F to C: subtract 32 then multiply by 5/9

C to F: multiply by 9/5 then add 32

I work in an industry where conversions in the same system or going from one to another is commonplace. Certain things you just remember. 7.48 gal/ft3, 1 inch/25.4 mm and so on.

How many mg/L of something to I need to add to soften 10 MGD of water. Fuck you metric.