The Quick Answer
A 10×10 slab at 4″ thick needs 1.23 cubic yards of concrete — about 56 eighty-pound bags, or roughly 74 sixty-pound bags if that's what your store stocks. That single number is enough to know whether you're buying a few bags or calling for a ready-mix truck. But it's only a starting point. The thickness you pour, the waste you build in, and the sub-base underneath all move the total, so the right answer for your patio, driveway, or shed pad may be quite different. Here's how to get it right for your specific project — with a worked example you can follow start to finish.
How to Calculate Concrete for Any Slab
Concrete is sold by volume, so every slab estimate comes down to length × width × thickness, converted into cubic yards. We'll use a 12×20 ft patio as the running example through each step.
- Measure length and width. Measure the footprint in feet. Our patio is 12 ft by 20 ft, which is 12 × 20 = 240 square feet. For L-shaped or irregular pours, break the area into rectangles, calculate each, and add them together.
- Determine thickness. A patio is typically poured 4″ thick. Thickness is the single biggest lever on your total volume — see the thickness guide table below to match your project to the right depth before you go further.
- Calculate volume. Use length × width × (thickness ÷ 12) to get cubic feet. Dividing thickness by 12 converts inches to feet so the units line up. For our patio: 240 × (4 ÷ 12) = 240 × 0.333 = 80 cubic feet.
- Convert to cubic yards. Concrete is ordered by the cubic yard, and one cubic yard is 27 cubic feet. So 80 ÷ 27 = 2.96 cubic yards.
- Add 10% waste. Spillage, uneven sub-grade, and over-dig at the edges mean you always use more than the theoretical volume. Adding 10% gives 2.96 × 1.10 = 3.26 cubic yards — round up to about 3.5 cubic yards when you order.
- Choose bags or ready-mix. At roughly 3.5 cubic yards, this patio is firmly ready-mix territory. In bags it would take about 147 eighty-pound bags — a punishing amount of hand-mixing. More on that decision below.
How Thick Should Your Slab Be?
Thickness is set by the load the slab carries, not by personal preference. Pour too thin and the slab cracks under weight; pour too thick and you waste concrete. These are the standard residential thicknesses and the reinforcement each one calls for.
| Application | Recommended Thickness | Reinforcement |
|---|---|---|
| Walkway / garden path | 3–4″ | Optional wire mesh or fiber mix |
| Patio | 4″ | Wire mesh or #3 rebar grid |
| Driveway (cars) | 4–6″ | #3–#4 rebar grid |
| Garage floor | 4–6″ | #4 rebar grid + control joints |
| Shed / storage pad | 4″ | Wire mesh |
| Hot tub pad | 6″ | #4 rebar grid (heavy point load) |
Notice how a driveway at 6″ uses 50% more concrete than the same area at 4″. If you bump our 12×20 patio from 4″ to 6″, the volume jumps from 2.96 to 4.44 cubic yards before waste — proof that thickness deserves a careful decision rather than a guess.
Bags vs Ready-Mix Concrete
Once you know your cubic yards, the delivery method almost decides itself. The dividing line is roughly 1 to 2 cubic yards.
Use bags for small projects — under about 1 cubic yard. A shed pad, a small landing, or a set of footings is easy to handle a bag at a time. An 80-pound bag yields about 0.6 cubic feet, so it takes 45 bags to make a cubic yard. At that scale you control the pace, buy only what you need, and skip the delivery minimum a truck charges.
Call for a ready-mix truck for anything over 1–2 cubic yards. A truck shows up with consistent, properly proportioned concrete and pours it in minutes. Our 12×20 patio would need roughly 147 eighty-pound bags mixed by hand — and that's the part people underestimate. Mixing 50-plus bags in a wheelbarrow or rented mixer is genuinely brutal, hours of heavy, time-sensitive labor where the first batches start curing before the last ones are poured. Check your bag count in the Concrete Slab Calculator before you decide; seeing "147 bags" on screen makes the case for a truck on its own.
On cost: bagged concrete runs roughly $4–$7 per 80-pound bag, which pencils out to about $180–$315 per cubic yard before your time. Ready-mix delivered is often $140–$200 per cubic yard, plus a short-load fee for orders under the truck's minimum (usually 1 cubic yard). For small jobs bags win; for a patio or driveway, ready-mix is usually both cheaper per yard and far less work.
What Goes Under a Concrete Slab
A slab is only as good as what it sits on. Skipping the base layers is the fastest way to end up with a cracked, heaving slab a few winters later.
- 4–6″ of compacted gravel base. A well-compacted gravel sub-base spreads the load and gives water somewhere to drain instead of pooling and freezing under the slab. Compact it in lifts with a plate compactor before you pour.
- A vapor barrier for enclosed slabs. Any slab inside a building — a garage, basement, or finished shed — needs 6-mil poly sheeting over the gravel to stop ground moisture from wicking up through the concrete and into your floor.
- Rebar or wire mesh for structural slabs. Driveways, garage floors, and hot tub pads need a steel grid set in the middle third of the slab thickness so the concrete can carry tension, not just compression. Patios and walkways can often use lighter wire mesh or a fiber-reinforced mix.
Common Mistakes
- Pouring without a gravel base. Concrete poured straight on dirt has no drainage and no stable footing. The first freeze-thaw cycle lifts and cracks it. Always build the compacted gravel base first.
- Skipping reinforcement. An unreinforced driveway or garage floor will crack under vehicle loads. Mesh and rebar are cheap insurance against a repair that costs far more than the steel ever did.
- Not ordering enough — and creating a cold joint. If you run out mid-pour and scramble for more, the first concrete starts setting before the rest arrives. The seam where old meets new is a weak "cold joint." This is exactly why you add 10% waste and round up.
- Pouring in extreme heat without shade. High heat flash-cures the surface before the slab is finished, causing cracks and a weak top layer. Pour early in the day, keep the concrete damp, and shade it while it cures.
Bringing It Together
Run your own numbers through the same six steps: measure, set a thickness from the table, calculate volume, convert to cubic yards, add 10%, then choose bags or a truck. The Concrete Slab Calculator does all of it instantly, including the bag count and reinforcement, and the Gravel Calculator handles the sub-base underneath. For post footings instead of slabs, see our Fence Post Concrete Calculator, which sizes the bags per hole. Plan it once on paper, order with a little margin, and you'll pour a slab that lasts decades.