Who actually has to pay estimates
If you'll owe at least $1,000 in federal tax beyond your withholding, the IRS expects quarterly payments. Most W-2 employees don't — withholding covers them. The most common quarterly payers are self-employed people, partners and S-corp shareholders receiving K-1s, retirees with pensions and IRAs, and anyone with material investment income or one-time large events (RSU vest, property sale, business exit).
The four due dates
April 15 (Q1: Jan–Mar), June 15 (Q2: Apr–May), September 15 (Q3: Jun–Aug), and January 15 of the next year (Q4: Sep–Dec). The 'quarters' are uneven by design. Pay on time — the IRS computes underpayment penalty as a daily simple-interest calculation, so even a few days late on a large payment compounds.
Two safe harbors
You avoid the underpayment penalty if you pay either (a) 90% of your current-year tax, or (b) 100% of your prior-year tax — 110% if your prior-year AGI was over $150,000 ($75,000 MFS). The prior-year safe harbor is the easier shield: divide your prior-year total tax by four and pay that each quarter. As long as you hit it, you're protected even if your current-year income explodes.
Calculating from scratch
Project current-year income, subtract deductions and credits, apply brackets, subtract expected withholding, divide what's left by four. If you have one big event (a property sale, an IPO), make a single targeted payment in the quarter the event happens rather than spreading it. Form 2210 reconciles uneven income at year-end if your payments don't match your earning pattern.
State estimates
Most income-tax states have parallel estimate systems with the same dates. Calculate them the same way. Some states differ: California weights its 'quarters' (30% / 40% / 0% / 30%); some don't require Q2 or Q3. Penalty math at the state level is just as expensive — when in doubt, ask.