A king box spring is almost always built as two split halves (each 38″ × 80″ — the same footprint as a Twin XL) that sit side by side under your king mattress (76″ × 80″). The split design makes a large king foundation easy to deliver, carry up stairs and set up, and it's exactly what a split-king adjustable base needs so each side can raise and lower independently. Shop split king box springs and foundations matched to Sealy, Serta, Beautyrest, Simmons, Stearns & Foster and more — free delivery across Canada. • What it is: a king box spring built as two 38″ × 80″ halves (each a Twin XL footprint) • Together they support a standard king mattress (76″ × 80″) • Easier to deliver and carry up stairs than a one-piece foundation • Required for split-king adjustable bases (each side adjusts independently) • Brands: Sealy, Serta, Beautyrest, Simmons, Stearns & Foster, Springwall, Galaxy & NM • From $176 — free Canada-wide delivery
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