How thick should a dorm mattress or topper be for a loft or bunk bed?

For lofted or bunk beds, overly thick mattresses can reduce guardrail height and make it harder to get in and out of bed safely. Many dorms recommend or require a certain maximum mattress thickness. Always follow the loft or bunk bed guidelines, and choose a mattress or topper thickness that keeps the top of the mattress safely below the rails.