Concrete Calculator: Slab, Bags & Cost
Cubic yards, bag counts and cost for slabs, footings and columns. Free, no sign-up.
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Concrete estimating guide
Concrete is ordered by volume, in cubic yards for ready-mix or in bags that each make a known number of cubic feet. Getting the volume right means fewer half-empty bags and fewer emergency runs to the store before the pour sets.
The formula
Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12. Divide cubic feet by 27 to get cubic yards. The ÷ 12 turns inches into feet. Forgetting it is the most common reason an estimate comes out twelve times too big.
Worked example: 10 × 10 ft patio, 4 in thick
10 × 10 × 4 ÷ 12 = 33.3 cu ft. 33.3 ÷ 27 = 1.23 cubic yards. With the typical bag yields below that's 56 bags of 80 lb, 75 bags of 60 lb or 112 bags of 40 lb. With 10% extra for an uneven base and spillage it's 36.7 cu ft: 62 × 80 lb, 82 × 60 lb or 123 × 40 lb.
Bag yields and bags per cubic yard
| Bag | Typical yield | Bags per cubic yard |
|---|---|---|
| 80 lb | ≈ 0.60 cu ft | 45 |
| 60 lb | ≈ 0.45 cu ft | 60 |
| 40 lb | ≈ 0.30 cu ft | 90 |
Yields vary a little by brand and mix, so check the yield printed on the bag you're buying.
Footings and columns
A footing is a long rectangle measured in feet along its length and inches across its width and depth: length (ft) × width (in) ÷ 12 × depth (in) ÷ 12. A 20 ft footing 12 in wide and 12 in deep is 20 cu ft, about 0.74 yd³. A round column or sonotube is π × (diameter in ÷ 24)² × height (ft). A 12 in tube 4 ft tall is 3.14 cu ft, which is 6 bags of 80 lb after rounding up.
How thick should a slab be?
4 in is a common thickness for patios, walkways and shed floors. Driveways and slabs that carry heavier loads are often 5–6 in, usually with reinforcement and a compacted base. Local code, soil conditions and your engineer or contractor decide what's right for a structural job. This calculator only does the volume math.
Bags or ready-mix?
Mixing by hand or in a small mixer is practical up to roughly a cubic yard. Past that, a ready-mix truck is usually faster and gives a more consistent pour. Ask local suppliers about minimum orders and short-load fees, then compare using the cost fields above.
Planning checklist
- Measure the formed area after the forms are set, not the sketch. Forms that end up a few inches larger add up over a whole slab.
- Check the base. Compacted gravel under a slab is common practice, and an uneven base changes the real thickness. Measure depth at several points.
- Plan the pour sequence. With bags, decide how many you can mix and place before the first batch stiffens, and where control joints will go.
- Weather. Hot, windy or freezing conditions affect curing. Follow the instructions on the bag or the advice of your ready-mix supplier.
Common mistakes
| Mistake | What happens | Fix |
|---|---|---|
| Multiplying by inches | Volume 12× too high | Divide thickness in inches by 12 |
| Forgetting ÷ 27 | Ordering cubic feet as yards | Cubic feet ÷ 27 = cubic yards |
| No extra for waste | Running short mid-pour | Add 5–10% |
| Rounding bags down | A few bags short | Always round up |
| Ignoring a sloped base | Thick spots use more | Measure depth in several places and average |
More: bags of concrete for a 10×10 slab, concrete for fence posts, bags per cubic yard, gravel calculator for the base.
How it works
Slab: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet. Footing: length (ft) × width (in) ÷ 12 × depth (in) ÷ 12. Column: π × (diameter in ÷ 24)² × height (ft). Multiply by quantity, add waste, then divide by 27 for cubic yards.
Bag counts use typical premixed yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³, rounded up. Check the yield printed on your brand's bag.
FAQ
How do I calculate cubic yards of concrete?
Multiply length (ft) by width (ft) by thickness (in) divided by 12 to get cubic feet, then divide by 27. A 10 × 10 ft slab 4 in thick is 33.3 cu ft, or about 1.23 cubic yards.
How many 80 lb bags of concrete do I need?
An 80 lb bag of premixed concrete yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft and a 40 lb bag about 0.30 cu ft. Divide your total cubic feet by the yield and round up. A 10 × 10 ft, 4 in slab needs about 56 bags of 80 lb.
How much extra concrete should I order?
Most contractors add 5 to 10 percent for spillage, uneven subgrade and form bowing. Use the waste field to add it.
Bags or ready-mix?
Bags are practical for small jobs up to roughly 1 cubic yard. Above that, ready-mix delivery is usually cheaper and faster. Check your supplier's minimum order.
Is a 4 inch slab thick enough?
4 in is common for patios, walkways and shed floors. Driveways and heavier loads are often 5–6 in. Follow local code and your contractor for structural work.
How many bags of concrete are in a yard?
At typical yields, a cubic yard (27 cu ft) is 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb.