The Paycheck Series · 11 min read
Moonlighting and 1099 Income
1099 income is taxed differently than W-2. Most residents who moonlight get surprised by a tax bill. Here is why — and what to do about it.
Reviewed by Jonathan Shafer, DO, July 2026. Disclosure: the reviewer is the founder and owner of Attending Financial LLC. This is education, not individualized financial advice.
The Q1 surprise
A senior resident moonlit two weekends a month during their PGY-3 year. The hospital paid via — $8,000 total across the year. In April, the resident files taxes and discovers they owe an additional $2,000. Why? No employer withheld federal, state, or FICA from the 1099 payments. The full tax bill came due at filing. This is the most common 1099 surprise in residency. Avoidable in 15 minutes if you know about it.
How 1099 differs from W-2
Tap each. Four mechanical differences with large consequences.
Self-employment tax (15.3 percent)
On W-2, you pay 7.65 percent FICA — your employer pays the matching 7.65 percent. On 1099, you pay both halves: 15.3 percent total on the first ~$184,500 of net earnings. This is on top of federal and state income tax. Plan for it.
Quarterly estimated payments
IRS expects you to pay tax quarterly on 1099 income — April 15, June 15, September 15, January 15. Failure to do so triggers underpayment penalties. Form 1040-ES is used. Many residents skip this and accept a small penalty — usually 4-8 percent of underpaid tax.
The Solo 401k opportunity
If you have 1099 income, you can open a Solo 401k. As a sole proprietor, the employer contribution is capped at 20 percent of net self-employment earnings after the deduction for half of SE tax, and it sits outside your hospital 403b limit (the $24,500 employee deferral is shared across both plans). For a resident moonlighting $20,000/year, the employer piece alone is about $3,700 of tax-deferred space.
Deductible business expenses
Travel to the moonlighting site, licensing fees, malpractice premium (if you pay it), DEA registration, board certifications, scrubs, and other directly related expenses are deductible against 1099 income on Schedule C. Track them in a simple spreadsheet.
See your moonlighting tax
Run a typical amount through the paycheck calculator to estimate the real take-home after self-employment tax.
Self-employment tax
A subtle but important difference between W-2 and .
When you receive 1099 income, who pays the employer portion of Social Security and Medicare?
When you are self-employed, you wear both hats. You pay the full 15.3 percent (12.4 percent Social Security + 2.9 percent Medicare) on the first ~$184,500 of net earnings, then 2.9 percent above that. Half is deductible against your income tax — but the cash outflow is real.
What a $1,500 1099 shift really nets
PGY-3 resident in the 22% federal bracket and a 5% state, takes a $1,500 weekend shift paid on a — nothing withheld.
Bottom line: About 41% of that $1,500 belongs to the IRS and your state — and none of it was withheld. Moving ~$600 into a separate "1099 taxes" account the day the check clears is how the April surprise never happens. (Half of the SE tax is later deductible against income tax, which softens the true cost a little.)
Skipping quarterly estimates quietly adds a penalty
Because nobody withholds tax from pay, the IRS expects you to send it in four times a year — April 15, June 15, September 15, January 15. Most residents never do and let it all come due in April. If you owe enough at filing, the IRS adds an underpayment penalty on top, calculated like interest on the tax you should have prepaid through the year.
How to avoid it: If your 1099 income is modest, the cleanest fix is to raise the withholding on your W-2 residency paycheck (file a new W-4) to cover the extra tax — withholding counts as paid evenly across the year and sidesteps the quarterly system entirely. If the 1099 income is larger, file Form 1040-ES quarterly from your dedicated tax-savings account. Either way the safe-harbor rule protects you: prepay at least 100% of last year's total tax (110% if you're a high earner) and no underpayment penalty applies, even if you still owe more in April.
Your first moonlighting shift
A PGY-3 resident takes their first shift. $1,500 for the weekend, paid via from the staffing company.
The check arrives. How much should the resident set aside for taxes before spending the rest?
Nothing — taxes will sort themselves out at filing
Common rookie move. In April, the tax bill arrives all at once. For $1,500 with no withholding: ~$212 self-employment tax + ~$180-330 federal income tax (12 or 22 percent bracket) + state tax. Total bill: roughly $450-650. If spent on lifestyle, you have to come up with it later.
Set aside 35-40 percent — about $525-600 — the better choice
Correct discipline. 15.3 percent self-employment tax on 92.35 percent of earnings (about 14 percent) + 12–22 percent marginal federal + 3-7 percent state = roughly 29-43 percent of every 1099 dollar owed at filing. Better to set aside slightly more and have a small surplus than scramble in April.
Set aside 25 percent — about $375
Likely insufficient. Self-employment tax alone is 15.3 percent. Add any federal/state income tax above zero and you cross 25 percent quickly. Short even in the 12 percent bracket once state tax is added (about 14 + 12 + 3 = 29 percent); well short in 22 percent.
The 1099 discipline
- A common discipline: 35-40 percent of every 1099 payment goes straight into a separate savings account, and quarterly estimated tax is paid from it.
- Form 1040-ES is the quarterly route. Skip it without a safe harbor (or enough extra W-2 withholding) and an underpayment penalty, computed like interest, applies.
- Track deductible business expenses against 1099 income — reduces the taxable amount.
- If 1099 income is substantial ($10,000+), a Solo 401k adds tax-deferred space worth pricing out.
Do this next: Take your last 1099 payment and multiply it by 35-40 percent. That is the reserve this module's arithmetic says it carried — compare it with what you actually set aside.
Residency curriculum · lesson 18 of 38
Next: Estimated Taxes Without the Panic.
Safe harbors, set-asides, and the quarterly calendar for 1099 income A free account keeps your place, records completion for you alone, and opens the interactive version with the calculators. Your program sees aggregate completion above five residents, and your own status only if you turn that on.
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