How much people weigh on Mars varies according to how much they weigh on Earth. Someone who weighs 160 pounds on Earth would weigh 59.5 pounds on Mars, according to the Athena Mars Exploration Rovers page hosted by Cornell University.
A person's weight depends on factors such as mass and gravity. Because Earth has a much greater mass than Mars, it has a stronger force of gravity. As a result, people weigh less on Mars than they do on Earth.
At one time, Pluto was the planet where a human would weigh the least. In 2006, Pluto was demoted from a planet to a dwarf planet, so as of May 2014, Mars is the planet where humans would weigh the least.