How to calculate gross annual income from net monthly income

Unsure if this is the appropriate forum, but hoping someone can help. I have been able to calculate net monthly income from gross annual, but struggling to reverse it so I can calculate gross annual from net monthly. Given the variables below, does anyone know how the equation should look?

  • taxRate1 applied to amounts up between 0 and taxBand1.
  • taxRate2 applied to amounts between band 1 and 2.
  • taxRate3 applied to amounts between band 2 and 3.
  • taxRate 4 applied to amounts above band 3.

    taxBand1 = 12500 taxBand2 = 45000 taxBand3 = 150000 - taxRate1 = 0% taxRate2 = 20% taxRate3 = 40% taxRate4 = 45%