Side Business Tax: Pay It Through Paycheck Withholding
How an employee with a side business can skip quarterly estimated payments by raising withholding at the day job, why withholding is treated as paid evenly all year, and the limits of the approach.
Reviewed by the Fairlight Accounting cross-border tax team — U.S. & Canadian Tax Desks
Employees with a side business can pay the tax on it through extra withholding at their job instead of quarterly estimated payments. Withholding is treated as paid evenly across the year no matter when it was taken, so even a large increase in December counts for all four quarters and can avoid the underpayment penalty.
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Why does withholding work differently?
| Payment | How the IRS credits it |
|---|---|
| Income tax withholding from wages | Spread evenly over the four quarterly periods, regardless of when withheld |
| Estimated tax payment | Credited to the quarter in which it is paid; late quarters incur penalties even if the year's total is right |
A side business that earns most of its profit in the fourth quarter cannot catch up with a January estimated payment without penalty exposure for the earlier quarters (unless the annualized income installment method on Form 2210 shows less was due earlier), but a December withholding increase covers the whole year.
How do you set it up?
- Project the side business's net profit for the year.
- Compute the tax: income tax at your marginal rate plus self-employment tax (15.3 percent on 92.35 percent of net profit, with half of it deductible for income tax; the 12.4 percent Social Security part stops once wages and self-employment earnings together reach $184,500 for 2026, or $176,100 for 2025), plus the 0.9 percent additional Medicare tax if combined wages and self-employment earnings exceed $200,000 ($250,000 married filing jointly, $125,000 married filing separately).
- Divide the total by the remaining pay periods and enter that amount on Form W-4, Step 4(c) (extra withholding), as additional withholding.
- Revisit mid-year and in November; adjust the W-4 again if profit has moved. An employer has until the start of the first payroll period ending on or after the 30th day after receiving a new W-4 to apply it.
The IRS Tax Withholding Estimator handles the arithmetic for most situations.
What are the safe harbors?
No underpayment penalty applies if total withholding and estimates reach 100 percent of last year's tax (110 percent if last year's adjusted gross income exceeded $150,000, or $75,000 if married filing separately) or 90 percent of this year's tax — or if less than $1,000 is owed after withholding. Withholding that meets a safe harbor by December 31 is enough, however uneven the side income.
When is this not enough?
When the side business profit is large relative to wages — the W-4 cannot withhold more than the paycheck — or when the employee leaves the job mid-year. Spouses can combine: extra withholding at either spouse's job covers a joint return. A business that grows into the main income source should move to estimated payments or an S corporation payroll.
Frequently asked questions
Does extra withholding have to be labeled for the side business?
No. All withholding is one pool credited against the total tax on the return.
Can I use withholding to cover a one-time capital gain?
Yes. The same even-spreading rule applies to any tax, not just side business tax.
Will my employer object to a large additional withholding amount?
No. Employers must honor a valid Form W-4 request for additional withholding, up to the pay left after other required withholding.
Does withholding cover state tax on the side business?
Only if the state has income tax and you add state withholding too; Florida has no state income tax.
Official sources
The IRS explains: “Use this tool to estimate the correct amount of tax your employer (W-2) or pension provider should withhold each year.” — Internal Revenue Service, Tax Withholding Estimator, https://www.irs.gov/individuals/tax-withholding-estimator
The IRS explains: “Generally, most taxpayers will avoid this penalty if they either owe less than $1,000 in tax after subtracting their withholding and refundable credits, or if they paid withholding and estimated tax of at least 90% of the tax for the current year or 100% of the tax shown on the return for the prior year, whichever is smaller.” — Internal Revenue Service, Topic no. 306, Penalty for underpayment of estimated tax, https://www.irs.gov/taxtopics/tc306
Next step
Fairlight Accounting handles U.S. domestic, cross-border (U.S.–Canada), and international tax returns, plus bookkeeping, payroll, and CFO advisory. Our U.S. Tax Desk sets the W-4 amount for side-business clients each spring and rechecks it in November. See pricing or book a free fit call.
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