How Much Concrete Do I Need? Slab Calculator in Yards and Bags

Ordering concrete is the one DIY purchase where coming up short is a real problem: half a slab that hardens before the rest arrives leaves a cold joint. The calculator below includes a waste allowance for exactly that reason. Read on to see how thick your slab really needs to be.

Concrete slab calculator

Bag yields used here: an 80 lb bag of typical premixed concrete yields about 0.6 cubic feet, and a 60 lb bag about 0.45 cubic feet. Check the bag; yields vary by brand.

How the math works

1. Convert thickness to feet: inches ÷ 12.
2. Volume = length × width × thickness (in feet) = cubic feet.
3. Add waste: multiply by 1.05 to 1.10 for uneven ground, spills and form bulges.
4. Cubic yards = cubic feet ÷ 27.

Example: a 10 × 10 foot slab, 4 inches thick: 100 × 0.333 = 33.3 cubic feet. With 10% waste, 36.7 cubic feet = 1.36 cubic yards, or about 62 bags of 80 lb.

How thick should a slab be?

• Walkways and patios: 4 inches is the common standard for foot traffic.
• Shed bases, hot tubs: often 4 to 6 inches depending on load; follow the manufacturer's requirements.
• Driveways: commonly 4 to 6 inches, thicker for heavy vehicles.
• Fence and deck post footings: depth is set by your local frost line and code.

Local building codes and permits may set requirements for thickness, reinforcement and base. Organizations like the American Concrete Institute publish the technical standards professionals follow; for anything structural, consult local code or an engineer.

Bags or ready-mix truck?

• Under about 1 cubic yard: bags are practical. You can mix in a wheelbarrow or rent a small mixer.
• 1 to 2 cubic yards: a judgment call. That is 45 to 90 bags of 80 lb, which is a lot of mixing and lifting. Some suppliers offer trailer loads or mini-mix trucks.
• Over 2 cubic yards: ready-mix delivery is usually easier and gives a more consistent pour, though small loads may carry a short-load fee.

Prep checklist

• Permits checked (if required locally)
• Utilities marked before digging (call 811 in the US, or local service)
• Area excavated and base gravel compacted (commonly about 4 inches)
• Forms built, level and staked, with slight slope for drainage
• Reinforcement (wire mesh or rebar) in place, if used
• Volume calculated with waste allowance
• Helpers, tools and a weather window lined up

Slope and drainage

Patios and walkways should slope slightly away from the house so water drains. A common rule of thumb is around 1/8 to 1/4 inch per foot. Set your forms with this slope before pouring.

Pouring and finishing (overview)

1. Dampen the base (not puddled) so it does not suck water from the mix.
2. Pour and spread, starting at the far end.
3. Screed with a straight board in a sawing motion across the forms.
4. Float with a bull float or hand float to bring cream to the surface.
5. Edge and cut control joints (roughly every 8 to 10 feet for a 4-inch slab, or per local guidance) to control where cracking happens.
6. Broom finish for outdoor slip resistance.

Curing: the step that matters most

Concrete gains strength by curing, a chemical reaction that needs moisture. Keep the slab damp (mist, wet burlap or curing compound) for about a week, and protect it from freezing. Rushing curing is a leading cause of weak, dusty surfaces.

Other shapes

• Round slab or column: π × radius² × height.
• Steps: calculate each step as its own rectangular block and add them.
• Footings: length × width × depth for each, times the number of footings.

Common mistakes on first slabs

• Skipping the gravel base: concrete poured straight on soft soil settles unevenly and cracks.
• Forms not braced: wet concrete is heavy and pushes forms outward, which also uses more concrete than calculated.
• Adding water to make it easier to spread: it weakens the surface.
• Finishing too early: troweling while bleed water is still on the surface traps water and leads to flaking. Wait until the sheen disappears.
• No control joints: concrete will crack somewhere; joints decide where.
• Forgetting the weather: rain on fresh concrete ruins the finish, and hot wind dries it too fast. Have plastic sheeting on hand.

Mixing bagged concrete well

Consistency matters more than speed. Add about three-quarters of the water recommended on the bag to a wheelbarrow or mixer first, then the dry mix, then the rest of the water gradually until the concrete is like thick oatmeal: it should hold its shape when you form a ridge with a hoe, but not crumble. Too much water makes concrete easier to pour but weaker and more prone to cracking and dusting. Mix each batch the same way and pour them close together so they knit into one slab.

A practical rhythm for a two-person crew: one person mixes while the other places and spreads. A small rented drum mixer handles roughly two 80 lb bags per batch and saves your back on anything over a dozen bags.

Ordering ready-mix: what to tell the supplier

• The total cubic yards, including your waste allowance (round up to the nearest quarter yard).
• The use (patio, driveway, footings) so they can suggest a suitable mix and strength.
• Whether you want fiber reinforcement or air entrainment (common in freezing climates to resist freeze-thaw damage).
• Access details: how close the truck can get, and whether you need a pump or a wheelbarrow crew. Trucks are heavy and can crack driveways and sidewalks.

Have everything ready before the truck arrives: forms checked, tools laid out, helpers present. Ready-mix drivers usually allow a limited unloading time before extra charges.