Illinois Quarterly Estimated Tax Calculator

Generally anyone who expects to owe $1,000 or more in tax for the year with no withholding to cover it — which describes most freelancers, gig workers, and small business owners without a W-2 job offsetting the balance.

April 15, June 15, and September 15 of 2026, then January 15, 2027 for the fourth quarter. Dates shift slightly if they land on a weekend or holiday.

Paying at least 90% of this year's tax (or 100% of last year's tax — 110% if your prior-year AGI was over $150,000) through withholding and estimated payments protects you from an underpayment penalty, even if you end up owing more at filing time.

The IRS charges an underpayment penalty based on how much you were short and for how long, calculated separately for each quarter — paying the full balance by the April filing deadline does not erase penalties for underpaying earlier quarters.

You can, but you'll likely owe an underpayment penalty on top of the tax itself. Spreading payments across the year at the IRS due dates avoids that extra cost.

Likely yes. Illinois has a flat 4.95% state income tax, and most states with an income tax expect their own quarterly estimated payments on a similar schedule to the IRS — check your state's department of revenue for its specific form and due dates.