The short version
A 10×12 shed slab poured 4 inches thick with a thickened perimeter edge takes about 92 bags of 80 lb concrete mix, or about 122 bags of 60 lb mix, once you allow 10 percent for waste. That is roughly 2 cubic yards of concrete. A plain flat 4 inch rectangle with no edge is only about 67 bags of 80 lb, which is why so many people come up short on pour day.
A shed slab is not just a rectangle
The number that matters is not the floor area, it is the total volume of the hole you are filling. On a shed slab that hole usually has three parts most calculators ignore.
- The thickened edge. Most shed slabs are poured monolithic, meaning the perimeter is turned down deeper than the field of the slab so the wall plates bear on something more substantial than 4 inches of concrete over gravel. A turndown adds real volume.
- Subgrade slop. Hand grading is never dead flat. Half an inch of extra depth across 120 square feet is 5 cubic feet, or eight extra bags of 80 lb mix.
- Form spring and spillage. Wet concrete pushes forms outward, and every wheelbarrow leaves something behind.
Turndown depth and width are code items in a lot of jurisdictions, and frost depth drives them. Before you dig, call your building department and ask for the local frost depth and whether a shed this size needs a permitted, frost protected footing instead of a floating slab. In many areas a small storage shed can float, but that is not universal and we would never assume it.
The formula
Work in cubic feet, then convert once at the end.
Field slab (cu ft) = length x width x (thickness in inches / 12)
Thickened edge (cu ft) = perimeter x (extra depth in inches / 12) x (edge width in inches / 12)
Total bags = (field + edge) x 1.10 / bag yield in cubic feet
Cubic yards is the total divided by 27. Note that the “extra depth” in the edge formula is the depth below the bottom of the field slab, not the full edge depth, or you will double count the top 4 inches.
Worked example, 10×12 with an 8 inch turndown
Field slab: 10 x 12 = 120 sq ft, times 4/12 of a foot, gives 40 cubic feet.
Perimeter: 2 x (10 + 12) = 44 linear feet.
Turndown: 8 inches total edge depth minus the 4 inch slab leaves 4 inches of extra depth, and we will make the turndown 8 inches wide. So 44 x (4/12) x (8/12) = 44 x 0.2222 = 9.8 cubic feet.
Total before waste: 40 + 9.8 = 49.8 cubic feet, which is 1.84 cubic yards. Measuring the perimeter on the outside face slightly overcounts the four corners, and we are fine with that. It is free contingency.
| Component | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| Field slab, 120 sq ft at 4 in | 40.0 | 1.48 | 67 | 89 |
| Thickened edge, 44 lf | 9.8 | 0.36 | 17 | 22 |
| Subtotal | 49.8 | 1.84 | 83 | 111 |
| With 10% waste | 54.8 | 2.03 | 92 | 122 |
Bag yields, and why 45 bags equals a yard
A bag of concrete mix yields far less finished concrete than its dry weight suggests, because the aggregate packs tight once water goes in. An 80 lb bag makes about 0.60 cubic feet. Divide 27 cubic feet in a yard by 0.60 and you get exactly 45. That is where the 45 bags per cubic yard rule comes from, and it is worth memorizing.
| Bag size | Approx yield (cu ft) | Bags per cubic yard | Bags for this shed slab |
|---|---|---|---|
| 40 lb | 0.30 | 90 | 183 |
| 50 lb | 0.375 | 72 | 147 |
| 60 lb | 0.45 | 60 | 122 |
| 80 lb | 0.60 | 45 | 92 |
Base prep, and the number it quietly changes
Under the slab you want compacted crushed stone, commonly 4 inches of clean angular material, placed and compacted in lifts of 3 to 4 inches rather than dumped all at once. Rounded pea gravel will not lock up. A plate compactor rental is cheap next to the cost of a slab that settles.
Here is the practical link to your bag count. If your base is compacted flat and screeded to grade with a straightedge, 10 percent waste is plenty. If you dug by hand and eyeballed it, use 15 percent. On this slab that is the difference between 92 and 96 bags of 80 lb.
Vapor barrier and anchor bolts
If the shed will store anything that can rust, warp, or mildew, put a vapor barrier under the slab. Six mil poly is the common minimum, thicker 10 or 15 mil sheet is far more puncture resistant when you are dragging rebar chairs and boots across it. Lap seams at least 6 inches and tape them. Run the sheet over the top of the gravel, not under it.
Anchor bolts go in wet, not drilled later, if you can help it. Half inch J bolts are the common choice, set within about a foot of each plate end, hooked end down, with enough thread exposed for a plate, washer, and nut. Diameter, embedment, and spacing are code items that vary, so confirm what your jurisdiction requires before the mixer shows up. Keep a tape and a chalked plate layout at the edge of the pour while the concrete is still plastic.
Two cubic yards is a decision point
Ninety two bags is roughly 7,400 lb of material to buy, haul, and mix, and it is more than one person can place in front of a setting edge on a warm day. At this size we would price a ready-mix delivery before committing to a pallet of bags. If the shed will carry anything heavier than shelving and a mower, or if the ground is soft or filled, that is engineer territory rather than a volume calculation.
Sources and Further Reading
Run your own numbers with the Concrete Slab Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 7, 2026
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