If you are getting a refund, there is not a penalty for filing past the deadline. If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.
Hi @patrick cornish !
TurboTax will offer to calculate your underpayment penalty, not interest, based on the information you enter, but the penatly and interest will be determined by the IRS at the exact time they actually process your return. This will still generate a letter from them advising you of the amount owed. If you "opt out" of having your penalty calculated within TurboTax, it won't change what your penalty or interest actually is.
Hope this helps!
Cindy
Here are some links with information about how much the interest and penalties would be.
https://www.irs.gov/taxtopics/tc653
If you have filed an extension, and are struggling to figure out how to pay your taxes, consider filing by the extended deadline and setting up a payment plan arrangement with the IRS. This will eliminate a failure to file penalty, and reduce the failure to pay penalty by 50%.
https://www.irs.gov/payments/payment-plans-installment-agreements
You may also want to consider requesting a penalty abatement (removal of the penalty completely). If you usually pay your taxes on time but messed up for the first time this year, or if there are extenuating circumstances, the IRS may waive the penalty. Here's some info about that. You would wait to receive the notice from the IRS, and then follow the instructions to apply.
https://www.irs.gov/payments/penalty-relief-due-to-first-time-abate-or-other-administrative-waiver