Mashzo
Every other abv calculator multiplies the gravity you lost by 131.25 and hands you a number. HMRC does not, and neither does this one: the alcoholic products technical guide says to multiply the degrees a beer attenuated by a factor from a table the Laboratory of the Government Chemist produced, because the yield of alcohol is not the same at every strength. A beer that fell from 1.048 to 1.011 dropped 37 degrees; the factor for that line is 0.130, and the answer is 4.8%: the shortcut says 4.86%. Three free calculators here work the three sums a brew day makes you do, and each one prints the constant it used and where it came from.