Enterprise
Individual & Small Business
| Payment | Montana due date |
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| 1 | |
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Federal 2026 estimated tax due dates, for comparison: April 15, 2026, June 15, 2026, September 15, 2026, January 15, 2027.
Who must pay Montana estimated tax?
If you estimate you will owe more than $500 in tax for last year, you should make quarterly estimated payments this year. (Official page wording; penalties/interest may apply if you owe more than $500 in tax and didn't make estimated payments.)
How do you avoid a Montana underpayment penalty?
To avoid underpayment interest, pay either 90% of the current year's tax liability or 100% of the previous year's tax liability (Form EST-I). No 110% rule stated.
How much should you pay?
Start with your federal estimate: the free Quarterly Estimated Tax Calculator works out your 2026 federal payments and safe harbor. Montana figures its own tax, so the state amount is separate.