To calculate the volume of a donut shaped footing you have to enter the inner circle diameter, the width of the footing and the required depth.
Donut Shape Volume Calculator - Imperial
Donut Shape Volume Calculator - Metric
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.