In a regular Octagon there are eight straight, equal length sides and eight inside corners that total 135°.
To calculate the volume of an Octagon shaped slab you need to provide the length of one of these sides and the required depth of the slab.
Octagon Shaped Volume Calculator - Imperial
Octagon Shaped 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.