Is the Serta Onyx firm enough for side sleepers?

Probably not. A firm innerspring with a thin comfort layer puts pressure on hips and shoulders, so side sleepers usually do better on a plusher model.

What sheets fit an 8.5-inch mattress?

Standard sheets fit, but deep-pocket sheets made for 14-inch mattresses will be loose. Look for sheet sets rated for mattresses up to 9 or 10 inches.

Does it need a box spring?

It works on a platform bed or slatted frame without one. On a traditional bed frame, a box spring or foundation is recommended to support the coils.

How long should the Serta Onyx last?

Budget innersprings typically last about 5 to 7 years with regular use. The 1-year warranty covers defects, not normal softening.

Is a king innerspring hard to move?

Yes. A king innerspring is bulky and does not compress like a foam bed-in-a-box, so plan for two people and a clear path.

Serta Onyx Firm King Innerspring 8.5" Mattress

Last price we saw

$279.95at Gardner-White Furniture, last seen Nov 1

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Our take on the Serta Onyx Firm King Innerspring 8.5" Mattress

The Serta Onyx is a low-profile innerspring mattress, 8.5 inches tall, with a firm feel. It is a basic, budget-oriented model: a simple coil core with a thin comfort layer, sold as a king. At 8.5 inches it sits low on the frame and suits platform beds, bunk setups, and guest rooms more than deep-pocket sheets and thick toppers.

Best forBack and stomach sleepers who want a firm surfaceGuest rooms and rentals