[CarolineW]

Very close over human ranges:

    C = (F-30) / 2
    F = 2 * C + 30

Exact conversions:

    C = (F-32) * 5 / 9
    F = C / 5 * 9 + 32

Table:

    C     F
  -40   -40
    0    32 (Approx water freezing point)
   10    50
   20    68 ( ~ 70)
  ~21    70
   30    86
   37    98.6 (body temperature)
   40   104
   50   122
  ...   ...
  100   212 (Approx water boiling point)