We are not married but are sharing a home mortgage on our primary residence. We intend to share the mortgage interest deduction. How do we go about splitting it?

[DoninGA · Accepted solution]

You only enter on your 2017 tax return the amount that you actually paid for the mortgage interest.  For example, if you paid half the mortgage payment each month then you can deduct half the mortgage interest.

[Hal_Al]

 You pretty much have a choice. One can claim it all or you can split it. It's usually best if only one claims it, allowing the other to use the standard deduction.

You have to meet the rules, which are:

1. You are legally obligated to pay it

2. You actually pay it. Paying from a joint account where you made sufficient deposits to cover the payments will usually meet this standard. However. paying from your own account would be a stronger audit defense. 

[SweetieJean]

The rules say that, since they are not married, they can each only claim what they actually each paid.