Concrete Shed Foundation Calculator
A 10×12 ft shed foundation at 4 inches thick requires 1.48 cubic yards of concrete — about 74 bags of 80-lb mix, manageable as a weekend DIY project. Enter your shed dimensions for exact volume, bag count, and material cost.
Pro Tips
- →Shed pads typically need 4 inches of concrete
- →Make the pad 6-12 inches larger than your shed footprint
- →Include wire mesh reinforcement for crack prevention
- →Ensure proper gravel base and compaction
- →Level the site carefully before pouring
Includes 10% waste factor
That's typically a professional pour. See costs ↓
4 short emails from Dave: what a fair quote should land at for your slab, the scope changes that swing it ±$500, and whether DIY is actually cheaper at your volume. Reply anytime — he'll review your real quote.
Cost Estimate
Estimated material costs for your project
Recommendation: Ready-Mix Concrete
For projects over 1 cubic yard, ready-mix is typically more economical and easier to work with.
74 bags × 80lb
1.63 cubic yards + delivery
120 sq ft × $3.00–$8.00/sq ft
Prices vary by location and time. Contact local suppliers for accurate quotes.
For general step-by-step instructions, read our complete How Thick Should Concrete Be and Gravel Under Concrete.
Choosing a Shed Foundation Type
The right shed foundation depends on shed size, soil stability, slope, and how hard your ground freezes. A poured concrete slab is the most common spec because it doubles as a finished floor, but it is not always the best fit. The table below compares the four mainstream options.
| Foundation Type | Typical Cost | Best For | When to Avoid |
|---|---|---|---|
| Poured concrete slab | $$$ | 8×10 to 12×20 sheds, workshops, anything heavy or permanent | Steep slopes, where you may relocate the shed |
| Pier / concrete footing | $$ | Sloped or uneven sites, elevated wood-floor sheds | Sites needing a finished floor surface |
| Gravel pad | $ | Small resin/metal sheds up to ~8×10 on level ground | Workshops, heavy storage, soft clay soil |
| Perimeter (stem-wall) footing | $$$$ | Large sheds, frost-prone regions, attached structures | Small budget sheds; overkill under 120 sq ft |
For most homeowners building an 8×10 to 12×16 shed or workshop, a 4-inch poured slab over compacted gravel is the sweet spot: durable, anchorable, and code-friendly. This page focuses on specifying that slab and its volume. If your priority is total installed price, use the concrete shed base cost calculator.
Concrete Volume by Shed Size
For a slab foundation, concrete volume is simply pad length × width × thickness. The table below shows ready-mix cubic yards and 80-lb bag counts for the most common shed sizes at 4 inches, sized to a pad 6 inches larger than the shed on each side (a 10×12 shed gets an 11×13 pad). All figures include a 10% waste buffer.
| Shed Footprint | Pad Size | Slab Area | Cubic Yards (4 in) | 80-lb Bags |
|---|---|---|---|---|
| 8×8 ft | 9×9 ft | 81 sq ft | 1.10 cu yd | 50 bags |
| 8×10 ft | 9×11 ft | 99 sq ft | 1.34 cu yd | 61 bags |
| 10×12 ft | 11×13 ft | 143 sq ft | 1.94 cu yd | 88 bags |
| 12×16 ft | 13×17 ft | 221 sq ft | 3.00 cu yd | ≥135 bags — use ready-mix |
| 12×20 ft | 13×21 ft | 273 sq ft | 3.71 cu yd | ≥167 bags — use ready-mix |
Bagged mix is practical up to roughly 1.5 cubic yards (about 60-70 bags). Above that, ordering ready-mix is cheaper and far less labor. For a deeper breakdown, run your exact dimensions through the concrete slab calculator or the general concrete cost calculator.
