Order too little concrete and you get a cold joint — a permanent weak seam through your slab. Order way too much and you've paid for a wheelbarrow of waste. This guide gets you to the right number, with every formula, table, and rule of thumb a real pour needs.
Every slab estimate is the same move: turn a flat area into a volume of concrete. Measure length and width in feet and thickness in inches, flip the thickness to feet by dividing by 12, multiply the three together for cubic feet, then divide by 27 for cubic yards — the unit ready-mix suppliers actually quote.
The two conversions trip people up, so keep them handy: a 4-inch slab is 4 ÷ 12 = 0.333 ft thick; a 6-inch slab is 0.5 ft. And to go the other way, multiply cubic yards by 27 — so 2.50 cu yd × 27 = 67.5 cubic feet.
20 × 20 = 400 sq ft. Thickness 4 ÷ 12 = 0.333 ft. Volume = 400 × 0.333 = 133.3 cu ft ÷ 27 = 4.94 cu yd. Add 10% waste → order 5.43 cu yd (a truck would round you to 5.5).
Here's how that scales for the slab sizes people ask about most, straight from the reference tables — exact volume plus the number you'd actually order with 10% waste baked in:
| Slab size (ft) | Thickness | Cubic feet | Cubic yards (exact) | + 10% waste |
|---|---|---|---|---|
| 10 × 10 | 4″ | 33.33 | 1.23 | 1.35 |
| 12 × 12 | 4″ | 48.00 | 1.78 | 1.96 |
| 20 × 20 | 4″ | 133.33 | 4.94 | 5.43 |
Most real slabs aren't a clean rectangle, but the trick never changes: break the footprint into simple shapes, calculate each, and add them up.
For a round pad — under a grain bin or a fire-pit patio — find the area with π × r², then multiply by thickness in feet and divide by 27 as usual. For an L-shaped or irregular slab, slice it into rectangles, get each volume, and sum them: a 12 × 20 section plus a 10 × 8 section is simply the two added together. This split-and-sum approach beats trying to "eyeball-average" the whole thing. And when the thickness changes — say 4″ over most of the slab but 6″ in a heavy-load zone — treat each depth zone as its own calculation and add the results.
Sloped or roughly excavated ground is where estimates fall short, because the real volume is bigger than any single depth reading suggests. The fix is to use an average depth rather than one number.
Plug that average into the core formula and you've accounted for the slope. This is also why a waste factor exists at all: the trench bottom quietly swallows concrete you never measured. Careful measurement shrinks the surprise, but you still carry margin.
A monolithic slab often has a thickened perimeter edge or an integral footing beam carrying the wall load — and forgetting it is a classic way to under-order. Estimate it in two parts. First, the main flat slab body at its surface dimensions and standard thickness. Second, the perimeter edge as a separate beam: its run around the perimeter × its width × the extra depth below the slab. Add the two. On a small slab a thickened edge can add 10–20% to the total — not a rounding detail.
With your volume in hand, the order depends on whether you're mixing bags or calling a truck. For bagged concrete, divide your total cubic feet by the bag's yield and round up to whole bags. These three yields cover almost every product on the shelf:
| Bag size | Yield per bag | Bags per cubic yard | Buy (rounded up) |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 45.0 | 45 bags |
| 60 lb | 0.45 cu ft | 60.0 | 60 bags |
| 50 lb | 0.375 cu ft | 72.0 | 72 bags |
Each 80 lb bag wants roughly 2.5–3 quarts of clean water. The practical bag-vs-truck line sits around 1 to 1.5 cubic yards: below it, hand-mixing is manageable; above it, the labor turns brutal (one cubic yard is 45 bags — well over 1.5 tons to haul and mix) and you risk a cold joint if you can't keep a continuous pour going. For anything bigger, order ready-mix.
Batch plants round orders up to the next ¼ or ½ yard for delivery efficiency and to guarantee you don't run short. That gap is normal — and it works in your favor, because running out mid-pour is far worse than a little leftover.
The slab is only part of the order. Most slabs sit on a compacted gravel base that uses the exact same volume math: a 4″ gravel base under a 10 × 10 area is the same 1.23 cu yd as the concrete above it, before compaction adjustments. Our gravel calculator handles the tonnage conversion.
For steel, a typical 10 × 10 slab with rebar on a 12″ grid set 3″ in from the edges takes roughly 18–20 pieces of 10 ft bar once you allow for overlaps at the splices. The rebar displaces a negligible amount of concrete, so don't subtract it from your volume — it's absorbed in the waste margin. Our rebar calculator gives an exact bar count and weight.
A waste factor is a deliberate safety margin covering everything that quietly eats concrete: subgrade depth variation, forms that flex outward under the hydraulic pressure of wet concrete, spillage during placement, and residue stuck in the mixer or truck chute. 10% is the default for a formed slab; on a rough subgrade many contractors push to 12–15%. A little extra concrete is cheap. Stopping a pour to wait for a second batch is not — that pause becomes a cold joint, a permanent weak line through the slab.
Thickness is set by what the slab carries:
| Application | Thickness | Notes |
|---|---|---|
| Sidewalks & patios | 4″ | Typical pedestrian foot traffic |
| Light shed bases | 4″ | With localized thickened edge borders |
| Residential driveways | 4–5″ | Use 5″ for larger SUVs / light trucks |
| Commercial / equipment pads | 6″+ | Engineered to the specific load |
For any outdoor slab, build in drainage: a minimum slope of about ¼ inch of fall per foot of run (roughly a 2% grade) directing water away from structures. A dead-flat slab pools water, which freezes and shortens its life.
Concrete doesn't dry, it cures, through a chemical reaction called hydration that takes weeks to finish.
| Time after pour | What's happened |
|---|---|
| 24–48 hours | Foot traffic generally OK |
| ~7 days | ~70% of design strength reached |
| 28 days | Full rated compressive strength (e.g. a 4,000 psi mix hits 4,000 psi) |
Keep the surface moist through that first week — cover or mist it — so hydration finishes properly and surface cracking stays down. Resist the urge to add water to the mix to make it easier to place: more water means weaker concrete, because final strength rides on the water-to-cement ratio.
Before you place an order, validate your math with this shortcut — it should land within a hair of your long-form answer.
If your detailed number and this shortcut disagree by more than a rounding error, recheck your inputs before the truck rolls.