Is the amount of water on Earth constant?

I wonder if the total quantity of water on Earth is always the same? Or can the water “leak” into space?

The total quantity of water on Earth has varied throughout our planet's geological history and it still does, but the changes are much more insignificant now than when the Earth was newly formed.

Earth’s water forms part of a cycle, in which there is a constant exchange between different reservoirs – rivers, lakes, oceans, atmosphere, glaciers, and ground water. Though the water changes state from solid to liquid and gaseous, it has no effect on the total quantity of water.

On the other hand, volcanoes continuously bring new water to the surface. Water from depths of 50+ km is not considered part of the water cycle, so when this water is forced towards the surface, the quantity of water on Earth is suddenly increased. The water could both have been inside the planet, since Earth was formed, or it could have come from underneath the continental plates. When continental drift pulls the plates into the abyss, they melt, and water is released.

Moreover, Earth is supplied with water from space via meteors that contain water. Just like Earth receives material from space, the planet also loses particles which escape Earth’s atmosphere; primarily hydrogen, of which we lose 3 kg per second. The particle loss influences the quantity of water on Earth, as water consists of one oxygen atom and two hydrogen atoms. So, every time the atmosphere sheds hydrogen, we lose one of the building blocks of water. At the existing rate, Earth will run out of water in three billion years.