I am on the IBR plan and currently have 280k in debt (yikes, I know).  I also have 170k in a mutual fund which has earned about 8-9% returns the last few years.  I am currently earning about 75k a year, so my monthly payments do not cover my accruing interest, and likely never will for the next 25 years.  I am trying to figure out what my principal and interest would be in 25 years, to try and work out what my income tax bill would be.  I am hoping that my 170k in my mutual fund will grow sufficiently in 25 years to cover this income tax bill and leave me with extra for retirement.  However, when I use various calculators online, they aren’t able to factor in my meager monthly payment contribution.  I realize it gets complicated trying to assume future earnings and recalculating the IBR payment every year- but I wonder if there is a way to calculate the ‘worst case scenario’ being I earn only what I earn now or less for the next 25 years to see what the tax bill would be.  The estimation on the studentloans.gov site gives me a forgiven amount of about 400,000, which I feel is an underestimation because they assume pay increases of 5% annually which I don’t think is realistic in my profession.

Are there any useful calculators online out there to use for the IBR scenario?