A 20x20 slab is one of the most common pours in residential work — a shed floor, a patio, a hot-tub pad, a workshop foundation. It's also the exact size where getting the concrete order wrong gets expensive fast. Concrete doesn't wait: once the truck is on site and the pour is underway, coming up half a yard short means a panicked call for a short load, a cold joint where the fresh batch meets the setting slab, or a stalled crew burning daylight.
So let's get the number right. We'll calculate the concrete for a 20x20 slab from scratch, apply a waste factor that reflects real jobsites, and cover the one decision that matters most: ready-mix versus bags.
The Quick Answer
For a 20 ft × 20 ft slab (400 square feet), here's the concrete volume before and after a 10% waste factor:
| Thickness | Bare volume | With 10% waste | Order (rounded up) |
|---|---|---|---|
| 4 inches (standard) | 4.94 yd³ | 5.43 yd³ | 5.5 yd³ |
| 5 inches | 6.17 yd³ | 6.79 yd³ | 7 yd³ |
| 6 inches | 7.41 yd³ | 8.15 yd³ | 8.25 yd³ |
If you just want the number and the order-ready figure, the Concrete Calculator does this for any dimensions and waste factor in a couple of seconds. If you want to understand where these numbers come from — so you can trust them and adjust for your own slab — read on.
How the Math Works
Concrete is ordered by the cubic yard, so every slab calculation is really a volume conversion. Three steps:
- Find the volume in cubic feet. Multiply length × width × thickness — but thickness has to be in feet, so divide the inches by 12.
- Convert to cubic yards. There are 27 cubic feet in a cubic yard, so divide by 27. This is the unit the ready-mix plant uses.
- Add a waste factor. Multiply by 1 plus your waste percentage. This is the number you actually order.
Cubic feet = Length × Width × (Thickness in inches ÷ 12) Cubic yards = Cubic feet ÷ 27 Order (yd³) = Cubic yards × (1 + Waste % ÷ 100)
Worked Example: The Standard 4-Inch Slab
Four inches is the default thickness for patios, shed floors, and general-purpose slabs, so let's run it in full.
- Area: 20 × 20 = 400 square feet
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Cubic feet: 400 × 0.333 = 133.3 cubic feet
- Cubic yards: 133.3 ÷ 27 = 4.94 cubic yards
That 4.94 is the bare volume — the concrete that fits a perfectly formed, perfectly flat 4-inch slab with nothing lost. No real pour works that way, which is where the waste factor comes in.
- With 10% waste: 4.94 × 1.10 = 5.43 cubic yards
Ready-mix is sold in quarter-yard increments, so you'd order 5.5 cubic yards. That's the figure you give the plant.
The Waste Factor: Why 5–10% Is Realistic
A waste factor isn't padding for sloppiness — it's an honest accounting of where concrete actually goes on a real slab pour:
- Subgrade that isn't flat. A hand-dug or machine-graded base always has low spots. Every dip below your 4-inch plane quietly drinks extra concrete. This is the single biggest source of overage.
- Chute and wheelbarrow losses. Concrete left in the truck's chute, stuck in the barrow, or dropped in transit never makes it into the forms.
- Edge spillage and over-dig. Forms flex, edges slump, and over-excavated perimeters take more than the geometry says.
- Spec variation. A slab screeded a touch thick across 400 square feet adds up quickly — a quarter-inch of extra average depth on a 20x20 is nearly half a yard.
Use 10% for a hand-formed slab on rough or hand-dug ground, and you can trim toward 5% for a tightly formed slab on a clean, machine-graded, compacted base. When in doubt, round up. The cost of a little leftover concrete is trivial next to the cost of running short mid-pour — a short load fee, a cold joint, and a crew standing idle.
Ready-Mix or Bags? For a 20x20, It's Not Close
It's worth seeing why bagged concrete is a non-starter at this size. An 80 lb bag of concrete mix yields about 0.6 cubic feet; a 60 lb bag yields about 0.45. For the 4-inch slab with waste included (5.43 yd³, or 146.6 cubic feet):
- 80 lb bags: about 245 bags
- 60 lb bags: about 326 bags
Mixing 245 bags by hand is days of brutal work, and you can't keep a 400-square-foot pour monolithic that way — the first section sets before you reach the last, creating cold joints that crack. For any slab over roughly one cubic yard, order ready-mix delivered by truck. A standard mixer truck holds around 10 cubic yards, so your 5.5-yard order is a single, comfortable load. Bags are for fence posts and small repairs — not slabs.
Don't Forget What Goes Under and In the Slab
The concrete order is only part of a slab estimate. A slab that lasts needs a proper base and, usually, reinforcement:
- Compacted gravel base. Most slabs sit on 4–6 inches of compacted crushed stone for drainage and support. Size it with the Gravel Calculator — and remember gravel loses volume when compacted, so it has its own allowance.
- Rebar or mesh. A 20x20 slab is large enough that reinforcement matters. Lay out a grid with the Rebar Calculator to get bar counts by spacing.
For the full sequence — forming, base prep, placing rebar, pouring, and curing — see the how to pour a concrete slab guide.
What Will It Cost?
Ready-mix concrete runs roughly $120–$170 per cubic yard in most U.S. markets, so a 5.5-yard order for a 4-inch 20x20 slab lands somewhere around $700–$950 in concrete alone — before base gravel, reinforcement, forms, and any short-load or delivery fees. Prices vary widely by region and by the day, so check concrete prices by state for a figure closer to your area.
Get Your Exact Number in Seconds
Now you know exactly how the 4.94-and-5.5-yard figures come together — area, thickness, cubic-yard conversion, and a realistic waste factor. For your own slab, you don't have to run it by hand.
Drop your dimensions into the Concrete Calculator, set the thickness, add a 5–10% waste factor, and you'll get an order-ready volume in cubic yards — plus the bag count if you're pricing a smaller pour. It handles slabs, footings, and round columns, so it's the one tool you need from the shed pad to the driveway.
Written by Daniel McCarney — AceCalc