Shed & Slab Calculator — Concrete, Gravel & Rebar in One Tool
Most concrete calculators stop at cubic yards. This one covers the whole slab: pick a shed size preset or enter your own dimensions, and get concrete volume, gravel sub-base, rebar or wire-mesh reinforcement, and a full cost breakdown — for sheds, AC pads, generator pads, hot tub pads, fire pit pads, dog houses, playhouses, and mailbox post footings.
Calculate your slab
No account needed. Sign up only if you want to save results.
One of the most common shed sizes.
44 ft³ · includes 10% waste factor
Editable material prices, not live local pricing — confirm with your supplier before ordering.
What the generic concrete calculators leave out
We checked the concrete calculators on Inch Calculator and Omni Calculator, plus the most feature-complete dedicated shed-base calculator we could find. Most skip gravel sub-base and reinforcement entirely, and the one that has them gates your results behind an email address.
| Feature | ConstructioncostAI | Inch Calculator | Omni Calculator | Best shed-base tool found |
|---|---|---|---|---|
| Basic concrete volume (cu yd / cu ft) | ||||
| Bags-needed calculation | ||||
| Ready-mix order rounding | — | — | — | |
| Gravel sub-base calculation | — | — | ||
| Rebar grid / wire mesh calculation | — | — | ||
| Shed & structure size presets | — | — | ||
| Circular pad support | — | — | ||
| Full project cost breakdown | — | — | ||
| Results free with no email required | — |
Checked directly against Inch Calculator, Omni Calculator, and concretecalculate.com's shed base calculator.
How much concrete do you need for a shed slab?
For a standard 4-inch slab, concrete volume scales directly with footprint. Here's what the most common shed sizes need before waste factor — the calculator above adds a 10% waste allowance by default, which you can adjust.
| Shed size | Footprint | Concrete @ 4in | 60 lb bags |
|---|---|---|---|
| 8×10 ft | 80 sq ft | 1.09 cu yd | 66 bags |
| 10×12 ft | 120 sq ft | 1.63 cu yd | 98 bags |
| 10×16 ft | 160 sq ft | 2.17 cu yd | 131 bags |
| 12×16 ft | 192 sq ft | 2.61 cu yd | 157 bags |
| 12×20 ft | 240 sq ft | 3.26 cu yd | 196 bags |
| 12×24 ft | 288 sq ft | 3.91 cu yd | 235 bags |
4 inches vs. 6 inches — which thickness?
4 inches is the standard for most sheds, garden buildings, and light storage. Go to 6 inches for a hot tub pad, a workshop that will hold heavy equipment, or anywhere a vehicle might drive or park on the slab — the extra 2 inches meaningfully increases load capacity and crack resistance.
Why a gravel sub-base matters
A compacted 4-inch layer of crushed gravel under the slab does three things a lot of DIY pours skip: it improves drainage so water doesn't pool and freeze under the slab, it reduces frost-heave risk in cold climates, and it gives you a level, stable surface to pour onto instead of raw soil that can settle unevenly. It's the single most common thing missing from small DIY slabs that crack or shift within the first year or two.
Bagged mix vs. ready-mix delivery
Under about 2 cubic yards, mixing bags yourself (or with a rented mixer) is usually the cheaper and more practical option. Above that, a ready-mix truck is worth the delivery minimum — it pours in minutes instead of hours of hand-mixing, and the finish is more consistent. The calculator above switches between both and rounds ready-mix orders to a realistic 0.25-yard increment, matching how most suppliers actually quote.
Wire mesh vs. rebar
Welded wire mesh is the standard, lower-cost choice for sheds and light-duty pads — it's quick to lay in a single sheet and controls surface cracking well. A rebar grid costs more in materials and labor but handles heavier loads and shifting soil better, which is why the calculator recommends it by default for hot tub pads and larger structures.
Frequently asked questions
How much concrete do I need for a shed slab?
For a standard 4-inch-thick slab with a 10% waste factor, a 10x12 shed needs about 1.63 cubic yards of concrete, and a 12x16 shed needs about 2.61 cubic yards. Use the calculator above with your exact dimensions — it also lets you switch between bagged mix and ready-mix delivery.
What thickness should a shed slab be?
4 inches is standard for most sheds, tool storage, and garden buildings. Use 6 inches for heavier loads — hot tubs, workshops with equipment, or any structure that will see vehicle or heavy machinery weight.
Do I need a gravel base under a shed slab?
Yes, in most cases. A 4-inch compacted gravel sub-base improves drainage, reduces frost heave risk in cold climates, and gives the slab a stable, even surface to cure on. Skipping it is one of the most common reasons small DIY slabs crack or settle unevenly.
Should I use rebar or wire mesh for a shed slab?
Wire mesh (welded wire fabric) is sufficient for most sheds and light-duty pads and is cheaper and faster to place. A rebar grid is worth the extra cost for larger slabs, heavier loads, or soil that's prone to shifting. Either is better than no reinforcement.
How many bags of concrete for a 10x12 shed slab?
At 4 inches thick with a 10% waste factor, a 10x12 slab needs roughly 98 bags of 60 lb concrete mix, or 74 bags of 80 lb mix. For anything over about 2 cubic yards, ready-mix delivery is usually cheaper and far less labor than mixing bags by hand — the calculator shows both options.
Is this calculator really free?
Yes — the calculator above is free with no signup, no credit card, and no usage limit. Creating a free account lets you save and revisit your slab calculations from your dashboard.