Montana Quarterly Estimated Tax Deadlines (2026)
If you own a business, have significant self-employment income, or receive substantial investment or K-1 income, you owe quarterly estimated taxes to both the IRS and the Montana Department of Revenue. Miss them and you owe interest-based underpayment penalties on top of the tax. Here are the 2026 quarterly deadlines and how to calculate what you actually owe.
2026 federal and Montana estimated tax deadlines
Q1 2026 (covers Jan–Mar 2026): due Wednesday, April 15, 2026.
Q2 2026 (covers Apr–May 2026): due Monday, June 15, 2026.
Q3 2026 (covers Jun–Aug 2026): due Tuesday, September 15, 2026.
Q4 2026 (covers Sep–Dec 2026): due Friday, January 15, 2027.
Montana estimated payments follow the same schedule. Both federal and Montana estimates can be paid electronically — IRS Direct Pay for federal and TransAction Portal (TAP) for Montana.
Who has to pay them
You generally owe federal estimated taxes if you expect to owe at least $1,000 in tax after subtracting withholding and refundable credits. That threshold catches most S-corp and LLC owners, most self-employed people, and W-2 workers with significant side income, capital gains, or K-1 income.
Montana's threshold is similar — most business owners owe Montana estimates as well.
The safe-harbor calculation (the easy way)
The IRS gives you a safe harbor: if you pay in at least 100% of last year's total tax through withholding and estimated payments (110% if your AGI was over $150,000), you owe no underpayment penalty — regardless of what you actually owe when you file.
This is the simplest approach for most business owners. Take last year's total tax, divide by four, and pay that amount each quarter. If your income has jumped this year you will owe more in April, but you will not owe a penalty.
The current-year projection (the accurate way)
The better approach — especially if income is growing — is to project your current-year income and tax, subtract expected withholding, and divide the difference by four. This avoids both underpayment penalties and a painful April surprise.
This is exactly what proactive tax planning does. We update the projection twice a year for planning clients and adjust the estimated payments so April is a formality, not a shock.
What happens if you miss one
The IRS charges an interest-based penalty for underpayment, currently around 8% annualized. It is calculated quarter by quarter, so paying late is better than not paying, and paying more in a later quarter does not undo an earlier miss.
The good news: the penalty is not enormous for a single missed quarter of a small business. It typically costs less than $200 per $10,000 of underpayment per quarter. Not catastrophic — but easy to avoid.