Yardtally is reader-supported. Some links on this page are affiliate links. As an Amazon Associate, Yardtally earns from qualifying purchases. See our Affiliate Disclosure.
Use this free concrete calculator to figure out exactly how much concrete your project needs. Pick a slab, footing, round column, or set of steps, enter your measurements, and you’ll get cubic yards plus the number of 80, 60, or 40 lb bags to buy.
How much concrete do you need?
Pick a shape, enter your measurements, and get cubic yards plus the number of bags to buy.
That’s a lot of bags. For this much concrete, ready-mix delivered by truck is usually faster and often cheaper. Call a local supplier and order by the cubic yard.
Bag counts are rounded up. Yields: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 40 lb = 0.30 cu ft.
| Slab size | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 × 4 ft | 0.20 | 9 | 12 |
| 10 × 10 ft | 1.23 | 56 | 75 |
| 12 × 12 ft | 1.78 | 80 | 107 |
| 10 × 20 ft | 2.47 | 112 | 149 |
| 12 × 24 ft | 3.56 | 160 | 214 |
Enter your exact measurements above for a precise number. Add 10% for waste and uneven ground.
How to Use the Concrete Calculator
The concrete calculator above works in 3 easy steps:
- Pick your shape. Choose slab for patios, walkways, and shed pads; footing for wall footings and trenches; round column for sonotube piers and deck footings; or steps.
- Enter your measurements. Length and width go in feet. Thickness, depth, and diameter go in inches, since that’s how you’ll measure them with a tape.
- Choose your waste factor. Leave it at 10 percent unless you have a reason not to. Uneven ground, a form that bows out, or a little spillage adds up fast.
The result shows cubic yards, which is how ready-mix trucks sell concrete, and the number of bags in each common size. If your project needs a yard or more, the calculator will point out that a truck delivery may be the better choice.
For fence posts, use our concrete per fence post calculator instead. It subtracts the space the post takes up and checks your state’s frost line depth.
How Thick Should Concrete Be?
It depends on what the concrete has to hold up. A few general rules of thumb:
- Walkways, patios, and shed pads: 4 inches is the most common thickness
- Driveways: usually 4 to 6 inches, thicker if heavy trucks or RVs will park on it
- Bagged concrete minimum: Quikrete says its standard concrete mix is designed for pours at least 2 inches thick
Your local building code may require a specific thickness, reinforcement, or base for driveways, steps, and footings, so check with your city or county before you pour anything structural. Source: Quikrete — Concrete Mix No. 1101 Data Sheet
How Many Bags of Concrete in a Yard?
One cubic yard equals 27 cubic feet. Based on Quikrete’s published yields, here’s how many bags of concrete it takes to make one yard:
- 80 lb bags yield about 0.60 cubic feet each, so you need 45 bags per yard
- 60 lb bags yield about 0.45 cubic feet each, so you need 60 bags per yard
- 40 lb bags yield about 0.30 cubic feet each, so you need 90 bags per yard
The concrete calculator uses these same yields and always rounds up, because coming up one bag short in the middle of a pour is a bigger problem than having one left over. Source: Quikrete — Concrete Mix No. 1101 Data Sheet
Bags or Ready-Mix Truck?
Bags make sense for small jobs: a few fence posts, a landing pad, a footing for a deck post, or a small walkway. You can mix them in a concrete mixing tub with a hoe, or speed things up with a handheld electric cement mixer and move it with a dual-wheel wheelbarrow that won’t tip under the weight.
Once you get past about a cubic yard, that’s 45 bags of 80 lb concrete, or 3,600 pounds to lift, mix, and pour by hand. At that point ready-mix delivered by truck is usually faster and often cheaper. It also arrives already mixed, so the whole slab sets up evenly instead of in batches. Call a local supplier, give them the cubic yards from the calculator, and ask about their minimum order.
How the Concrete Calculator Does the Math
If you want to check the numbers yourself, here’s the math behind each shape:
- Slab or footing: length × width × thickness, with thickness converted from inches to feet (divide by 12)
- Round column: 3.14 × radius × radius × height, all in feet
- Steps: each step is figured as a solid block from the ground up, so a 3-step stair is 1 + 2 + 3 = 6 risers’ worth of concrete times the step width and tread depth
Then the total is multiplied by your waste factor, divided by 27 to get cubic yards, and divided by each bag’s yield to get the bag count. For a 10 × 10 slab at 4 inches thick, that’s 33.3 cubic feet, or 1.23 cubic yards, which is 56 bags of 80 lb concrete before waste.
Not sure of your square footage? Our square footage calculator handles L-shapes, circles, and odd-shaped areas.
Tips for a Better Pour
Getting the amount right is half the job. A few tips from Quikrete’s instructions keep the concrete itself from cracking or failing:
- Fill the form to full depth in one pour. Partial fills can leave weak cold joints between layers.
- Cut control joints about 1 inch deep every 6 to 8 feet so the slab cracks where you want it to.
- Keep it moist while it cures, about 5 days in warm weather and 7 days when it’s cooler, using plastic sheeting, wet burlap, or a cure-and-seal product.
- Protect new concrete from freezing for the first 48 hours. In the fall, watch the forecast before you pour.
Source: Quikrete — Concrete Mix No. 1101 Data Sheet
Most slabs also go on a compacted gravel base so water drains away instead of sitting under the concrete. Our gravel calculator tells you how much base material to order. If you’re fixing old concrete instead of pouring new, see how to repair sidewalk cracks.
Frequently Asked Questions About the Concrete Calculator
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 slab 4 inches thick takes 1.23 cubic yards of concrete. That’s 56 bags of 80 lb concrete or 75 bags of 60 lb concrete before waste, or about 62 bags of 80 lb concrete with 10 percent extra.
How many 80 lb bags of concrete are in a yard?
About 45. Each 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet.
How much area does a yard of concrete cover?
At 4 inches thick, one cubic yard covers about 81 square feet. At 6 inches thick, it covers about 54 square feet. Enter your measurements in the concrete calculator above for the exact amount.
Should I add extra concrete for waste?
Yes. Plan on 5 to 10 percent extra. Ground is rarely perfectly level, forms can bow outward, and some concrete always ends up on the tools and in the wheelbarrow. The calculator adds 10 percent by default.
When should I order a concrete truck instead of bags?
Once your project needs about a cubic yard or more. Mixing 45 or more bags by hand is a lot of work, and a truck delivers it all at once so the slab sets evenly.