There's no 'right' depth of trundle mattress that you should get; it entirely depends on how tall your trundle bed is and how much space you have to work with beneath your bed. As such, we sell the following trundle bed mattress depths:
15cm trundle mattresses
16-20cm mattresses
If you need some help measuring your bed to find out which size trundle mattress is right for you, look at our bed measuring guide for more helpful information.