Key Facts
- Most foam mattress warranties specify a maximum slat gap of about 3 inches, and some 2.5 inches for thicker builds.
- Exceeding it is commonly treated as improper support and can void the mattress warranty.
- Any bed wider than a twin needs a centre rail with legs reaching the floor. Queen, king and California king all require it.
- A frame's weight capacity must carry the mattress too — a queen foam mattress alone can exceed 100 lb.
- Platform beds do not need a box spring. They support the mattress directly.
- A foundation is rigid; a box spring flexes. Foam mattresses generally require the rigid option.
- Standard US sizes: Twin 38×75, Twin XL 38×80, Full 54×75, Queen 60×80, King 76×80, California King 72×84 inches.
- A California King is narrower than a King by four inches, and four inches longer.
Full sourcing: the Slat Spacing & Warranty Index.