A king mattress measures roughly 76" x 80" and a California king 72" x 84". Neither will fold, and a hybrid or innerspring king should never be bent sharply because the coil unit and its perimeter can be permanently distorted. So the job is not about strength. It is about whether the shape can physically pass the tightest point on the route, and about not damaging the mattress in the attempt.
MEASURE FIRST, LIFT SECOND. Walk the route with a tape. Note the narrowest doorway width, the stairwell width, the ceiling height above the stairs, and — the one that catches people out — the diagonal clearance at the landing where you have to swivel. A mattress passes a turn if the landing's diagonal is greater than the mattress's thickness plus a little slack, because you are pivoting it on its edge, not carrying it flat. If the numbers do not work, no technique will fix it and you need one of the alternatives at the end of this guide.
THE FOUR-STEP METHOD THAT WORKS. One: strip it bare and put it in a mattress bag, then tie two or three ratchet straps or cargo straps around the width, snug but not crushing. The bag stops the cover snagging on door frames and the straps stop the mattress flopping. Two: carry it on edge, not flat — a mattress on its side presents a 10–14" profile instead of 76". Three: two people minimum, the stronger person below on the stairs taking the weight, the person above steering and calling the turns. Four: at the landing, walk the top corner round the turn first and let the bottom follow, pivoting rather than pushing.
WHAT NOT TO DO. Do not fold a hybrid, innerspring, latex or pocket-sprung king to force a turn; coil damage and structural sag from folding are excluded by essentially every warranty. Do not drag it up the stairs on its cover. Do not use the handles as lifting points if the mattress has them — most manufacturers state that side handles are for positioning the mattress on the base, not for carrying its full weight, and a torn handle is cosmetic damage you paid for.
WHEN THE STAIRS SIMPLY DO NOT WORK. Three real options remain. A split king — two twin XL halves — goes up any staircase a single mattress fits and reads as a king once assembled, which is also why split kings pair with adjustable bases. A boxed foam or hybrid king carried up compressed solves the geometry entirely, because the box is a fraction of the finished footprint; this is the strongest argument for buying bed-in-a-box if you live upstairs. Or hire white glove delivery and let a trained two-person crew, who do this daily and carry the liability, take it up.
MOVING A KING OUT OF THE HOUSE. The same rules apply in reverse, plus one: never re-compress and re-roll a mattress that was not designed for it. Home vacuum-bagging an innerspring or a latex bed to save van space will damage it. If you are storing rather than moving, store it flat and covered, not on its side against a wall, where the core will distort over weeks.