The interest is calculated by your HSA custodian, who may or may not send you a statement (more likely not).
Why do you want to know your interest? The are two likely options:
If it is because you made an excess contribution that you are going to withdraw along with the earnings on the excess, don't try to figure out what the interest is.
When you withdraw an excess contribution, withdraw only the excess that is reported to you by TurboTax. Then, next year, you will receive from the HSA custodian a 1099-SA with a distribution code of '2' that will list the amount of earnings on the excess, and you will pay tax on that then (on your 2020 tax return).
Why? Because you probably have earnings on the excess in 2019 and 2020, so the IRS lets you wait to declare it until you do your 2020 return.
If it is because you live in California or New Jersey where HSA deductions are not allowed, then you must call your HSA custodian and find a customer support person who understands that you need to report the earnings as income, because in these two states, HSAs are like regular taxable financial accounts.
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