Filing Quarterly Estimated Taxes on Side Income

Why this matters

Your paycheck has tax taken out before you see it. Side work does not. A tech who does weekend jobs all year, spends the cash, and then meets the bill in April gets hit three ways: a balance due nobody budgeted for, self-employment tax most W-2 workers have never heard of, and an underpayment penalty for not paying during the year. The federal system expects tax to be paid as income is earned, through withholding or through quarterly estimated payments. This article shows how to size those payments, when they are due, and the safe-harbor rule that lets a W-2 tech with side income avoid the penalty with very little guesswork.

Figures and rules here are current as of 2026. Tax law changed in 2025, including a temporary deduction for qualified overtime pay and changes to other deductions, and the annual limits are indexed, so check the current year's Form 1040-ES instructions before relying on any of it. This is not personal tax advice.

Whether the side work itself is a good idea (insurance, licensing, your employer's rules) is its own subject. See related: Side Work: The Tax, Insurance and Employer Risks.

Step 1: Decide whether you need to pay estimates at all

You generally owe estimated payments if you expect to owe at least the small-balance threshold in the Form 1040-ES instructions after your withholding and credits are subtracted, and your withholding will not reach the safe harbor in Step 5.

Why first: a lot of techs with modest side income can skip quarterly payments entirely by meeting the safe harbor through their paycheck. Knowing that before you open a payment account saves effort; finding it out in April after paying four installments you did not need is only an inconvenience, but missing installments you did need costs a penalty.

Step 2: Track side profit, not side income

What is taxed is net profit: what customers paid you minus your real business expenses for the side work (materials you bought, a proportion of tool costs, mileage for side jobs at the IRS standard mileage rate or actual vehicle costs, a share of your phone). That profit goes on Schedule C, and self-employment tax on it goes on Schedule SE.

Why: estimating on gross receipts overstates your tax, sometimes badly on material-heavy jobs; estimating on a guess understates it. Keep a running ledger by job: date, customer, amount received, materials, miles. See related: Going 1099: The Tax Math a Tech Misses for the full list of what counts as an expense and what does not.

Step 3: Work out the percentage to hold back

Side profit carries two federal taxes.

• Self-employment tax is the Social Security and Medicare tax an employer would normally split with you, paid in full: 15.3 percent, charged on 92.35 percent of net self-employment earnings, so about 14.1 percent of profit. The Social Security part stops at the annual wage base, which is indexed, and your W-2 wages count toward it first; few techs reach it. It applies once net self-employment earnings pass a small floor set in the code.
• Income tax at your marginal bracket, on profit after two deductions: half of the self-employment tax, and for most techs the qualified business income deduction, up to 20 percent of the remaining side profit, which the 2025 law made permanent.

For a tech in the 12 percent bracket, those combine to about 23 percent of profit for federal (the arithmetic is in the worked example below). Add your state income tax rate if your state has one. A common starting point: hold back 25 percent of each side job's profit for federal tax, plus your state rate. Tune it after your first full year, when your own return tells you what the real figure was.

Step 4: Move the hold-back the day you get paid

Open a separate savings account and transfer the percentage from each payment the same day it lands.

Why: a percentage left in your everyday account is spent by the time the due date comes. A separate account turns the quarterly payment into moving money that is already sitting there. If you are also running a seasonal reserve, keep the tax account separate from it; see related: Building an Emergency Fund on Seasonal Income.

Step 5: Pick the safe harbor you will aim at

Under the federal rules (Internal Revenue Code section 6654), you owe no underpayment penalty if your withholding plus timely estimated payments reach the smaller of:

• 90 percent of this year's total tax, or
• 100 percent of last year's total tax, shown on last year's return. If last year's adjusted gross income was above the high-income threshold in the instructions, this becomes 110 percent.

The prior-year path is the one that removes guesswork: last year's total tax is a known number on a filed return. The current-year path can need less money in a year when side work drops off, but you are betting on an estimate of a year that is not over.

Withholding has an advantage estimated payments do not: federal withholding is treated as paid evenly through the year, whenever it actually came out of your check. So a tech who missed a spring installment can raise the extra withholding on a new Form W-4 in the fall, and that withholding counts as if it had been spread across all four quarters. Estimated payments only count from the date you make them.

