The Formula Behind This Calculator
Volume (cubic feet) = length (ft) x width (ft) x thickness (ft)
Cubic yards = cubic feet / 27
Bags = volume with waste / yield per bag (80 lb = 0.60, 60 lb = 0.45, 40 lb = 0.30 cubic feet)
Concrete is sold by volume, so every estimate starts by turning your slab into cubic feet and then into cubic yards, the unit ready mix trucks use. Thickness has to be in feet for the multiplication, which is why 4 inches becomes 0.333 feet and why small thickness changes matter more than people expect: going from 4 to 5 inches raises the volume, and the bill, by 25 percent. Bag yields come straight from manufacturer packaging. An 80 lb bag of standard concrete mix makes about 0.60 cubic feet of placed concrete, a 60 lb bag about 0.45 and a 40 lb bag about 0.30. Divide your volume by the yield, round up to whole bags, and add waste for uneven ground, and you have the same number a supplier would give you at the counter.
Worked Example: A 10 by 12 Patio Slab
You are pouring a backyard patio, 10 feet by 12 feet, at the standard 4 inches thick, and you want a 10 percent waste allowance.
- Volume: 10 x 12 x (4 / 12) = 40 cubic feet.
- Cubic yards: 40 / 27 = 1.48 cubic yards.
- With waste: 40 x 1.10 = 44 cubic feet to actually have on site.
- Bags: 44 / 0.60 = 74 bags of 80 lb mix, or 44 / 0.45 = 98 bags of 60 lb, or 44 / 0.30 = 147 bags of 40 lb.
- Cost check: 74 bags at $5 to $7 is roughly $370 to $520. Ready mix at $140 to $190 per yard is roughly $210 to $285 for the concrete itself, plus delivery and a likely short load fee at this small volume.
That example shows the real decision this calculator helps with. It is not just how much concrete, it is which way to buy it. Seventy four 80 lb bags weigh close to three tons, and every one has to be lifted, mixed, carried and placed before the batch starts to stiffen. For this patio, most homeowners are better served by a ready mix truck and a couple of helpers with wheelbarrows.
A Complete Guide to Estimating Concrete for a Slab
Concrete forgives almost nothing. It arrives or it is mixed, the clock starts, and the amount you have is the amount you get. Underestimating is therefore the expensive direction to be wrong: a slab finished a yard short has a visible seam, a weaker plane where the two pours meet, and a color difference that never quite fades. Overestimating by a sensible margin costs a few bags you can return or a small overage charge on a truck. Every habit in this guide points the same way: measure carefully, round up, and plan for the ground to be slightly less perfect than your tape says it is.
Start With the Base, Not the Slab
The volume math assumes your excavation is exactly the thickness you entered, everywhere. Real soil is not. A compacted base of 2 to 4 inches of gravel does three jobs: it levels the site so the slab thickness stays honest, it drains water away from under the slab, and it reduces cracking from soft spots. If you skip the base and pour onto uneven dirt, the low spots silently consume your concrete and the high spots become the thin, weak parts of the slab. Pros screed the base to grade for exactly this reason. If your base is hand dug and roughly leveled, choose the 15 percent waste option and be glad for every spare bag.
Choosing a Thickness You Will Not Regret
Four inches handles foot traffic, patio furniture, walkways and shed floors comfortably, and it is the thickness most residential codes and suppliers treat as the default. Cars and pickup trucks are fine on 4 inches over a well compacted base, but many builders step up to 5 or 6 inches where heavy vehicles, delivery trucks or an RV will park, or where soil is clay heavy and moves with the seasons. Thickness is not the whole story: reinforcement matters just as much. Welded wire mesh or a grid of No. 3 or No. 4 rebar holds cracks tight so they stay hairline instead of opening and offsetting, and fiber reinforced mixes add a measure of the same protection. Control joints cut or tooled every 8 to 12 feet tell the slab where to crack, because it will crack somewhere, and a straight planned line looks infinitely better than a wandering random one.
Bags vs Ready Mix: The Honest Comparison
Bagged concrete wins on control. You mix what you need, when you need it, there is no minimum order and no truck clock running. For fence posts, a mailbox, a small equipment pad or a repair, bags are simply the right tool. The costs hide in your body and your afternoon: a cubic yard is 45 bags of 80 lb mix, each to be loaded at the store, loaded into your vehicle, unloaded, lifted into a mixer or wheelbarrow, and placed. A pallet delivery solves the store part and is worth the fee past about 20 bags. Ready mix flips the trade. The concrete is consistent, the placement is fast, and big volumes are cheaper per yard, but you will pay a delivery fee, often a short load fee under 4 or 5 yards, and you must have forms, base, reinforcement, tools and helpers completely ready when the truck arrives, plus a washout spot agreed in advance. As the worked example shows, the crossover for most homeowners sits between half a yard and a yard and a half.
What Concrete Costs in 2026
As of October 2026, an 80 lb bag of standard mix runs about $5 to $7 at national home centers, with 60 lb bags near $4.50 to $6 and 40 lb bags near $4 to $5.50, which is why small bags cost more per cubic foot. Ready mix concrete runs roughly $140 to $190 per cubic yard in much of the country before delivery, with delivery commonly $60 to $150 and short load fees on small orders. A professionally finished plain patio slab, including excavation, base, forms and finishing, typically lands between $8 and $15 per square foot in published contractor cost surveys. Use those figures to budget, then confirm with two local quotes, because concrete is one of the most regional prices in all of construction.
Pour Day Habits That Protect the Result
- Order or stage 10 percent more than the math says, and keep the receipt so unopened bags can go back.
- Never add extra water to make mixing or finishing easier. A wetter mix is a weaker slab; use the water amount printed on the bag.
- Pour in mild weather where you can. Hot, windy days steal moisture and rush your finishing; freezing nights can ruin a young slab.
- Cure it properly: keep the surface moist or covered for at least 3 to 7 days. Most of the strength concrete will ever have is decided in that first week.
- Wait before loading it: foot traffic after a day, cars after at least 7 days, and full design strength at about 28 days.