Concrete Slab Calculator
To figure concrete for a slab or patio, measure the length and width of the form in feet and the thickness in inches. Enter them above and you get cubic yards, bags and a rough cost. A 10′ × 10′ slab at 4″ takes 1.23 cubic yards, or 1.36 with 10% extra.
Measure inside the forms. For an L-shaped patio, split it into two rectangles and add a second section. If you already know the area, use By area and type the square feet, for example 100, then the thickness.
Check the thickness at a few spots. Ground is rarely flat, so go with the deeper readings, not the average.
| Use | Typical thickness |
|---|---|
| Walkway, patio | 4″ |
| Small slab, shed base | 4″ to 6″ |
| Driveway, passenger cars | 4″ to 5″ |
| Driveway, heavy vehicles | 5″ to 6″ |
Quikrete says sidewalks and small slabs should be at least 4″ thick, small slabs normally 4″ to 6″. Concrete Network puts car driveways at 4″ to 5″ and heavier loads at 6″. Check local code for anything structural. Thickness sources
How Many Bags of Concrete Do I Need?
Pick your bag size in the calculator and it counts the bags for you. For a 10′ × 10′ × 4″ slab with 10% extra, that is 62 bags of 80 lb, 82 bags of 60 lb or 123 bags of 40 lb. One cubic yard is 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb.
| Bag size | Yield | Bags per cu yd | 10′ × 10′ × 4″ slab |
|---|---|---|---|
| 90-lb bag | 0.675 cu ft | 40 | 55 |
| 80-lb bag | 0.60 cu ft | 45 | 62 |
| 60-lb bag | 0.45 cu ft | 60 | 82 |
| 50-lb bag | 0.375 cu ft | 72 | 98 |
| 40-lb bag | 0.30 cu ft | 90 | 123 |
Yields from the Quikrete Concrete Mix data sheet. The last column includes 10% extra, rounded up to whole bags. Sakrete’s 90-lb bag yields 0.66 cu ft, so it takes 41 per cubic yard. All bag yields and sources
Bags are always rounded up. You can’t buy half a bag, and running short mid-pour leaves a cold joint.
The yield is printed on the bag. We use the Quikrete and Sakrete data sheets, but a different mix or brand can yield a bit more or less, so check yours. High-yield mixes like Sakrete Maximizer get more out of a bag, which means fewer bags.
Concrete for Post Holes and Sonotubes
A fence post set in a 10″ hole, 3 ft deep, takes about 2.3 bags of 80-lb mix, so plan on 3 per post. Ten posts take about 26 bags with 10% extra.
Quikrete’s rule for setting posts: dig the hole 3 times as wide as the post and 1/3 as deep as the post is long, plus 6″ of gravel underneath. For structural posts, such as a deck, go at least 6″ below the frost line.
In Posts, enter the hole diameter, depth and number of posts, then pick the post size: 4×4 (3 1/2″ square), 6×6 (5 1/2″ square) or none. The calculator subtracts the post, because the post takes up room the concrete doesn’t need to fill.
For a sonotube (form tube), use Column and enter the tube diameter and height. A 12″ tube 4 ft deep needs 0.12 cubic yards, which is 6 bags of 80 lb with 10% extra. The calculator counts the full hole depth, so the gravel base may save you a little.
Footings, Walls and Steps
A footing is a long, narrow slab. Enter the total length, the width and the depth of the trench. If the trench changes width or depth, add one section per run.
For a wall, enter length, height and thickness. If the wall sits on a footing, add the footing as a second section. The calculator adds everything up and rounds the bags once at the end.
For steps, enter width, rise, run and the number of steps. If the top step is deeper than the rest, enter its depth in Top step depth.
Round footings and curbs have their own shape, Ring. Enter the outer diameter, inner diameter and depth.
How Much Does a Yard of Concrete Cost?
Ready-mix runs about $119 to $147 per cubic yard delivered, with $133 as the typical figure (HomeGuide, Jan 2026). That price assumes a delivery of 20 miles or less.
A full mixer truck carries about 10 cubic yards. Order less and many plants add a short-load fee. HomeGuide lists $53 extra per cubic yard under 10. Delivery past 20 miles, Saturday pours and minimum orders vary by plant, so call yours.
Bags cost more per yard. At $6.47 for an 80-lb bag, 45 bags come to about $291. For a patio or a few post holes, bags are still the easy choice. Once the job reaches around 1 cubic yard, price a truck.
Prices vary by region and change often. The defaults in the calculator are national figures with their dates; put your local price in the fields and the cost updates.
How to Calculate Concrete
Convert every dimension to feet, multiply, then divide by 27 to get cubic yards. Add 4% to 10% for waste. NRMCA recommends that range for spills, over-excavation and uneven grade.
Cubic yards = L (ft) × W (ft) × T (ft) ÷ 27 10′ × 10′ × (4″ ÷ 12) = 33.33 cu ft 33.33 cu ft ÷ 27 = 1.23 cu yd + 10% extra = 36.67 cu ft = 1.36 cu yd
Round columns and post holes use π × r² × depth, with r in feet. A 12″ tube 4 ft deep: 3.14 × 0.5 × 0.5 × 4 = 3.14 cu ft, or 0.12 cubic yards.
To get bags, divide the cubic feet by the yield on the bag and round up. Type measurements the way your tape reads: 3 1/2, 3-1/2 and 3.5 all mean 3 1/2″, and 10' 6 means 10 ft 6 in.
Concrete Questions
How much concrete do you need for a 10x10 slab?
At 4″ thick, 1.23 cubic yards, or 1.36 with 10% extra. In bags, that is 56 bags of 80 lb before any extra and 62 with it.
How many 60 lb bags of Quikrete are in a yard?
60. A 60-lb bag yields 0.45 cubic feet, and a cubic yard is 27 cubic feet. Add extra on top for waste.
How much does 1 yard of concrete cover at 4 inches thick?
81 square feet. A cubic yard is 27 cubic feet, and 4″ is one third of a foot, so 27 ÷ 1/3 = 81.
Is it cheaper to buy bags of concrete or mix your own?
Bagged mix is already blended, so you skip buying cement, sand and gravel separately. We don’t price site-mixed batches. The real question for bigger jobs is bags against a truck: about 45 bags of 80 lb make a yard, and ready-mix is usually cheaper per yard from around 1 cubic yard up.
How many cubic feet are in a bag of concrete?
An 80-lb bag of Quikrete Concrete Mix yields 0.60 cubic feet, a 60-lb bag 0.45 and a 40-lb bag 0.30. The yield is printed on the bag.
How much does a yard of concrete weigh?
About 3,900 lb, or nearly 2 tons. Normal-weight concrete runs about 145 lb per cubic foot, and a cubic yard is 27 cubic feet. Weight source
How much does a 50 lb bag of concrete mix cover?
About 1.1 square feet at 4″ thick. A 50-lb bag of Quikrete Concrete Mix yields 0.375 cubic feet.
How many bags of concrete per fence post?
About 3 bags of 80 lb for a 4×4 in a 10″ hole, 3 ft deep. For other post sizes or depths, run your numbers through the Posts shape.