Open a business checking account this week and run every 1099 deposit and every business expense through it. The calculator needs net profit. You cannot see net profit in a personal checking account that also buys groceries.
Pick a bookkeeping floor, not an app personality. A spreadsheet with date / payee / amount / category is enough at $40k. Mileage log in the phone the day you drive. Home office is optional — simplified $5/sq ft up to 300 is the one that does not invite a reconstruction of your floor plan. Do not wait for “real software.”
Estimate net profit for the year, then re-estimate every month. January optimism is not a tax position. Plug the current trailing run-rate into the calculator. If Q2 was half of Q1, do not keep sending Q1’s number.
Create the IRS payment rail before the first due date. Direct Pay for one-off, EFTPS if this is the job. Put April 15, June 15, September 15, and January 15 on a calendar with the quarterly amount. A reminder on April 14 is how people miss it. Same-day wires have cutoffs; “I clicked on the 15th at 9pm” is a late payment.
Decide W-4 extra vs 1040-ES, on purpose. If a W-2 still covers most of the year and the side hustle is under ~$15k, Step 4(c) extra withholding can eat the SE tax without four extra payments. Once 1099 is the main check, estimated payments are cleaner than stuffing a W-4. Mixing both without a spreadsheet double-pays. See the W-4 guide.
Use last year’s return as the penalty shield, this year’s profit as the bill. 100% of prior-year tax (110% if AGI was over $150k) avoids the underpayment penalty. First year off a $0 return has no shield — pay toward 90% of current. Write both numbers on the calendar: “send $X to avoid penalty” and “I will still owe $Y in April if I only do that.”
Choose the retirement wrapper before Halloween, not on April 14. Want the $24,500 employee deferral or Roth or a plan loan? The solo 401(k) must exist by December 31. A SEP can be opened next year with an extension, and that is the only reason to pick it if you already missed the year. Run SEP vs solo on the calculator at your actual net, not at the $72,000 marketing cap.
If a SEP already exists from a prior freelance year, treat it as a traditional IRA. It sits in the backdoor Roth pro-rata pile. Roll it into the new solo 401(k) if the provider accepts incoming IRAs and you still want a clean backdoor. Leaving $4,200 in a forgotten SEP is how a $7,500 conversion becomes mostly taxable.
Do not elect S-corp at $40k of profit to “save SE tax.” Reasonable salary, payroll filings, state franchise tax, and a payroll provider usually cost more than the SE tax you imagined cutting. Revisit entity when net is high enough that a CPA can price the payroll against the savings — and when you can actually pay yourself a salary on time. This playbook is Schedule C on purpose.
Health insurance is a tax and a cash item. Marketplace premium, spouse’s W-2 plan, or an HSA HDHP if you qualify. Self-employed health insurance is an above-the-line deduction if you are not eligible for a subsidized employer plan. The HSA still needs the HDHP; do not open one on a copay plan. Keep the cash buffer at 3–6 months of personal essentials plus one bad invoice cycle.
December is for the plan contribution, not for shopping a custodian. Employee deferral cash has to be in the solo 401(k) by year-end. Profit-sharing / SEP can wait until the filing deadline. Do not “max it from the refund.” The refund is next year’s money. If cash is tight, fund the deferral first (bigger tax cut per dollar on modest profit), then employer money if anything is left.
Close the year with documents, not vibes. 1099-NEC / 1099-K as received, a profit and loss that ties to the business account, estimated-tax payment confirmations, and the plan adoption agreement. File. Pay. An extension is extra time to assemble the PDF, not extra time to earn the tax money. If a W-2 401(k) and a solo 401(k) both exist, the $24,500 employee cap is shared — payroll withholdings already used count against the side-hustle deferral.
One-page decision
Separate the money. Estimate net monthly. Pay the four dates (or 4(c) if the W-2 still dominates). Open the solo 401(k) by December 31 unless you are fine with a SEP’s 20% ceiling. Do not S-corp out of a tweet. Keep enough cash that a late invoice is not an IRS installment plan.