Knowing the amount of water on Mars over the past several billion years is critical in trying to understand the climate and atmosphere of Mars today, especially in light of the recent increased interest in human exploration of the red planet. There are several processes that can affect the amount of water on the surface of a planet. For Mars over the past three billion years, the two dominant processes are (a) escape to space through stripping of the upper atmosphere which reduces the amount of available hydrogen and (b) release from the interior of the planet through volcanism in a process called outgassing, which increases the amount of water on the surface. These processes not only alter the amount of water but also affect the deuterium to hydrogen (D/H) ratio in the water, which can help quantify how much water was added or removed. In this study, we use the evolution of the D/H ratio in computer modeling simulations to help narrow down the range of the amount of water on Mars in the past three billion years, including how much water was added and removed.