardware stores and yields about 0.6 cubic feet. For small slabs (under 1.5 cubic yards), bags can be economical if you have the time and labor. For a 10×10 shed slab at 4 inches thick, you'd need about 56 bags at roughly $6 each = $336 in materials.

Ready-mix concrete delivery typically costs $125–$175 per cubic yard with a minimum order of 1 yard and a short-load fee of $50–$100 for orders under 3–4 yards. For the same 10×10 slab, ready-mix would cost about $185–$275 including the short-load fee — actually cheaper than bags, and you skip hours of mixing.

The breakeven point is typically around 1.5 cubic yards: below that, bags might save money. Above that, ready-mix almost always wins on both cost and time. Use our concrete calculator to get exact quantities for your specific dimensions.

Preparing the Ground Before Pouring

A proper shed slab requires more than just pouring concrete on dirt. First, excavate 8–10 inches deep to allow for a 4-inch gravel base plus your concrete thickness. The gravel base provides drainage and prevents the slab from heaving in freeze-thaw cycles.

Compact the gravel base thoroughly — rent a plate compactor for $50–$75 per day rather than trying to compact by hand. Place 6×6 welded wire mesh or rebar on 2-inch chairs to reinforce the slab. For sheds, #3 rebar on 24-inch centers or WWM is sufficient.

Build forms from 2×4 or 2×6 lumber, staked every 2–3 feet. Make sure forms are level — check diagonals to ensure square corners. Apply form release oil so you can strip the forms cleanly after curing. Use our gravel calculator to determine how much base material you need.

Frequently Asked Questions

How much concrete do I need for a shed slab? Multiply length by width by thickness in feet, then divide by 27 for cubic yards. A 10x12 slab at 4 inches is about 1.5 cubic yards.

Do I need rebar in a shed slab? For a standard 4-inch shed slab, welded wire mesh or a light rebar grid helps control cracking and is recommended. Heavier slabs that carry equipment benefit from 6 inches and rebar.

How thick should a shed slab be? Four inches is fine for a typical garden or storage shed. Step up to 5 or 6 inches if you will store a vehicle, heavy tools, or machinery. Our concrete calculator gives exact yardage, bag counts, and cost for your slab.

Thickness, Reinforcement, and Ordering Tips

For most backyard sheds, a four-inch-thick slab is the standard and is plenty strong for storage buildings, garden tools, and light workshop use. Step up to five or six inches only if you plan to store a vehicle, heavy equipment, or run machinery that vibrates the floor. Each additional inch of thickness across a 10-by-12 slab adds roughly half a cubic yard of concrete, so over-building 'just in case' carries a real cost.

Reinforcement extends the life of the slab and helps control cracking. Welded wire mesh or a grid of #3 or #4 rebar set on chairs in the middle third of the slab is inexpensive insurance against the slab shifting and cracking as the ground freezes, thaws, and settles. A compacted gravel base four to six inches deep beneath the slab provides drainage and a stable platform that prevents the most common cause of premature failure.

When ordering, always add 10 percent to your calculated volume to account for uneven subgrade, spillage, and the slight over-dig that happens at the edges. If your total is under about one cubic yard, bagged concrete may be cheaper than a ready-mix delivery, which often carries short-load fees below a full truck. Above one yard, ready-mix is almost always more economical and far less labor than mixing dozens of bags by hand.