Step 6: Pay on the four due dates

Estimated tax is due in four installments, and the periods they cover are uneven:

When a date falls on a weekend or legal holiday, it moves to the next business day. You can skip the January installment if you file your return and pay the full balance by January 31.

Why the dates matter: the penalty is figured installment by installment, as interest at the federal short-term rate plus 3 percentage points (section 6621) on the amount short for the days it was short. A big payment in December does not erase a missed April payment; it stops the interest from that day forward.

If your side work is lumpy, heavy in summer and dead in winter, equal installments over-pay early in the year. The annualized income installment method (Form 2210, Schedule AI) lets each installment follow the income actually earned in that period, at the cost of more paperwork at filing time.

Step 7: Pay online and keep the confirmation

Pay through the IRS's own systems: IRS Direct Pay (from a checking or savings account, no fee), the Electronic Federal Tax Payment System (EFTPS, requires enrollment), or your IRS online account. Card payments go through IRS-authorized processors and carry a fee. Select "estimated tax" and the correct tax year (a January payment belongs to the previous year), and save the confirmation number with your records. Your state's estimated payments are separate, on its own website, often on similar dates.

What breaks if you pick the wrong year: a January payment applied to the new year leaves the old year looking unpaid, and you will spend time on the phone getting it moved.

Worked example: the first two years of side work

Call one week of the tech's W-2 gross pay W, so the figures hold at any wage. The tech is single, in the 12 percent bracket, and the state is ignored to keep the federal arithmetic visible.

The year before side work. Total federal tax on the return: 4.0 W, all withheld from paychecks.

Year 1. The tech nets 10 W of side profit. The extra federal tax:

• Self-employment tax: 10 W times 0.9235 times 0.153 is 1.413 W.
• Half of that is deductible: 0.707 W. Profit after it: 10 minus 0.707 is 9.293 W.
• Qualified business income deduction at 20 percent of 9.293 W is 1.859 W.
• Taxable addition: 9.293 minus 1.859 is 7.434 W, times 12 percent is 0.892 W of income tax.
• Extra federal tax: 1.413 plus 0.892 is 2.305 W, which is 23 percent of the 10 W of profit.

Total federal tax for Year 1 is 4.0 plus 2.305, or about 6.31 W. Withholding stayed at 4.0 W. The tech made no estimated payments and still owes no penalty, because 4.0 W of withholding is 100 percent of the prior year's 4.0 W total tax. The tech does owe the 2.31 W balance with the return in April, and the 25 percent hold-back, 2.5 W, covers it with about 0.19 W to spare.

Year 2. Now the prior year's tax is 6.31 W, and withholding of 4.0 W no longer reaches it. Three ways forward:

• Prior-year safe harbor by estimates: pay 6.31 minus 4.0, or 2.31 W, in four installments of about 0.58 W each (0.58, 0.58, 0.58 and 0.57). Certain, whatever the side work does this year.
• Prior-year safe harbor by withholding: add 2.31 W of extra withholding across the 26 biweekly checks, about 0.089 W per check, with a new W-4. Same protection, nothing to remember on four dates.
• Current-year safe harbor: if side profit falls to 6 W, the extra tax is 6 tenths of 2.305 W, or 1.383 W, and total tax is 5.383 W. Ninety percent of that is 4.845 W, so only 0.845 W beyond withholding is needed, about 0.21 W a quarter. That is under half of the 2.31 W the prior-year path asks for, but if the side work comes back strong in the fall, 90 percent of the larger total may not be met and the penalty applies.

What changes the answer. If Year 1's adjusted gross income had crossed the high-income threshold, the prior-year path would need 110 percent of 6.31 W, about 6.94 W, so 2.94 W beyond withholding. If the tech lived in a state with an income tax, a separate state estimate on the state's own schedule would sit beside all of this, and state safe-harbor rules do not always match the federal ones, so check your state's instructions. And if the tech's W-2 job ended partway through the year, withholding would stop with it and the whole Year 2 plan would need resizing.

References

• IRS Form 1040-ES and instructions (current year), including due dates and the small-balance threshold
• Internal Revenue Code section 6654 (underpayment of estimated tax and safe harbors) and section 6621 (underpayment interest rate); IRS Form 2210 and Schedule AI
• IRS Schedule C and Schedule SE instructions
• See related: Going 1099: The Tax Math a Tech Misses; Side Work: The Tax, Insurance and Employer Risks; Building an Emergency Fund on Seasonal Income