Concrete Calculator
Calculate concrete volume and bags needed for slabs, footings, columns and stairs.
Inputs
Enter in inches.
Typically 5–10%. Accounts for spillage and uneven sub-base.
Multiply to pour several identical shapes.
Cubic Yards
1.36
Cubic Feet (with waste)
36.67
Cubic Feet (net)
33.33
Bags Needed
62
Rounded up to whole bags.
Bag Size Used
80 lb
Step by step
Net volume
10 ft × 10 ft × 4 in ÷ 12
= 33.333 cu ft
Add 10% waste
33.333 × 1.100
= 36.667 cu ft
Convert to cubic yards
36.667 ÷ 27
= 1.36 yd³
Bags needed (80 lb, 0.6 cu ft yield)
⌈36.667 ÷ 0.6⌉
= 62 bags
How it works
Concrete volume is calculated in cubic feet for the selected shape. A waste factor (typically 5–10%) is added to account for spillage, overfill and uneven sub-grade. The total is divided by 27 to get cubic yards — the standard unit used when ordering ready-mix. For bag mixes, dividing the cubic-foot total by the published per-bag yield gives the number of bags. Bags are always rounded up because you cannot use a partial bag.
Formulas
Slab / Footing
Volume (cu ft) = Length × Width × (Thickness_inches ÷ 12)
- L
- Length (ft)
- W
- Width (ft)
- T_{in}
- Thickness (inches)
Column (cylinder)
Volume (cu ft) = π × (diameter_in / 24)² × height_ft
Stairs
Volume = (n × rise_ft / 2) × (n × run_ft) × width_ft
- n
- Number of steps
- r_{ft}
- Rise in feet
- t_{ft}
- Tread in feet
- w_{ft}
- Width in feet
Bags
Bags = ceiling(volume with waste ÷ per-bag yield)
Frequently Asked Questions
How many cubic yards do I need for a 10×10 slab at 4 inches thick?
10 × 10 × (4/12) = 33.33 cu ft. With 10% waste: 36.67 cu ft ÷ 27 = 1.36 cubic yards. The calculator gives you this instantly — just set Length 10 ft, Width 10 ft, Thickness 4 in.
Why is 80 lb a better choice than 60 lb?
An 80 lb bag yields 0.60 cu ft vs 0.45 cu ft for a 60 lb bag. You mix fewer bags for the same volume, reducing labor. On small pours, the weight difference is manageable.
What waste factor should I use?
5% for well-prepared, flat sub-grade. 10% for typical residential work. 15% for rough or sloped terrain. The default 10% is a safe choice for most jobs.
When should I order ready-mix instead of using bags?
Ready-mix is typically more economical above 1 cubic yard. Below that, bags are practical. The calculator shows cubic yards so you can make that call directly.