The sheet problem arrives about a week after the bed does. The FlexTop King is set up, both remotes are paired, and the first search for a second set of sheets returns four different names for the size you own — plus a wall of ordinary king sets that will not fit it. If that is where you are, this page is the map: what Sleep Number sells for this bed, what any other set has to get right, and the exact measurements to check before you spend anything.
Sheets for a Sleep Number FlexTop King need three things: a king-width fitted sheet (76 by 80 inches) that splits at the head, pockets deeper than the mattress plus everything sitting on it, and retention that holds while the head sections move. Sleep Number's own Lyocell Ultra set fits; so does any split top king set that clears the checklist below.
One mattress, two head sections.
Start with what Sleep Number publishes about its own bed. A FlexTop King is a king mattress — 76 inches wide by 80 inches long, per Sleep Number's mattress and base specifications — with one difference that governs everything: each sleeper raises or lowers the head of their own side while the lower half of the bed moves as one piece.
That geometry is the entire sheet problem. At the foot, the bed behaves like any king. At the head, it behaves like two beds. A fitted sheet therefore has to be one sheet at the bottom and, functionally, two sheets at the top. An ordinary king fitted sheet — one uninterrupted pocket across the head — either ties the two head sections together so neither articulates cleanly, or works itself off a corner the first night one of you raises your side. The size that solves this is sold under four names: split top king, flex top king, top split king, and split head king. Four names, one sheet. And it is not a split king, which is a different bed entirely — two separate twin XL mattresses taking two separate fitted sheets. The naming maze has its own pages here if you want it settled properly: flex top king vs split top king and split king vs split top king.
What Sleep Number's own set gets right.
The default answer is the one Sleep Number sells. Their Lyocell Ultra set comes in FlexTop King, and the fit engineering is real: by their published description, the sheets stretch and recover as the bed flexes, with a fully elasticized edge, elastic corner seams and anchor bands holding the fitted sheet while the head moves. It is made for the bed by the people who made the bed. As the zero-research option, it is a good one.
Two facts from their own product page are worth weighing before you decide. First, the sheets are designed to fit mattresses up to 15 inches thick — fine against the bare mattress, tighter once a mattress pad or topper goes on. Second, the set comes in one fibre, lyocell, at one price point: $349.99 list for the FlexTop King set, on sale at $279.99 as we write this. Neither fact is a criticism. But if you want deeper pockets, a different fibre, or a second and third set for the rotation, you are shopping third party — and then the checklist below is what separates sheets that fit from sheets that almost fit.
What any set must get right, in numbers.
We sell sheets in this size, so read what follows knowing that. The checklist itself is brand-neutral: it is the same three measurements whether the label says ours or anyone else's.
| Check | What to look for |
|---|---|
| Footprint | 76 by 80 inches — king width and length, per Sleep Number's own spec page. The flat sheet and the fitted sheet are both cut king-size. |
| Head split | The fitted sheet must divide at the top so each half moves with its own head section. If the listing does not say split top, flex top, top split or split head, it is not this sheet. |
| Pocket depth | Measure from the top of everything on the bed — pad, topper — down past the mattress edge. Sleep Number publishes a 15-inch fit for its own set; our pockets are cut at 17 inches and fit mattresses up to 16 inches. |
| Retention | An adjustable base articulates the head section every night, and that motion is what pops corners. Look for corner straps or anchor bands, not elastic alone. |
| Not this | Split king sets — two twin XL fitted sheets for two separate mattresses. Wrong bed, no matter how similar the name reads. |
The pocket-depth row earns one extra sentence, because it is the quiet failure. Most people measure the mattress the day it arrives, bare, and buy against that number. Then the waterproof pad goes on, then perhaps a topper, and a pocket that cleared the mattress by half an inch now does not clear it at all — which gets misread as the sheet shrinking. Measure the bed as it is actually dressed, and measure at the corner, tape running from the top surface down to where the fitted sheet should tuck.
A fitted sheet for this bed has to be a king at the foot and two sheets at the head. Every fit failure starts by ignoring half of that sentence.
The whole checklist, compressedThe bamboo case, stated once.
Fit qualifies a sheet; fibre is the tiebreak. Ours are 100% rayon (viscose) from bamboo in a sateen weave — a smooth, drapey hand that runs cooler through the night because the fibre wicks moisture instead of holding it against you. The mechanism, including its honest limits, is laid out in do bamboo sheets keep you cool. On a bed bought so two people can finally sleep at two different angles, temperature tends to be the other half of the argument, which is why fibre comes second and not first.
And price, third, once: our Split Top King set is $169.95 — head split already cut, 17-inch pockets, corner straps sewn in, seven colours. Free shipping applies at $75 and up. It is checked against the same three measurements above, and so should anything else you consider, including ours: measure your own bed as it is dressed today, then buy.
Split Top King, made for this bed
100% rayon (viscose) from bamboo · 17-inch pockets · corner straps · $169.95
The questions people also ask.
Do Sleep Number sheets fit a split top king?
FlexTop King and split top king are the same geometry, so the house set fits the house bed. The checks that matter for any other brand are the head split, the pocket depth measured over your pad and topper, and corner retention.
