No one withholds tax on retainer income
An employee's taxes are handled invisibly through withholding. An agency owner's are not: clients pay retainers and project invoices gross, contractors and software get paid, and whatever profit remains has had exactly zero tax collected on it. The IRS does not want to wait until April for its share of that profit, so it requires owners of pass-through businesses, sole proprietors, partners, LLC members, and S corp shareholders alike, to pay as the year goes, once they expect to owe $1,000 or more at filing.
The mechanism is quarterly estimated payments, calculated with Form 1040-ES and paid most easily through IRS Direct Pay or an EFTPS account. Each payment covers both layers of an agency owner's federal bill: income tax on profit, and self-employment tax for owners of sole proprietorships and partnerships. Most states run a parallel estimated-payment system on their own vouchers, so budget for both.
Apr 15, Jun 15, Sep 15, Jan 15
The four federal due dates land around April 15, June 15, September 15, and January 15 of the following year. Notice the uneven spacing: the second payment arrives only two months after the first, which surprises owners in their first year. When a date falls on a weekend or holiday it rolls to the next business day.
Missing or shorting a payment does not trigger a dramatic penalty letter; it accrues an underpayment penalty that works like interest on the shortfall from that quarter's due date until paid. It is quiet, but on an agency owner's five-figure or six-figure tax bill it becomes real money, and it applies even if you pay everything in full at filing time. The system rewards paying evenly through the year, which matches how retainer income actually arrives anyway.
Safe harbor: 100% of last year makes you penalty-proof
Agency profit swings, and nobody wants to re-forecast taxes every quarter. The safe harbor rule fixes that: pay in at least 100% of last year's total tax liability, in four even installments (110% if your adjusted gross income was over $150,000), and you owe no underpayment penalty regardless of how much this year's profit grows. In a scaling year you settle the difference at filing; in a down year, you can instead pay 90% of the current year's actual liability and keep cash in the business.
S corp owners get an extra lever: taxes withheld from your own W-2 salary count as paid evenly through the year, no matter when withheld. Bumping withholding late in the year can retroactively cover earlier quarters in ways an estimated payment cannot, a genuinely useful correction tool if the year ran hotter than planned. However you get there, the operating habit is the same one that makes agencies durable: sweep a fixed share of profit into a tax account monthly, and the quarterly dates become transfers, not emergencies.