Three scales: Celsius (°C, used almost everywhere), Fahrenheit (°F, mostly the US), and Kelvin (K, science). They measure the same thing, just from different starting points.
C to F
F = C × 9/5 + 32
F to C
C = (F − 32) × 5/9
C to K
K = C + 273.15
Try it
100 °C in °F?
100 × 9/5 + 32 = 180 + 32 = 212 °F. (Boiling water.)
Try it
−40 °C in °F?
−40 × 9/5 + 32 = −72 + 32 = −40 °F. The two scales meet at −40°.