Quarterly estimated tax payments for freelancers – the four dates, the two safe harbors, and the math Marginmoth does for you.

If you earn a living as a freelancer, sole proprietor, or single-member LLC, the IRS treats you as both the employer and the employee. There is no payroll department withholding federal income tax from each invoice for you. There is also a second tax the W-2 world never sees: self-employment tax, which is the Social Security and Medicare your old employer used to split with you. Ignoring both of them for a year is the most common way a high-earning freelancer ends up writing a check they cannot write in April.

The fix is the quarterly estimated payment system – four checks a year, on a fixed calendar, under IRS Form 1040-ES. Miss the cadence and the IRS charges you an underpayment penalty (Form 2210) on the shortfall, computed per quarter with interest that compounds in your disfavor. The penalty is small relative to the bill, but the bill itself is the problem, and the four checks are how the bill is softened into something the cash flow can actually absorb.

The four dates are not negotiable. They are April 15, June 15, September 15, and January 15 of the following year. Each covers a three-month income period; the January payment covers the back half of November and December. If the 15th lands on a weekend or federal holiday, the deadline slides to the next business day, which is what the IRS publishes each year in a one-page notice that nobody reads until they need it.

Now the part that matters: the safe-harbor rule. The IRS does not require you to estimate your current-year tax perfectly. It only requires that you pay, on time, one of two safe-harbor amounts:

1. Ninety percent of your current-year total tax liability (income tax plus SE tax, less any credits). This is the "true up" path – you owe roughly what you owe, and the April return settles the difference.

2. One hundred percent of your prior-year total tax liability, or 110 percent if your prior-year adjusted gross income was above $150,000. This is the path almost every freelancer should default to, because the number is known, fixed in last years return, and the IRS computes the penalty against this floor, not against your best guess.

The second safe harbor is the one that saves people. Suppose your 2025 return showed $24,000 of total tax. Your 2026 quarterly payments are $6,000 each, four times a year. If 2025 AGI was above $150K, bump each payment to $6,600 – $26,400 on the year. That is the floor that removes Form 2210 from your life, regardless of how 2026 actually shakes out. A worked example matters more than the rule in the abstract: a freelancer whose 2025 AGI was $185,000 and total tax was $24,000 should be sending $6,600 every quarter, not $6,000, and the extra $2,400 across the year is the cost of certainty.

The alternative – paying 90 percent of your current-year liability – only makes sense if your income has dropped materially since last year. Even then, it is a projection, and the penalty still applies to any shortfall between the projection and the actual. The annualized-income method, which the IRS permits on Form 2210 Schedule AI, lets you weight the four quarters unevenly, so a freelancer whose income arrives in Q3 and Q4 can avoid overpaying in Q1 and Q2. It is paperwork your CPA knows how to file and worth it once your income is uneven enough to make the flat $6,000 feel like dead cash.

State estimated taxes are a reminder, not a footnote. California, New York, and most other states with an income tax have their own estimated-payment system with their own due dates (California is the same four dates as the federal for most filers; New York is on a different cadence the first year). The federal safe harbor does not protect you from a state underpayment penalty, and a state return in April can bring its own surprise check. Your CPA files both; what your books owe you is the same running projection at the state marginal rate.

The penalty mechanics, for the record. The IRS computes Form 2210 quarterly. Each underpaid quarter accrues interest at the federal short-term rate plus 3 percent, until either the underpayment is zeroed out or the April return catches up. The annualized-income method is the only legal way to recognize that Q1 income was small and Q3 income was large, so the IRS does not charge you for "missing" a Q1 payment the cash was never going to make.

This is where Marginmoth earns its keep. The nightly bank sweep and the per-line categorization from our second post produce a continuous P&L – the running scoreboard that the monthly close from our third post signs off on. Once the P&L is continuous, the SE-tax bite and the federal blended marginal rate project forward from every payout, and the quarterly payment number falls out of the close. You do not estimate once a year in a panic in March. You see the next quarters check, dated April 15 or June 15, on the cash-flow view the day after each invoice clears. The deduction side of the same problem is what our fourth post on expense tracking solves – the home-office square footage, the mileage, the half of SE tax – and a clean deduction set feeds the same projection back the other direction.

The pattern is the same one we keep writing about here. A continuous book, line-scored and human-reviewed, projects the tax liability as it accrues instead of as a surprise. That is how the four IRS checks turn from a March panic into a line item you already budgeted for, and how Form 2210 becomes a form you never have to read.

Marginmoth is $79 a month, no setup fee, no per-transaction surcharge. Tools that promise purely AI bookkeeping with no human review produce a close with a guessed-at quarterly figure, and the guess is what the IRS measures the penalty against. A close your CPA will sign off on is the bar. The tax reserve is one output of that close, and it is updated the day after every payment lands. If you do not yet have a CPA, this is the conversation to have. If you do, send them your April P&L and ask them to spot-check the reserve projection – the one we already wrote.