Four deadlines, zero withholding
If you're self-employed, nobody withholds taxes for you. The IRS expects four payments a year instead: April 15, June 15, September 15, and January 15. Underpay and you're charged a penalty that keeps accruing until you catch up.
Most self-employed people handle this one of two ways. Either they pay a flat number every quarter (a guess made back in January that ignores how the year is going), or they open their banking app the night before the deadline and try to reconstruct three months of income from memory and a search box.
Your bank account already knows the real answer. Every client payment, every software renewal, every equipment purchase is sitting in the transaction history. The data was never the problem. The problem is the hour of copying numbers into a spreadsheet four times a year.
What you need
Two things: a BankBridge account with your business bank connected, and an AI agent that speaks MCP. Claude, ChatGPT, Cursor, Gemini CLI, and a couple dozen other hosts all qualify. Setup takes a few minutes and costs $5 a month per connected bank.
Three BankBridge tools do the work here. list_transactions pulls the raw deposits for any date range. get_spending_summary totals your expenses by category. get_monthly_cashflow gives you the income-minus-expenses view as a sanity check.
Everything is fetched live from the bank when you ask. Nothing sits in a cache, so the number you compute on June 14 includes the deposit that cleared on June 13.
Step 1: Pull the period's real deposits
Start with income. Ask your agent something like:
List every deposit into my business checking from April 1 through May 31. Separate client payments from transfers, refunds, and anything else that isn't income.
Note those dates. IRS payment periods aren't even quarters: the June 15 payment covers only April and May, while the January payment covers four months, September through December. Getting this wrong is the most common quiet error in DIY estimated taxes.
The agent calls list_transactions with that range, then does the part that's tedious by hand: sorting real revenue from noise. A transfer from your own savings looks exactly like a client payment until something with context reads the description.
Step 2: Subtract the business expenses
Summarize my business spending for the same period by category. Exclude credit card payments and transfers between my own accounts.
get_spending_summary returns the categorized totals. The exclusions matter: if you charge expenses to a business card and then pay the card from checking, counting both the swipes and the payment doubles your deduction. The agent should count the swipes and skip the payment.
This is also where you catch deductions a night-before scramble would miss. The $47 domain renewal from April. The subscription that changed its billing descriptor mid-quarter. It's all in the history, and the agent reads all of it.
Step 3: Run the 1040-ES math
Say the period shows $28,400 in client deposits and $6,150 in deductible expenses. Net profit: $22,250.
Self-employment tax applies to 92.35% of that, so $20,548, taxed at 15.3%. That's $3,144. Half of it ($1,572) is deductible against your income tax. Then income tax applies at your bracket: at 22%, roughly $4,550 on the remaining profit. Total for the period: about $7,700.
That's the blunt version. Your real number comes down once the standard deduction and the 20% QBI deduction enter the picture, which is why you tell the agent your situation and let it carry the details:
Using those deposit and expense numbers, calculate my estimated payment for this period. Single filer, no W-2 income, include the QBI deduction and my standard deduction spread across the year. Show every step.
The output you want isn't just a dollar figure. It's the worked math: income, expenses, SE tax, deductions, bracket. Save it. If the IRS ever questions a payment, a documented calculation built from real bank data beats a shrug.
Cross-check against the safe harbor
There's a second valid answer to "how much should I pay": the safe harbor. Pay 100% of last year's total tax in four equal installments (110% if your AGI was over $150,000) and the IRS won't penalize you no matter what you earn this year.
So which do you use? If your income is up, safe harbor is the smaller check today, but you'll owe the difference next April. If your income is down, the actual-deposit math means you stop overpaying now instead of waiting a year for the refund.
My total tax last year was $31,000. Compare the safe harbor installment against the payment based on this period's real profit, and tell me the tradeoff.
An agent with both numbers in front of it lays out the comparison in one reply. For a lot of freelancers, that comparison was the whole reason to keep an accountant on retainer.
Deposits that aren't income
The biggest risk in the deposit-based approach is counting money that isn't revenue. Four patterns to watch.
Transfers between your own accounts: moving $5,000 from savings to checking lands as a deposit but isn't income. Refunds and reimbursements: a returned purchase, or a client paying you back for a pass-through cost, isn't profit. Loan proceeds and credit line draws. And your own contributions, if you topped up the business account from personal funds.
A good agent separates these by reading descriptions and asking when it's unsure. Spot-check its buckets the first quarter. After that, the pattern holds.
One more trap: don't annualize a hot streak. If April was your best month ever, that doesn't mean the year quadrupled. Estimate from the period's real numbers and let each quarter correct the last one.
Make it a fifteen-minute ritual
The whole flow, four times a year, is one conversation:
It's estimated tax week. Pull this period's deposits and expenses from my business checking, run the 1040-ES math with the assumptions we used last quarter, and compare it against my safe harbor amount.
No CSV exports, no statement PDFs, no rebuilding a spreadsheet from scratch. The connection is read-only, so the agent can compute the payment but can't touch the money. You make the payment yourself at IRS Direct Pay or EFTPS.
Guessing four times a year is how people end up with penalties, or with interest-free loans to the government. The deposits are right there. Use them.