# Donut Shape Concrete Calculator

Calculate the amount of concrete needed in cubic feet and cubic yards for a donut shaped footing or slab.
Note: this is a circular area with a circular hole on the inside, not a torus!!

To be able to calculate the concrete volume needed for this type of slab your need to know the outer diameter of the circle, the inner diameter and the required depth. ### Formula's used:

Total Area
A = π * D² ÷ 4
where A is the area in square feet, π = 3.14159265358979, oD = outer diameter of circle
Perimeter
P = π * D ÷
where P is the perimeter in feet, π = 3.14159265358979, D = diameter of circle
Volume
V = π * D² ÷ 4 * (d ÷ 12)
where V is the volume in cubic feet, π = 3.14159265358979, D = diameter of circle, d = depth of slab in inches ÷ 12 converting this to feet

Example calculation for volume:
```outer diameter  = 24 feet
inner diameter  = 22 feet
depth of slab = 4 inches
3.14159265358979 * 24 *  24 ÷ 4 ( 4 ÷ 12) = 150.796 cubic feet
3.14159265358979 * 22 *  22 ÷ 4 ( 4 ÷ 12) = 126.711 cubic feet
150.796 -  126.711 =  24.1 cubic feet of concrete needed for this footing
```
To convert cubic feet to cubic yards multiply cubic feet * 0.037037037037037
To convert cubic feet to cubic metres multiply cubic feet * 0.028316846592
To convert cubic yards to cubic metres multiply cubic yards * 0.764554857984

### Donut Shaped Slab / Footing

Outer Diameter ft in
Inner Diameter ft in
Depth in

 Width of Footing in Surface Area of Footing ft² Inner Perimeter ft Outer Perimeter ft Total Length of Perimeter ft Amount of Concrete Needed in: Cubic Feet ft ³ Cubic Yards yd ³ Cubic Metres m³

#### Calculation

1. enter the outer diameter in feet and inches
2. enter the inner diameter in feet and inches
3. the required depth in inches

#### Results

1. footing or slab width in inches
2. square footage of this area. The surface area of the footing or slab
3. inside perimeter in feet
4. outside perimeter, the circumference in feet
5. combined perimeter the outside plus the inside added together
6. amount of concrete needed in cubic feet
7. concrete needed in cubic yards
8. concrete needed in cubic metres

### When Ordering Concrete

This concrete calculator will help you in estimating the amount of concrete needed for your project. The amount given as needed, does not include any waste. It is recommended, depending on the job you are doing, to add anywhere from an extra 4% to 10% to your concrete order to make sure you have enough concrete to finish the job. Ask when ordering the concrete. They will be able to give you a good idea of what is needed.