Not all rice cookers work exactly the same.
In my rice cooker 1.5 times as much water as rice is just right for white rice and 2 times as much water works best for brown.
The trick to success with rice cookers is to rinse the rice well before cooking. Especially true with brown rice. Otherwise it will turn out gummy.
There's usually a line marker in a rice cooker. In general 1.5 cups of water to 1 cup of rice works.