Slab Thickness and Reinforcement
A 4-inch slab is correct for the vast majority of sheds. This satisfies the IRC R506.2.3 minimum slab-on-ground thickness of 3.5 inches with a comfortable margin. Increase thickness only when load or span demands it.
| Shed Use | Slab Thickness | Reinforcement |
|---|---|---|
| Standard storage (mowers, tools, bikes) | 4 in | 6×6 W1.4 wire mesh or fiber mix |
| Workshop with benches and power tools | 4-5 in | Wire mesh; #3 rebar grid for large spans |
| ATV / tractor / vehicle storage | 5-6 in | #3-#4 rebar at 18 in on center |
| Large shed (over 200 sq ft) on soft soil | 5 in + thickened edge | Rebar grid + thickened perimeter |
Each additional inch of thickness adds roughly 25% more concrete volume, so jumping from 4 to 6 inches on a 12×16 pad adds about 1.5 cubic yards. See the 4-inch vs 6-inch comparison and our when to use rebar guide before upgrading.
The Thickened-Edge (Monolithic) Detail
For sheds larger than about 12×12, or any shed in a freeze-thaw climate, a thickened perimeter edge dramatically improves performance without pouring a full thick slab. This is a monolithic slab: the footing and floor are cast in one pour.
- Field thickness: keep the interior at 4 inches
- Edge turndown: thicken the perimeter to 8-12 inches deep and 8-12 inches wide
- Edge reinforcement: run two continuous #4 rebar bars in the turndown
- Why it works: the stiffened edge carries the concentrated wall loads, resists curling, and reduces edge cracking from frost movement
The thickened edge adds only the turndown-trench volume while giving the slab the behavior of a much thicker pour. Account for it when ordering: a 12×16 monolithic pad with a 12-inch turndown needs roughly 0.4-0.6 cubic yards beyond the flat-slab figure.
Gravel Base and Vapor Barrier
No shed slab should be poured directly on soil. The layered base below the concrete controls drainage, frost, and moisture.
| Layer | Spec | Purpose |
|---|---|---|
| Subgrade | Stripped of topsoil, compacted | Stable bearing |
| Gravel base | 4-6 in compacted crushed stone | Drainage, frost relief, even support |
| Vapor barrier | 6-10 mil polyethylene | Blocks ground moisture wicking up |
| Concrete | 4 in min, mesh or fiber | The slab itself |
A compacted 4-6 inch gravel base is the single most important defense against settling and frost heave. A 6-10 mil poly vapor barrier keeps stored items dry and is essential if the shed is heated or used as a workshop. Budget roughly 1-1.5 tons of crushed stone per 10×12 pad; see the gravel under concrete guide for depth math.
Frost: Floating Slab vs. Frost-Protected
In cold regions, how the slab handles frost determines whether it cracks. Two approaches are code-accepted for sheds:
- Floating slab: the most common shed solution. The slab sits on 6+ inches of gravel and is free to rise and settle as a unit with seasonal frost. Most building departments exempt small accessory sheds (commonly under 120 or 200 sq ft) from frost-depth footings, permitting a floating slab.
- Frost-protected / footing below frost line: required for large sheds, attached structures, or where local code mandates it. Footings extend below the local frost line per IRC R403.1.4, or insulation is added as a frost-protected shallow foundation.
Always confirm requirements with your building department before pouring, since frost-line depth and exemption thresholds are local.
Anchoring and DIY Feasibility
Once the slab cures (about 7 days before anchoring), fasten the shed down to resist wind uplift. Use 3/8 or 1/2-inch wedge anchors through the bottom plate or base rail, placed within 12 inches of each corner and roughly every 4 feet along the walls. Cast-in J-bolts or anchor straps set in wet concrete are an alternative if you plan the layout before pouring.
A shed slab is one of the most DIY-friendly concrete projects: the form is a simple rectangle and most sizes stay under 2 cubic yards. A 10×12 pad is a realistic one-weekend job with two or three helpers. Step up to a contractor or ready-mix once volume exceeds about 1.5-2 cubic yards, the design calls for a monolithic edge, or code requires inspected frost footings. To compare DIY versus hired-out price, use the concrete shed base cost calculator, then enter your final dimensions in the calculator above for an exact volume and bag count.