She's moving out and says that it's permanent

[MJontheMend]

I have the same but opposite issue with mortgage payment. What I did which seemed fair was to figure out what I'd be paying to rent something like my 2bx is renting and then make him responsible for paying half the mortgage above that amount. It's not fair to make the person who moved out pay half of the whole mortgage because you are using the house for your current shelter needs for which you would have to pay otherwise if the house was sold tomorrow. It makes sense if you think about the fact that you could move out too and the two of you could rent out the house to a third party and still equally share in any gain or loss in equity. Basically, you would be living rent-free in her half of the house if she were paying half the mortgage.