How to use the concrete calculator
Start with the shape and measurements. The calculator converts a rectangular slab or footing to volume, adds the waste percentage, and estimates bags from the selected yield. Its result also shows the formula and the intermediate values behind the numbers.
- Choose Shape: Slab or Footing. The selection changes the quantity and thickness labels, while the volume calculation remains rectangular.
- Enter the number of identical shapes in Number of slabs or Number of footings.
- Enter Length, Width, and Slab thickness or Footing depth. Choose a unit beside each measurement.
- Set Waste / overage %. It starts at 10%, and you can enter 0% when no allowance is needed.
- Choose Bag size. If the product label lists another yield, select Custom yield and enter Bag yield (cu ft).
- Click Calculate. Reset clears the dimensions, restores the defaults, and hides the result.
Concrete volume and bag calculator example
Each measurement is converted to feet before the rectangular volume calculation. The calculator multiplies one shape by the quantity, adds waste, then divides total cubic feet by the selected bag yield. Bags are rounded up to a whole number.
Input: 1 Slab, 10 ft x 10 ft x 4 in, 10% waste, 80 lb bag
Output: 36.67 cu ft, 1.36 cu yd, 1.04 cu m, 62 bagsSlab concrete calculator inputs and bag sizes
- Length and Width accept ft, in, yd, m, or cm.
- Slab thickness or Footing depth accepts in, ft, cm, or mm.
- Bag size presets use about 0.30 cu ft for 40 lb, 0.45 cu ft for 60 lb, and 0.60 cu ft for 80 lb. Check the yield on the product label.
- Select Custom yield to edit Bag yield (cu ft). A preset makes that field read-only.
Concrete calculator limits and assumptions
Length, width, and thickness/depth must be at least 0.0001 in the entered unit. Quantity accepts whole numbers from 1 to 1000. Waste / overage % accepts 0 to 50, and Bag yield (cu ft) accepts 0.01 to 10. The calculation treats each slab or footing as a uniform rectangle.
The calculator does not model irregular or tapered shapes, reinforcement displacement, site conditions, supplier data, prices, or mix designs. Its result comes from the values you enter, so it is not certified engineering, structural, or purchasing advice.
Concrete calculator examples
For a rectangular slab, enter its plan dimensions and finished thickness. Then choose a bag size to see the volume and bag estimate.
For repeated footings, choose Footing, enter the shared dimensions, and set Number of footings to the count covered by the pour.
If a bag label gives a yield different from the presets, select Custom yield so the estimate uses that value.
Concrete calculator or another construction calculator?
Use the Construction Calculators category for other project types. The Asphalt Calculator estimates asphalt tonnage. The Construction Materials Calculator covers other material modes, while the Deck Calculator uses deck dimensions and spacing.
Frequently asked questions
How do I calculate concrete for a slab?
Enter Length, Width, Slab thickness, Number of slabs, and Waste / overage %. The calculator converts the measurements to feet, multiplies the rectangular dimensions, adds the waste allowance, and shows the volume in cubic yards, cubic feet, and cubic meters.
How many concrete bags do I need?
The calculator divides total cubic feet by Bag yield (cu ft) and rounds the result up to a whole bag. Bag size presets supply the yield. Custom yield lets you enter the value printed on the product label.
Can I use this as a concrete footing calculator?
Yes. Choose Footing, enter the shared dimensions in Length, Width, and Footing depth, and set Number of footings. The calculator applies the rectangular volume formula and changes the labels for the footing inputs.
What units can I enter in the concrete calculator?
Length and Width accept ft, in, yd, m, or cm. Slab thickness and Footing depth accept in, ft, cm, or mm; results are shown in cubic yards, cubic feet, and cubic meters.