Flex top king, split top king, split head king, top split king, half split king. Five names, one bed — and if you've just bought an adjustable base, the name on the box is quietly deciding which sheets will actually fit it.
Here's what a flex top king really is, how it differs from a split king, and why the sheets are the part everyone gets wrong.
The short version
- Flex top king and split top king are the same bed: one 76" × 80" king mattress, split only through the top third.
- A split king is different — two completely separate Twin XL mattresses.
- A flex top king needs a special fitted sheet with a split at the head. A standard king fitted sheet will not sit right — and two Twin XL sheets won't work either.
One of you is reading. One of you is asleep.
That's the whole reason this mattress exists. The head of your side rises, their side stays flat, and nobody negotiates. The bed splits at the top — and only at the top — so below your shoulders it's still one seamless king. No gap in the middle of the marriage.
Then the first sheet you put on it pops off at 2 a.m., because it was cut for a mattress that doesn't bend.
What a flex top king actually is.
A flex top king is a king-size mattress — 76 inches wide, 80 inches long — with a split that runs from the head of the bed down through roughly the top 34 inches. Each sleeper can raise their head independently on an adjustable base, while the lower half of the mattress stays connected.
Mattress brands never agreed on a name for it. Sleep Number sells it as FlexTop. Other makers say split top king, split head king, top split king, or half split king. Different labels, identical bed. If your mattress is king-sized and only the head splits, everything in this article applies to you.
Flex top king vs. split king: one bed or two?
A split king is two separate Twin XL mattresses (38" × 80" each) pushed together. The split runs the full length of the bed, so each side can raise the head and the feet independently — and each sleeper can even choose a different firmness. The trade-off is a seam you can feel in the middle of the bed.
A flex top king keeps the bottom two-thirds as one piece: no center seam where you actually lie, but only the head adjusts. Couples who cuddle pick the flex top. Couples who want zero-gravity on one side and flat on the other pick the split king.
The flex top is for couples who want the adjustability without the gap down the middle of the bed.
The one-line answerThe sheets are the catch.
A standard king fitted sheet can't follow the head split — raise one side and the sheet tents, strains, and pops a corner. Twin XL sheets don't fit either, because the bottom of the mattress is one piece. You need a flex top king fitted sheet: king-sized, with a matching split sewn into the head end and separate pocketed corners for each half of the top.
Ours is cut exactly for that. The Split Top King Bamboo Luxe set is 100% rayon (viscose) from bamboo in a cooling sateen weave, with 17" deep pockets and corner straps on every corner — including the two middle pieces at the split, which is where lesser sheets give up. The flat sheet and pillowcases are standard king, because those don't need to bend.
How to order the right set.
Check your mattress, not the base. If the mattress is one piece with a head split, order split top king sheets — whatever your brand calls the bed. If it's two separate Twin XL mattresses, you want split king sheets instead. Still unsure? The size guide maps every name to the right set, and bamboo runs cool for anyone who sleeps warm — the fiber breathes instead of trapping heat, which is its own story.
Split Top King · 100% Bamboo Luxe
Cut for flex top & split head king beds · 100% bamboo viscose · 17" deep pockets · corner straps on every corner, split included.
Quick answers, no scrolling.
Is a flex top king the same as a split top king? Yes. Identical bed, different brand vocabulary — split head king, top split king, and half split king too.
Will regular king sheets work on a flex top king? The flat sheet and pillowcases will. The fitted sheet won't — it needs the sewn-in head split.
Do flex top king sheets fit Sleep Number FlexTop beds? Yes — FlexTop is Sleep Number's name for the same split, and here's what actually fits a Sleep Number FlexTop. See real customer experiences on our reviews page.
