Building a Monthly Take-Home Calculator

By Charlie Groves

I have always been frustrated by the lack of a simple tool to help me figure out what salary I need to earn to achieve a desired monthly take-home pay. Something like this would be really useful when planning future finances and running retirement projections.

This is why I built howmuchdoineedtoearn.com - a simple tool that lets you input your desired monthly take-home pay and tells you what salary you need to earn it.

Try the Calculator →

Technical Implementation

The most interesting technical challenge was implementing the reverse calculation - going from monthly take-home to required salary. Since tax calculations involve multiple bands and thresholds, there's no simple formula to reverse it.

Instead, I implemented an iterative approach that tries different salaries until it finds one that results in the desired take-home pay:

def calculate_gross_income_for_net_income(target_net_income)
  gross_income = 0
  net_income = 0

  while net_income < target_net_income
    gross_income += 25
    net_income = calculate_net_income(gross_income)
  end

  gross_income
end

The calculator takes into account:

International Support

While the tool started with just UK support, I've since expanded it to cover several US states, each with their own tax systems:

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