Excel

Create the cells above and create names for each of the values from the names in the left column.

Two functions are available, one for employee national insurance, one for employer.

The calls are

=NationalInsurance(50000.0,Lower_Earnings_Threshold,Upper_Earnings_Threshold,Class_1,Class_1___4)

=EmployerNationalInsurance(50000.0,Lower_Earnings_Threshold,Class_1___Secondary)

The code for these functions is this

Function NationalInsurance( _
   income As Double, _
   lowerearnings As Double, _
   upperearnings As Double, _
   classonerate As Double, _
   secondaryrate As Double _
   ) As Double
   NationalInsurance = 0#
   If income <= lowerearnings Then        
       Exit Function    
   End If    
   If income <= upperearnings Then        
       NationalInsurance = (income - lowerearnings) * classonerate    
   Else        
       NationalInsurance = (upperearnings - lowerearnings) * classonerate + (income - upperearnings) * secondaryrate    
   End If     
End Function 

Function EmployerNationalInsurance( _
   income As Double, _
   lowerearnings As Double, _
   secondaryrate As Double _
   ) As Double
   EmployerNationalInsurance = 0#
   If income <= lowerearnings Then
       Exit Function
   End If
   EmployerNationalInsurance = (income - lowerearnings) * secondaryrate
End Function

With this as a table, the following function call can be made from Excel

=IncomeTax(50000.0,TaxRates)

To calculate the tax paid. TaxRates is the range above.

The code for this is as follows.

Function IncomeTax(income As Double, rates As Range) As Double
    Dim i As Integer
    Dim tax As Double
    tax = 0
    For i = 1 To rates.Rows.Count - 1
        tax = tax + Application.Min(income, rates.Cells(i, 1)) * rates.Cells(i, 2)
        income = Application.Max(0, income - rates.Cells(i, 1))
    Next i
    IncomeTax = tax + income * rates.Cells(rates.Rows.Count, 2)
End Function