In the context of sustainability, a footprint is considered an easy-to-perceive equivalent amount your own individual actions have on the planet's finite resources.
We recommend a few different calculators that explain your footprint in different ways:
The Earthday Network Footprint Calculator gives results in number of earths required to support your lifestyle, commonly known as an Ecological Footprint.
The EPA Personal Emissions Calculator gives results in pounds of carbon dioxide generated per year, also known as a Carbon Footprint.
The EPA also provides a list of other individual emissions calculators that specialize in calculating your emissions.