One thing I find frustrating about loan calculators is that it asks you how much you can pay a week/month towards your loan and then it calculates how long the loan period will be. But in reality (and I know I am making an assumption here) is that your earning power will increase let's say with inflation. So yes I might be able to put $500 a week towards a loan in 2015 but in 2016 I might be able to pay $515 a week and therefore reducing the loan period. How in Excel could I chart this? I am familiar with the nper function but it only takes a static payment amount to work out the loan length. How can I find out the remaining loan period at different time intervals based on increasing payments?

Thanks in advance!