Bunk LedgerFree tool
Truck mattress size checker
Enter the sleeper bunk you measured and see which standard mattress sizes drop in, which are tight, and which do not fit.
How a size gets a yes
The page compares the bunk you measured with a short list of sizes people actually try to order: the household names, plus the truck cuts 32×80, 36×80, 39×80, and 42×80. A size drops in when it leaves about an inch on the width and half an inch on the length. Tight means it is no bigger than the pan, with less spare than that. Anything else does not fit, even if a listing calls it “truck size.”
Measure the bare pan at both ends and the middle and use the narrowest width. Catalog letters from one mattress maker are that maker’s letters. SleepDog and another seller do not always agree on which Volvo or Cascadia pan is which. The tape on your bunk beats both of them.
Thickness is the clearance you have while sitting up. It does not change the width match. Seller ranges often put an upper bunk near 3 to 5 inches and a lower bunk near 6 to 9 inches. Your cabinet is the limit. Twin XL sheets do not fit a 42 inch width.
Related: how to measure a bunk.