A split king is two Twin XL mattresses (each 38" x 80") placed side by side, totaling 76" x 80" — identical to a standard king. Same footprint, very different functionality. Split king costs 20–40% more on average. Worth it for some couples, wasted money for others.
WHO BENEFITS FROM SPLIT KING. (1) Couples with very different firmness preferences — each side can be a different mattress. (2) Anyone using an adjustable base who wants independent head/foot positioning. (3) Light sleepers disturbed by a partner's motion — physical separation gives perfect motion isolation. (4) Couples with significant weight differences (40+ lbs) — heavier partner sinks less when on their own mattress.
WHO SHOULD STICK WITH STANDARD KING. (1) Couples with similar firmness preferences. (2) Solo sleepers who like to spread out. (3) Anyone bothered by the center seam — a noticeable gap can form between the two mattresses, even with seam-bridging straps. (4) Budget-conscious shoppers.
THE CENTER GAP PROBLEM. Two Twin XL mattresses don't fuse — they sit side by side. Even with bridging foam strips, couples who sleep "in the middle" or who cuddle frequently will feel the gap. Mitigations: (a) gap-filling pads (King Maker, Bed Bridge, $30–$60), (b) one-piece king sheet over both, (c) split king with a non-split top layer (Sleep Number flexFit, Saatva Solaire).
COST COMPARISON. Standard king queen mattress: $1,200–$3,500. Split king: $1,600–$5,000 (two Twin XL at premium prices). Adjustable base: split king bases cost $200–$800 more than non-split kings.
BEDDING IMPLICATIONS. Sheets: split king sheets exist (top sheet is one piece, fitted sheets are two separate Twin XL) but are harder to find and cost more. King-size top sheet works fine. Mattress protectors: must buy two Twin XL. Comforter: standard king works.
NOT THE SAME AS SPLIT CALI KING. Split California King is two 36" x 84" custom-cut halves — much rarer, almost always custom order, and significantly more expensive. Standard split king (two Twin XL) is the practical choice for most.
BOTTOM LINE. If you and your partner agree on firmness and don't plan to use an adjustable base, save the money — buy a standard king. If you have meaningful firmness disagreements OR want adjustable functionality, split king is one of the highest-impact sleep upgrades a couple can make.