Quick answer
Multiply length by width by thickness in feet to get cubic feet, then divide by 0.6 for 80 lb bags or 0.45 for 60 lb bags. A 10 x 10 foot slab at 4 inches thick is 1.23 cubic yards, or about 62 bags of 80 lb concrete with a 10 percent buffer.
How many bags of concrete for common slab sizes
Volumes and bag counts for 4 inch thick slabs, including a 10 percent buffer.
| Slab size | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 x 4 ft | 0.22 cu yd | 10 bags | 14 bags |
| 6 x 6 ft | 0.49 cu yd | 22 bags | 30 bags |
| 8 x 8 ft | 0.87 cu yd | 40 bags | 53 bags |
| 8 x 10 ft | 1.09 cu yd | 49 bags | 66 bags |
| 10 x 10 ft | 1.36 cu yd | 62 bags | 82 bags |
| 10 x 12 ft | 1.63 cu yd | 74 bags | 98 bags |
| 12 x 12 ft | 1.96 cu yd | 88 bags | 118 bags |
| 10 x 20 ft | 2.72 cu yd | 123 bags | 163 bags |
| 20 x 20 ft | 5.43 cu yd | 245 bags | 326 bags |
For a 6 inch slab, multiply these numbers by 1.5. Above roughly one cubic yard, price a ready-mix delivery as well.
How do you calculate how many bags of concrete you need?
Concrete is ordered by volume. The calculator converts thickness from inches to feet, multiplies by the slab area, and adds a buffer for uneven subgrade and spillage. It then reports the same volume three ways: cubic yards for a ready-mix order, 80 lb bags, and 60 lb bags.
Bag counts use the yield printed on standard concrete mix: about 0.6 cubic feet for an 80 lb bag and 0.45 cubic feet for a 60 lb bag. The cost line compares bagged mix against your local ready-mix price so you can see where the crossover is.
The formula
- Cubic feet = length x width x (thickness in inches / 12)
- Adjusted cubic feet = cubic feet x (1 + waste %)
- Cubic yards = adjusted cubic feet / 27
- 80 lb bags = adjusted cubic feet / 0.6, rounded up
- 60 lb bags = adjusted cubic feet / 0.45, rounded up
Worked example
With the starting values in the calculator (Length: 10 ft, Width: 8 ft, Depth: 4 in, Waste buffer: 10%, 80 lb bag price: $7, Ready-mix price: $165/cu yd), the result is 49 80 lb bags at an estimated $343 bagged or $179 ready-mix before fees.
- Cubic feet = 10 x 8 x (4 / 12) = 26.7
- After waste = 26.7 x 1.1 = 29.3 cu ft
- 80 lb bags = 29.3 / 0.6, rounded up = 49
What this calculator assumes
- An 80 lb bag yields 0.6 cubic feet and a 60 lb bag yields 0.45 cubic feet.
- Waste defaults to 10 percent.
- The pour is a rectangle of even thickness. Thickened edges and footings need to be added separately.
- The ready-mix price excludes delivery, short-load, and pump fees.
Tips for a more accurate estimate
- One cubic yard is 45 bags of 80 lb mix, which is 3,600 pounds to carry and mix. Plan the labor before you plan the price.
- For round post holes, volume is 3.14 x radius x radius x depth, all in feet.
- Ask the ready-mix supplier about minimum loads and short-load fees before you compare costs.
- Compact the base and check form height in several places. A slab that averages half an inch too thick uses about 12 percent more concrete.
Concrete Bag vs Ready-Mix Calculator FAQ
How many bags of concrete do I need for a 10 x 10 slab?
A 10 x 10 foot slab at 4 inches thick is 33.3 cubic feet. That is 56 bags of 80 lb mix or 75 bags of 60 lb mix with no waste, and 62 or 82 bags with a 10 percent buffer.
How many 60 lb bags of concrete make a cubic yard?
A 60 lb bag yields about 0.45 cubic feet, so one cubic yard, which is 27 cubic feet, takes 60 bags.
How many bags of concrete do I need per fence post?
A 10 inch wide hole that is 30 inches deep with a 4x4 post holds a little over 1.1 cubic feet of concrete, which is about two 80 lb bags. Wider or deeper holes need more.
How many 80 lb bags are in one cubic yard?
An 80 lb concrete bag yields about 0.6 cubic feet, so one cubic yard takes about 45 bags before waste.
When should I order ready-mix?
Ready-mix often becomes more practical around 1 cubic yard or more, especially when time and labor matter.
How thick should a slab be?
Many patios and walkways use 4 inches. Driveways, garages, and structural work need project-specific guidance.
Use this estimate as a planning starting point. Product coverage, waste, local prices, code rules, and site conditions can change the final amount.