We would always strongly recommend you to have your stairs measured by an experienced flooring professional, however, if there is no alternative, this guide should help you roughly calculate how much carpet you will need.
Firstly, it helps to know what carpet you are planning to install on your stairs.
All the details of the carpet are important in the planning stages, for example, you will need to know what widths it is available in. If there is a pattern, you will need to know how many times it repeats on the width and length to make sure it lines up correctly.#