Quarterly Estimated Taxes: A Small Business Owner's Guide

Who owes quarterlies

The technical trigger: you expect to owe $1,000 or more in tax for the year beyond any withholding. In practice, that's nearly every profitable LLC owner, partner, S corp shareholder (for income above their W-2 withholding), and side-business earner. The payments cover income tax plus, for default LLC owners, self-employment tax, which is the part that surprises people: 15.3% applied to 92.35% of net self-employment earnings, before income tax even starts. (Why profit, not draws, is what's taxed: How to Pay Yourself From an LLC (Without Creating IRS Trouble) ↗.)

The due dates (memorize the shape, diary the dates)

Four payments, unevenly spaced, the 'quarters' are 3, 2, 3, and 4 months long: mid-April (Q1), mid-June (Q2), mid-September (Q3), and mid-January of the following year (Q4). The June date ambushes everyone new, it arrives two months after April, not three. When a date falls on a weekend or holiday it rolls to the next business day.

Safe harbor: the rule that ends the guesswork

You don't have to predict your year perfectly. Pay in, through the year, in even quarterly chunks, either 90% of what you'll actually owe this year, or 100% of last year's total tax (110% if your prior-year adjusted gross income topped $150K), and you're penalty-proof no matter how the year ends. That second option is the workhorse: last year's tax is a number you already know.

The strategy this creates: in growing years, pay the prior-year safe harbor (the smaller number), bank the difference, and settle up in April penalty-free. In shrinking years, switch to estimating 90% of the current year so you're not lending the IRS money you need. The choice each quarter is exactly the kind of small decision that's easy with current books and impossible without them.

How to actually calculate a quarter

The simple method (safe harbor)

Take last year's total tax (line item on your 1040), multiply by 100% or 110% per the rule above, divide by four, pay on the dates. Done. This is the right method for anyone whose income is stable or growing.

The current-year method (annualizing)

For lumpy or shrinking income: each quarter, take actual profit-to-date from your books, project the year, compute tax on the projection (income tax at your bracket plus self-employment tax: 15.3% applied to 92.35% of net self-employment earnings, less the SE deduction), subtract what you've paid, and pay the difference. It's more work, and it's only possible if the books are current, which is the quiet argument for monthly bookkeeping hiding inside every tax topic.

The set-aside habit that makes both work

Whatever method: move 25–30% of profit into a separate tax account as the profit happens, weekly or monthly, automatically. Quarterlies then become transfers, not crises. Owners who fund taxes from the same account as operations are one strong quarter away from spending the IRS's money on a truck.

How to pay

Federal: IRS Direct Pay or an EFTPS account (worth the setup, it keeps a payment history), applied as estimated tax for the correct year. State: nearly every income-tax state runs its own parallel estimated system with its own portal and dates, the state payment is the one owners forget. S corp owners: your quarterly math should also check whether simply increasing W-2 withholding covers you, since withholding is treated as paid evenly all year, a clean fix for a late-year income surprise.

What the penalty actually is

Not a fine so much as interest, computed quarterly on the shortfall at the IRS underpayment rate, which has been high enough in recent years (7–8%) to stop being ignorable. A $40K tax bill paid entirely in April instead of quarterly can cost four figures in penalty. Avoidable, every time, with the safe harbor and a calendar.