The IRS Charges a Penalty, Not a Crime
If you earn Substack income (subscriptions, paid tiers, tips) and don't have taxes withheld anywhere, the IRS expects you to prepay through quarterly estimated taxes using Form 1040-ES. Skip them and you don't go to jail or get audited automatically. What happens is simpler and quieter: the IRS calculates an underpayment penalty using Form 2210, treats it like interest on a loan, and adds it to your tax bill when you file.
The penalty is based on how much you owed each quarter, how late you paid it, and the current IRS interest rate, which changes quarterly and is usually a few points above prime. So the cost isn't fixed. A writer who owes $6,000 in tax for the year but pays nothing until April will owe more in penalties than one who pays even a rough estimate each quarter, even if the total tax bill is identical.
Why Substack Income Triggers This
Substack doesn't withhold anything from your subscription revenue. Whether you're paid through Stripe or get a 1099-NEC or 1099-K at year end, that income arrives gross. If you also have podcast sponsorships, affiliate links, or brand deals layered on top, none of those payors withhold either. The IRS still wants roughly your tax liability paid in on a rolling basis throughout the year, not all at once in April. That's the entire reason quarterly taxes exist for self-employed writers: there's no employer doing it for you.
The general rule: if you expect to owe $1,000 or more in tax for the year after subtracting any withholding, you're supposed to make estimated payments. For most full-time or side-hustle Substack writers, that threshold is easy to cross once you clear a few thousand dollars in net income.
What Actually Compounds the Damage
A few things make this worse than people expect:
- The penalty stacks quarterly. Each missed due date (roughly mid-April, mid-June, mid-September, mid-January) starts its own penalty clock. Catching up in Q4 doesn't erase what accrued in Q1 through Q3.
- Self-employment tax adds up fast. Substack income is subject to Schedule SE self-employment tax (15.3% on net earnings) on top of regular income tax, so the total owed is often higher than writers expect when they only budget for income tax.
- Underestimating quarterly amounts still helps. Paying something close to accurate each quarter, even if imperfect, reduces the penalty compared to paying nothing. There's also a safe harbor: if you pay at least 100% of last year's total tax liability (110% if your prior year income was high) spread across the four payments, you generally avoid the penalty even if you owe more at filing time.
What to Do If You've Already Missed a Quarter
Missing one due date doesn't mean giving up on the rest of the year. Send in a payment as soon as you can through Form 1040-ES or IRS Direct Pay, using your Schedule C net income (revenue minus deductible expenses like a home office, recording gear, editing software, or research subscriptions) to estimate what you owe. Paying late is still better than paying nothing, since the penalty calculation is based on how long each dollar sat unpaid, not just whether you technically missed the deadline. If you consistently guess wrong, recalculating your estimate each quarter based on actual year-to-date Substack and freelance income keeps the gap, and the penalty, smaller.