Concrete calculator: cubic yards, bags and cost
Enter the size of the pour and get the volume in cubic yards, the number of bags you would need to mix it by hand, and a rough concrete bill — before you commit to a pallet or a truck.
Concrete volume & bags
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Concrete only — gravel base, forms, rebar or mesh and labour are extra. Bag counts already include your waste allowance.
Concrete is the one material on this site where guessing is expensive in both directions. Order too little and you get a cold joint in the middle of the slab. Order too much and you pay for a partial yard you cannot return, because concrete starts setting the moment water hits it. The calculator exists to turn three measurements into one number you can read off a delivery ticket.
How this calculator works
- Slab volume = length × width × thickness. A 12 × 12 ft slab at 4 inches is 144 sq ft × 0.333 ft = 48 cubic feet.
- Footing volume = number of footings × length × width × depth. Six footings 4 ft long, 12 inches wide and 12 inches deep give 6 × 4 × 1 × 1 = 24 cubic feet.
- Waste is added before converting: 5% for a flat formed pour, 10% for a typical subgrade, 15% if the ground is uneven or the concrete is being pumped.
- Cubic yards = cubic feet ÷ 27. That is the unit ready-mix plants sell in.
- Bags = cubic feet ÷ yield per bag, rounded up. An 80 lb bag yields 0.60 cu ft, a 60 lb bag 0.45 cu ft, a 50 lb bag 0.375 cu ft.
The calculator shows both answers on purpose. Bags are what you can carry home today; cubic yards are what a truck delivers. The crossover point is around one cubic yard, which is about 45 bags of 80 lb mix.
Bags per cubic yard
| Bag size | Yield per bag | Bags per cubic yard | Typical price |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | $3.50–$5.50 |
| 50 lb | 0.375 cu ft | 72 | $4–$6.50 |
| 60 lb | 0.45 cu ft | 60 | $5–$7.50 |
| 80 lb | 0.60 cu ft | 45 | $5.50–$9 |
Multiply the bag price by the bag count and the appeal of bagged concrete usually disappears. A cubic yard mixed from 80 lb bags costs $250–$400 in material alone, plus a mixer, water and several hours of work. Ready-mix delivered runs $150–$210 per cubic yard in most US markets, with a short-load fee of $40–$150 on small loads.
Slab thickness by use
| Project | Thickness | Reinforcement |
|---|---|---|
| Garden path, stepping pads | 3 in | None needed |
| Patio, walkway, shed floor | 4 in | Wire mesh or fibre |
| Driveway, cars and light trucks | 5 in | #3 rebar grid or mesh |
| RV pad, heavy truck parking | 6 in | #4 rebar at 18 in centres |
| Garage slab | 4–5 in with thickened edges | Mesh plus 12 × 12 in edge beam |
| Commercial or point-loaded slab | 6–8 in | Engineered |
Thickness is the single biggest driver of volume. Going from 4 to 5 inches on the same 144 sq ft slab adds 12 cubic feet — about a third of a yard, or roughly 20 extra bags. If you are unsure, see how thick a concrete slab should be.
Footing mode
Footings are calculated differently because they are deep rather than wide. A deck post footing is often a 12 inch diameter tube 36–48 inches deep; a porch footing might be 18 × 18 inches and 12 inches deep. Enter the count, the length of each run and the cross-section, and the volume falls out the same way.
Depth is the number people underestimate. In cold climates the footing must sit below the frost line — 36–48 inches across most of the northern US, deeper in the far north — or frost heave will lift and crack whatever sits on top of it. The visible part above grade is not the pour.
Five mistakes that waste concrete
- Measuring the slab, not the base. The gravel base is wider than the slab and the forms sit outside the finished edge, so the pour is slightly bigger than the room dimensions.
- Ordering the exact volume. Subgrade settles, forms bow and some concrete stays in the chute. Five to ten percent extra is not waste, it is insurance.
- Adding water to stretch a load. Extra water makes concrete easier to place and weaker once cured. A 6 gallon bag mix needs about 3 quarts, not a hose.
- Forgetting the base. A 4 inch gravel base compacted in two lifts matters more than an extra inch of concrete on top of dirt.
- No control joints. Concrete cracks. Joints cut or formed at 2–3 ft per inch of thickness — 8–12 ft apart on a 4 inch slab — decide where it cracks.
Frequently asked questions
How many bags of concrete do I need for a 12x12 slab?
A 12x12 ft slab at 4 inches thick is 48 cubic feet, or 1.78 cubic yards. At 0.60 cubic feet per 80 lb bag that is 80 bags before waste, or 88 bags with a 10% allowance. That is more than one pallet, which is why most people order ready-mix for anything this size.
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. An 80 lb bag yields about 0.60 cubic feet when mixed, and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags. With 60 lb bags it takes 60 bags per yard, and with 50 lb bags it takes 72.
How thick should a concrete slab be?
Four inches for patios, walkways and shed floors. Five inches for a driveway carrying cars and light trucks. Six inches for RVs, heavy trucks or a garage that will see loaded trailers. Anything poured over poor soil or with heavy point loads should be engineered rather than guessed.
Is it cheaper to mix concrete by bag or order ready-mix?
Bagged mix is cheaper for pours under about half a cubic yard, roughly 23 bags of 80 lb. Between half a yard and one yard the two are close once you count the mixing time. Above one yard ready-mix wins on both price and quality, because you get consistent slump and no cold joints.
How much extra concrete should I order?
Add 5% for a flat, formed slab on level ground and 10% for anything with an uneven subgrade, curved edges or a pump. Never order the exact calculated volume: coming up short mid-pour creates a cold joint that cannot be fixed later.