Concrete Calculator
Calculate how much concrete you need for a slab, footing, or column. Get the volume in cubic yards and cubic feet, a complete bag count by bag size, and a ready-mix cost estimate so you can compare both options before your pour.
Concrete Volume
| 40 lb bags (0.30 cu ft): | 0 bags |
| 60 lb bags (0.45 cu ft): | 0 bags |
| 80 lb bags (0.60 cu ft): | 0 bags |
How to Use This Calculator
Choose your project shape from the dropdown. Most residential jobs are slabs: patios, driveways, garage floors, and sidewalks. Select Column for posts, piers, or tube forms. Wall footing uses the same inputs as a slab but describes a narrow, deep trench.
Enter your dimensions. Depth is in inches because that is how most contractors specify slab thickness. A 4-inch patio slab is standard residential. A 6-inch slab handles vehicles and heavier loads. Edit the ready-mix price if your local rate differs from the $125/cu yd default.
How Concrete Volume Is Calculated
Slab or footing: Volume (cu ft) = Length x Width x (Depth in inches / 12)
A 12 x 16 ft patio at 4 inches deep: 12 x 16 x (4/12) = 12 x 16 x 0.333 = 64 cu ft. Divided by 27 = 2.37 cubic yards.
Column: Volume (cu ft) = pi x radius squared x height. A 12-inch diameter, 3-ft post: pi x (0.5)^2 x 3 = 2.36 cu ft = 0.09 cu yards.
Bag count: Bags needed = Volume in cu ft / cu ft per bag. Round up to nearest whole bag. Standard bag yields: 40 lb = 0.30 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft. Always add 10 percent for spillage and waste.
Concrete Calculation Examples
- 12 x 16 ft patio, 4 inches thick: 2.37 cu yd. About 133 x 40-lb bags or 89 x 60-lb bags. Ready-mix at $125/cu yd would cost $296. At this volume, bags and ready-mix cost similarly.
- Driveway, 20 x 40 ft, 6 inches thick: 14.81 cu yd. Ready-mix at $125/cu yd = $1,851. Using bags would require 822 of the 80-lb size and take far longer to mix properly. Ready-mix makes far more sense here.
- Deck post, 12-inch diameter tube, 3 ft deep: 0.09 cu yd = about 2.36 cu ft. Five 40-lb bags are enough for one post with some left over for rounding.
Frequently Asked Questions
Should I order extra concrete?
Always order 5 to 10 percent more than your calculated volume. Forms are rarely perfectly level, subgrade can have low spots, and some concrete is lost in the truck chute or spills at the pour site. Running short means a cold joint in your slab, which is a weak point that cracks over time.
When should I use bags instead of ready-mix?
Bags work well for projects under one cubic yard. That is small fence post holes, repair patches, or a single deck post. Once you hit one yard and above, ready-mix delivers faster, mixes more consistently, and usually costs less per cubic foot than buying and hand-mixing 50-plus bags.
How thick should a concrete slab be?
Four inches is the residential standard for patios, walkways, and shed floors. Six inches handles driveways and any surface that sees vehicle traffic. Eight inches or more is for heavy commercial loads. Reinforcing with rebar increases structural strength regardless of thickness. Use our Rebar Calculator to size the reinforcement for your slab.
