Please explain the difference. My understanding is that a Split KG mattress is basically 2 pcs of TXL mattress 39x80…but yet a King mattress is 76x80. My math tells me that 2x39=78" and not 76".
I want to buy a SOFT and a FIRM TXL so that my wife and I can have two different comfort levels.
How does this work on a bed?
I want to buy KG fitted sheet to put over both TXL mattresses. Will the KG fitted sheet be too small?
Hi Sully,
Welcome to the Mattress Forum! 
Your math is excellent. This sizing question pops up every so often on the forum, so thank you for giving me a chance to respond to it again.
The ISPA (International Sleep Products Association) standard sizing for mattresses are as follows:
Size Dimensions
Twin 38 x 74½
Twin Extra Long 38 x 79½
Full 53 x 74½
Full Extra Long 53 x 79½
Queen 60 x 79½
Eastern King 76 x 79½
California King 72 x 83½
Even though these are the “standard” sizes for ISPA … there can still be some variation between the “standard” sizing made by different manufacturers in the industry. With soft goods like mattresses there can also be variations between different mattresses as well (normally in the range of a +/- 1" variation from their standard sizing) depending on the dimensions of the materials when they are cut (there can be some variance with cutting flexible materials) and the components they use in their mattresses and on the thickness of the cover that encloses them (a cover that is 1/2" thicker or thinner for example would change the length and width of the mattress by 1").
So the “short and long” of this (pun intended :lol: ) is that both you and ISPA are correct.
You king mattress pad and fitted sheet should be just fine over two twin extra long products. If you’re concerned, make sure that the pocket of your fitted sheet is a bit larger than your mattresses (which I would normally recommend you do anyway) to allow for some extra flexibility.
Phoenix