Concrete Shed Base Calculator
Size a poured concrete slab foundation for a storage shed, playhouse, or outbuilding.
Concrete needed
1.65 yd³
for a 10 × 12 ft shed base
- Slab footprint
- 120 ft²
- Cubic feet
- 44 ft³
- Cubic yards
- 1.65 yd³
- Bags needed
- 74 bags
Short answer
A common 10×12 foot shed base poured 4 inches thick needs about 1.65 cubic yards of concrete, or roughly 74 bags of 80 lb premix. Larger sheds or bases meant to hold heavy equipment should be poured thicker and often benefit from wire mesh reinforcement.
Recommended thickness by shed size
| Shed size | Typical thickness | Notes |
|---|---|---|
| 8×10 ft or smaller | 3.5–4 in | Light storage, garden tools |
| 10×12 to 12×16 ft | 4 in | Standard residential storage shed |
| 12×20 ft or larger, workshop use | 4–6 in | Heavier equipment, add wire mesh |
How to plan and pour a shed base
- Confirm the shed’s footprint dimensions from the manufacturer or your build plans, and add a few inches of margin on each side if you want the slab slightly larger than the shed.
- Excavate and compact the area, then add a 3–4 inch gravel base for drainage under the slab.
- Enter your slab dimensions and thickness above to get the concrete volume and bag count.
- Form the perimeter, place wire mesh if the slab will support heavy equipment, then pour and screed level.
- Slope the surrounding grade slightly away from the slab so water drains away from the shed.
- Let the slab cure for at least a few days — ideally closer to a week — before setting the shed on it.
Worked example
- Slab size
- 10 ft × 12 ft × 4 in thick
- Volume
- 10 × 12 × 4/12 = 40 ft³, plus 10% waste = 44 ft³
- Bags (80 lb, 0.60 ft³ each)
- ceil(44 / 0.60) = 74 bags
Common mistakes
- Skipping the gravel base, which lets the slab settle or heave over time, especially in freeze-thaw climates.
- Pouring the slab flush with surrounding grade instead of slightly raised, letting water pool against the shed’s base.
- Undersizing the slab thickness for a heavy workshop or equipment shed, leading to cracking under load.
- Setting the shed on the slab before the concrete has cured enough to bear the full weight.
Assumptions
- Standard premix concrete at roughly 150 lb per cubic foot.
- Bag yields follow manufacturer averages: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 50 lb = 0.375 ft³, 40 lb = 0.30 ft³.
- Slab is treated as a flat rectangle matching your entered dimensions.
- Does not include gravel base material, wire mesh, forms, or vapor barrier.
Frequently asked questions
How thick should a shed base be?
4 inches is standard for most residential storage sheds. Larger sheds or those used as a workshop with heavy equipment often benefit from 5 to 6 inches with wire mesh reinforcement.
Do I need a gravel base under a shed slab?
Yes, in most cases. A 3 to 4 inch compacted gravel layer improves drainage under the slab and reduces the chance of frost heave or settling over time.
What size base do I need for my shed?
Match the base to your shed’s footprint, or add a few inches of margin on each side. Check your shed manufacturer’s foundation requirements, since some specify a minimum base size or thickness.