Measuring alcohol content!!

[Mickchap]

Hi everyone. I’m a bit confused on which method to use when you measure your alcohol content. I have done a bit of research and they all say to subtract the FG from the the SG. But…. Then some say divide the answer by 7.36 and some say multiply the answer by 131.25. For example
1040-1010=30.
30 divided by 7.36 = 4.07 (first method)
30 multiplied by 131.25= 3.93 (second method)
Which one is correct????

 

[MHB]

The first gives the right answer in % ABV (alcohol by volume), when working in points, what most home brewers use in Australia

So both are right (ish), what most home brewers use in Australia is the first or close enough.
People tend to use 7.5 rather than 7.36 as the equation is actually polynomial rather than linear and 7.5 is closer around 5% ABV.
Standard form is
Change in gravity in points (like the 30 you have)/7.5
1.040 being 40 "points", likewise 1.010 being 10 points. Giving 30 points divided by 7.5 (or 7.36)
Second equation is when you use 1.040-1.010=0.030*131.25, not 30.
Mark