Hi Toro,
I would keep in mind that coil gauge by itself wouldn’t be a reliable indicator of the firmness of the innerspring although if all the other variables in two innersprings are identical such as coil number, coil shape, coil height, number of turns, coil diameter, coil arrangement, type of innerspring (linked or pocket coils) then a lower gauge innerspring with thicker wire will be firmer. The type and thickness of any padding above and below the innerspring and the specifics of the cover will also have a significant effect on how firm a mattress feels as well.
Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science. In other words … the only reliable way to know whether a mattress will be “firm enough” or “soft enough” for you will be based on your own careful testing or your own personal experience.
Having said that … there is a table here that you can use to convert the diameter of the wire to AWG (American Wire Gauge). The 1.28 mm diameter wire would be 16 gauge and the 1.42 mm diameter wire would be 15 gauge.
Phoenix