I am a back sleeper who presently owns an old sleep number bed which I like, but it’s really old. I’d like to get another backup bed, because I do have a bad back and if my sleep number bed stops working, I’ll have an alternative bed to use. I cannot consider sleep number beds anymore because all their 2022 beds come with at least 2" of foam, and my back won’t tolerate that. So I’m looking for a traditional bed instead. My next step will be to visit several mattress stores and try out whatever firm beds they have without foam. Once I narrow it down to several I like, I have several questions. Since I’d like a quality bed that won’t start sagging, am I looking for a mattress with low gauge coils? And what other traits should I be looking for (or avoiding) in a good firm mattress? And is a box spring really needed? Also, if I were choosing between a twin and full size mattress, is one of these more susceptible to sagging? Also, if it matters, I’m 5’9" and weigh 150 lbs. Thanks in advance for any responses. Recommendations and suggestions are most welcome.
Hi Marcus.
Welcome to our Mattress Forum. 
The lower the gauge number of the mattress coil, the thicker the coil. This means that the lower the gauge number is, the firmer and stiffer the mattress will feel, offering a harder surface for the mattress.
Since you’re looking at innerspring models, you may still require a box spring unless you prefer the functionality of a different type of bed base (such as a platform bed).
No. Sagging comes from wear over time or an inappropriate base (like a slatted base for your innerspring mattress). At your weight and height, you wouldn’t need to worry about coil sagging. Are you planning to buy based on your in-person testing or are you also interested in options you may be able to purchase online?
NikkiTMU