Cost = estimate from regional index, not a quote.
Enter your slab dimensions to get the concrete volume in cubic yards, the number of 40, 60, or 80 lb bags, and an estimated material cost for your area — with the formula and waste factor shown.
V (ft³) = length_ft × width_ft × (thickness_in ÷ 12)
Slab volume is length times width times thickness, with thickness converted from inches to feet, then divided by 27 to get cubic yards. To get bags, divide the volume by each bag's yield and round up.
For formed pours, ACI and RSMeans guidance puts spillage and over-pour at roughly 5–10%. We default to 10% so your order covers an uneven subgrade.
Below about one to two cubic yards, bagged concrete is usually practical. Above that, ready-mix tends to win on cost and labor.
Slab thickness is the biggest single driver of how much concrete you'll buy, and it's set by what the slab has to carry. A 4-inch slab is the residential standard for patios, walkways, shed floors, and garage floors carrying normal passenger vehicles. Step up to 5-6 inches for driveways that see heavier vehicles, and 6 inches or more for anything carrying trucks, RVs, or point loads.
Going from 4 to 6 inches isn't a small change — it's a 50% increase in concrete volume and cost, so it pays to confirm the spec before you order. At the same time, under-building a slab that cracks and fails costs far more than the extra concrete would have. When in doubt for a load-bearing slab, have the thickness specified by an engineer rather than guessing.
The calculator lets you enter any thickness, but the standard increments — 4, 5, 6, and 8 inches — cover the vast majority of residential pours.
The crossover between bagged and ready-mix concrete sits around one to two cubic yards, but the real decision involves more than volume. Bagged concrete makes sense when the pour is small, access is tight, or you're working alone and want to mix at your own pace. The downside is labor: a cubic yard is 45 bags of 80-pound mix, which is over 1.5 tons to haul, open, and mix by hand.
Ready-mix wins on everything above a yard or two: cost per yard drops, the mix is batched consistently at the plant, and a single truck can place several yards in minutes. The catch is that you must be ready — the truck waits for no one, and most plants charge a short-load fee below a full truckload of around 9-10 yards. There's also a time limit: concrete should be placed within about 90 minutes of batching before it starts to set.
For a middle-ground pour of 2-4 yards, get quotes both ways. Sometimes the short-load fee makes ready-mix and bags surprisingly close, and the labor savings tip it toward the truck.
Say you're pouring a two-car garage slab, 24 ft by 24 ft at 4 inches thick. First find the volume: 24 x 24 = 576 square feet, times 4 inches (0.333 ft) = 192 cubic feet. Divide by 27 and you get 7.11 cubic yards before waste.
At this size you are well past the bag-versus-ready-mix crossover, so you would order ready-mix. Add 10% for over-pour and an uneven subgrade and you'd order about 7.8 cubic yards. Most suppliers sell in quarter-yard increments and have a short-load fee below a full truck (around 9-10 yards), so rounding up to 8 yards is sensible here.
If you tried to bag this instead, 192 cubic feet at 0.6 cubic feet per 80 lb bag is 320 bags before waste, around 350 with waste. That is roughly 14 tons of concrete to mix by hand, which is why ready-mix wins above a yard or two.
Running out mid-pour is far worse than having a little extra. A cold joint from waiting on a second truck is a permanent weak line in the slab. The standard 10% waste covers this, but on a rough subgrade many contractors go to 12-15%.
Ready-mix plants charge a short-load fee below a full truck and may have a minimum order. If your job is 3-4 yards, ask about the fee — sometimes rounding up to avoid it costs the same.
Bag yields (0.30 / 0.45 / 0.60 ft³ for 40 / 60 / 80 lb) match Quikrete's published calculator and the ASTM C387 packaged-mix standard. A standard bagged mix reaches about 4,000 psi at 28 days.
About 45. A cubic yard is 27 cubic feet and each 80 lb bag yields roughly 0.60 cubic feet, so 27 ÷ 0.60 ≈ 45 bags before waste.
About 1.23 cubic yards before waste; roughly 62 bags of 80 lb mix with 10% waste.
One cubic yard covers about 81 square feet at 4 inches thick (27 cubic feet ÷ 0.333 ft). At 6 inches it covers about 54 square feet, and at 5 inches about 65 square feet.
Concrete is typically walkable in 24-48 hours and ready for vehicle traffic in about 7 days, but it continues gaining strength for 28 days, which is when its rated strength (such as 4,000 psi) is measured. Keep it moist during the first week for best results.
Most slabs benefit from reinforcement to control cracking. Wire mesh or fiber is common for patios and walkways; rebar on a grid is used for driveways and load-bearing slabs. Use the rebar calculator to estimate the steel.
A 3,000-4,000 psi mix covers most residential slabs. Driveways and garage floors are commonly 4,000 psi; sidewalks and patios can use 3,000-3,500 psi. Bagged mixes are usually around 4,000 psi at 28 days.
Batch plants round orders up to the next quarter or half cubic yard to ensure you do not run short and to suit delivery efficiency. A little extra is intentional — running out mid-pour creates a cold joint, so the rounding protects your slab.
Multiply the total square footage by the thickness in inches, then divide by 324. A 400 sq ft slab at 4 inches is 400 x 4 / 324 = 4.94 cubic yards, which should match the long method. The 324 is 12 inches per foot times 27 cubic feet per yard.