Shed base calculator
Shed base size calculator
For a concrete shed base, size the slab from the shed footprint plus any required door, anchor, and drip-edge clearance, then calculate length x width x thickness. A common 10 ft by 12 ft shed base at 4 inches thick needs about 1.49 cubic yards of concrete before waste, or about 74 80-lb bags with a 10% buying allowance.
Quick answer
| Typical slab thickness | 4 inches for light sheds; 5-6 inches for heavier loads |
|---|---|
| Common overhang | Use the shed maker's clearance, often 0-6 inches per side |
| 10x12 base example | 1.49 yd3 before waste / 74 80-lb bags with 10% waste |
| Cost basis | $150/yd3 ready-mix and $5 per 80-lb bag |
Common shed base concrete sizes
Use this table as a first pass before entering exact dimensions. The order and cost columns include a 10% material allowance.
| Scenario | Dimensions | Base volume | 80-lb bags | Ready-mix with 10% | Bagged with 10% |
|---|---|---|---|---|---|
| Small tool shed Good for a compact resin or mower shed when the base is well drained. | 8 ft x 8 ft x 4 in | 0.80 yd3 | 36 bags before waste | 0.87 yd3 / $131 | 40 bags / $200 |
| Mower shed Common narrow backyard utility pad. | 8 ft x 10 ft x 4 in | 0.99 yd3 | 45 bags before waste | 1.10 yd3 / $164 | 49 bags / $245 |
| Standard shed Common storage shed footprint and the main example on this page. | 10 ft x 12 ft x 4 in | 1.49 yd3 | 67 bags before waste | 1.63 yd3 / $244 | 74 bags / $370 |
| Workshop shed Better for a shed with workbench space or heavier storage. | 12 ft x 16 ft x 4 in | 2.38 yd3 | 107 bags before waste | 2.61 yd3 / $392 | 118 bags / $590 |
| Heavy shed base Use when loads, mower storage, or manufacturer details justify extra thickness. | 12 ft x 20 ft x 5 in | 3.71 yd3 | 167 bags before waste | 4.08 yd3 / $612 | 184 bags / $920 |
Interactive calculator
Shed base concrete calculator
Enter the finished slab dimensions below. If your shed plan calls for a gravel base under the concrete, calculate the concrete slab here and calculate the gravel layer separately.
Use Quick estimates for common slab, patio, and driveway sizes.
How to decide what to buy
Start from the shed plan, not the catalog name
A shed sold as 10x12 may have a different skid, wall, roof, or door footprint. Use the manufacturer's foundation drawing when available, especially if anchor bolts or a ramp must land on the slab.
Keep water off the shed walls
A slab that is too large can catch roof runoff and hold water near siding. If the shed needs a larger working pad, slope the exposed concrete and keep the shed wall line dry.
Separate gravel from concrete
A compacted stone base may be 3 to 6 inches deep. That stone is not part of concrete yardage, but it affects excavation depth, grade, and the finished threshold height.
Material checklist
- Concrete volume from slab length, width, and thickness
- 10% extra concrete for base variation, spillage, and form tolerances
- Compacted gravel base if soil, drainage, or the shed plan requires it
- Forms, stakes, vapor barrier when needed, anchors, and control-joint plan
Cost assumptions
The table uses $150 per cubic yard for ready-mix and $5 per 80-lb bag. It is a planning comparison, not a delivered quote.
- Ready-mix is usually easier once the slab approaches 1 cubic yard, but short-load fees can affect small shed pads.
- Bagged concrete is possible for small 8x8 pads, but a 10x12 slab can mean more than 70 80-lb bags once waste is included.
- Do not compare a bare concrete price with a contractor quote that includes excavation, base, forms, reinforcement, and finish.
Common mistakes
- Using the shed model name instead of the actual foundation footprint.
- Forgetting door ramp clearance, roof drip line, anchors, or local setback rules.
- Making the slab wide enough to collect water against wood siding.
- Ordering exact calculated volume with no allowance for uneven base depth.
Formula and methodology
Volume in cubic feet equals length x width x thickness in feet. Cubic yards equal cubic feet / 27. An 80-lb premix bag is estimated at 0.60 cubic feet. Ready-mix order size and bagged cost use a 10% buying allowance for field variation.