Calculating the interest for partial year loans

If your loan does not last the full calendar year, be forewarned the tax rules and possibly the tax software seem broken/wrong on how to handle this case. For example, you (a) bought a property mid year or (b) you refinanced from loan L1 to L2, so that L1 stopped and L2 started mid year, You can calculate the “mathematically correct” deduction by calculating the eligible principal per day and applying the interest rate. Namely, for every day you have a loan, determine the max deductible principal P = min(1M or 750K, actual principal). Multiply P by the daily interest rate of that loan. E.g. a 4% loan charges 4%/365 a day. Add this up for every day you had the loan. Sounds hard but it is not. This calculation for a single loan L usually involves two phases of the form

1.

You have a loan L for D days of the year. Figure out P and multiply by the daily interest rate DIR, getting P * D * DIR.

2.

The remaining 365-D days of the year, you did not have loan L, which adds 0 (zero) to your interest deduction.

If you had two loans L1 and L2, say due to a refinancing, do the above steps for L1 and L2.

When calculating your average balance for tax purposes, TurboTax defaults to using the average of your first and last balance. However, you’re free to choose any reasonable method, such as a month-by-month average based on statements from your lender. For a more accurate calculation, especially if you’ve made a significant payment in March, consider computing the interest paid and average balance separately for the periods before and after this payment. This approach also helps if you’ve sold one home and bought another within the year, a situation that requires some clever workarounds to accurately report in TurboTax.