Foundations · King
King box spring dimensions: split foundation sizes
A King foundation is commonly supplied as two pieces around 38–39 × 79.5–80 inches each. Together they support the nominal 76 × 80 inch King footprint.
Semrush US · 12,100 searches/mo · KD 16| Label or calculation | Dimensions | What it means |
|---|---|---|
| TEMPUR example | 2 pieces, each 38 × 80 in (96.5 × 203.2 cm) | Split construction for King. |
| Beautyrest example | 2 pieces, each 39 × 79.5 in (99.1 × 201.9 cm) | Shows finished-size variation. |
| Sealy example | 2 pieces, each 38 × 79 in (96.5 × 200.7 cm) | Current listed finished dimension. |
Order the support configuration specified for the mattress and frame; the two-piece pattern is common but not a universal compatibility promise.
Why the foundation is split
Two narrower pieces are easier to move and can fit a King frame while providing a combined support surface. The joined width may differ slightly from the mattress’s nominal width.
Measure the frame’s support ledges
Record the internal opening, ledge depth, center support, and required foundation height. Outer room footprint comes from the frame, not the foundation alone.
Measurement method
Start with the product and space you actually have.
- 1
Measure the frame opening. Record inside width, length, support ledges, and center support.
- 2
Check finished dimensions. Use the foundation specification, not only the size label.
- 3
Add vertical layers. Combine frame support height, foundation, mattress, pad, and topper.
- 4
Verify the support system. Follow the mattress and frame manufacturers’ current requirements.
Questions, answered
Frequently asked questions
Is a King box spring two Twin XL box springs?
Its two pieces are often close to Twin XL dimensions, but current examples vary by one inch or more. Buy the matched King foundation set specified by the manufacturer.
Can one King mattress sit on two foundations?
Yes when the mattress and frame manufacturers approve that configuration and the frame has the required center support.