Side Income Taxes for Software Engineers

Your W-2 Has Withholding. Your Side Income Does Not.

Every paycheck, your employer quietly handles your tax bill. They withhold federal income tax, state income tax, and — crucially — half of your Social Security and Medicare taxes. You see the net deposit and move on.

The moment you start freelancing, consulting, building a SaaS on the side, or doing anything that generates 1099 income, all of that changes. Nobody is withholding anything. The IRS does not get a copy of your Stripe dashboard. But come tax time, they expect every dollar of that income reported, and they expect you to have been paying it along the way.

This guide covers the mechanics: what taxes hit side income, how to run the math, which deductions actually move the needle, and how to use tax-advantaged accounts to soften the blow.

The 1099 Situation

If you earn $600 or more from a single client in a calendar year, they’re required to send you (and the IRS) a 1099-NEC by January 31st. That form reports non-employee compensation.

Important nuance: you owe taxes on ALL self-employment income, even if you don’t get a 1099. The $600 threshold is the client’s filing requirement, not your reporting threshold. Made $400 doing contract work for your neighbor’s startup? That’s still taxable income. The IRS has no sense of humor about this.

The good news is that 1099 income is also business income, which means you get to deduct legitimate business expenses against it. More on that shortly.

Self-Employment Tax: The Double Hit

Here’s where side income gets expensive. W-2 employees pay 7.65% of their wages in FICA taxes (Social Security + Medicare). Their employer pays an equal 7.65% on top of that — a cost that workers never see but that economists will tell you ultimately comes out of worker compensation anyway.

When you’re self-employed, you pay both halves. That’s 15.3% of your net self-employment income.

But the IRS isn’t completely heartless. They let you exclude the “employer’s share” from the SE tax base by multiplying your net income by 92.35% before applying the rate. The logic is that if you were an employer paying that 7.65%, it would be a deductible business expense for you, so this adjustment approximates that.

The math on $30,000 of net freelance income:

• Net SE income: $30,000
• Apply the 92.35% factor: $30,000 × 0.9235 = $27,705
• SE tax: $27,705 × 15.3% = $4,239
  • Social Security portion (12.4%): $3,435
  • Medicare portion (2.9%): $804

That $4,239 is what you owe in SE tax before a single dollar of federal income tax enters the picture.

The above-the-line deduction for half of SE tax

You get to deduct half of that SE tax as an adjustment to income on Schedule 1 of Form 1040. In this example, that’s roughly $2,120 that reduces your AGI — not just your taxable income, but your Adjusted Gross Income, which affects everything from IRA eligibility to ACA subsidy calculations.

The deduction doesn’t eliminate the SE tax, but it does partially offset it by reducing your income tax burden.

Income Tax on Top: The Full Picture

Self-employment income doesn’t just get hit by SE tax. It also stacks on top of your W-2 income for federal (and state) income tax purposes.

If you’re a software engineer earning $150,000 in salary, you’re already well into the 22% federal bracket (24% once you cross ~$201k as a single filer in 2026). Every dollar of net freelance income — after deducting half the SE tax — gets taxed at your marginal rate.

So that $30,000 in freelance income costs you roughly:

• SE tax: ~$4,239
• Federal income tax at 22% on ($30,000 - $2,120): ~$6,133
• Total: ~$10,372

That’s about 34.6% of the gross freelance income gone before state taxes. Most tech workers in high-income states are looking at an effective rate of 37–42% on side income once you add California’s 9.3%, New York’s 6.85%, or whatever local flavor of “we also want some” your state charges.

This is not a reason to avoid side income. It’s a reason to track every deductible expense and max out your retirement contributions.

Schedule C: Your New Best Friend

Schedule C is where self-employed income and expenses get reported. It’s attached to your Form 1040 and is the document that transforms “gross freelance receipts” into “net self-employment income.”

The math: Revenue - Business Expenses = Net SE Income

That net SE income is what gets hit by SE tax and added to your AGI. Reducing it is worth real money at a 35%+ combined marginal rate.

Legitimately deductible business expenses:

Equipment — Computers, monitors, keyboards, external drives, tablets. If you bought a MacBook for freelance work, the cost is deductible. You can take it all in the first year via Section 179 expensing, or depreciate it over time. Section 179 is simpler and usually better for small-ticket items.

Software and subscriptions — GitHub Copilot, JetBrains IDEs, Figma, domain registration, hosting, email services, design tools, project management software. If it’s used in your freelance work, it’s a business expense. Pro-rate anything you use for both personal and business purposes.

Home office — If you have a dedicated space used exclusively and regularly for business, you can deduct it. The simplified method: $5 per square foot, up to 300 sq ft ($1,500 max). The actual expense method: calculate the percentage of your home used for business and apply that to rent/mortgage interest, utilities, internet, and repairs. The actual method is more math but often yields a larger deduction.

Professional development — Online courses, technical books, conference fees, certifications related to your freelance work. A course on AWS architecture for a client project is deductible. Your third re-read of Clean Code probably is too.

Health insurance premiums — If you’re not eligible for employer-subsidized health insurance (or your spouse’s plan), you can deduct 100% of health, dental, and vision premiums you pay for yourself and your family. This is an above-the-line deduction, so it reduces AGI directly, not just taxable income.

What doesn’t fly: general clothing (“business casual” wardrobe), commuting to a client site you use regularly, meals with yourself while working. The IRS has seen all of these and is not amused.

SEP-IRA: Retire Your Tax Burden

The single best tax move for side income is contributing to a retirement account that didn’t exist for you as a pure W-2 employee: a SEP-IRA.

SEP-IRA contributions are deductible against your freelance income, reducing your federal income tax dollar-for-dollar. They grow tax-deferred and get taxed as ordinary income in retirement (when you’ll presumably be in a lower bracket).

2026 contribution limit: The lesser of 25% of your net self-employment income (after the SE tax deduction) or $69,000.

For $30,000 in net freelance income, working through the calculation:

• Net SE income: $30,000
• Deduct half SE tax: -$2,120
• Adjusted SE income: $27,880
• Max SEP-IRA contribution: 25% × $27,880 = $6,970

That $6,970 contribution:

• Reduces your federal income tax by $1,533 (at 22%)
• Keeps growing tax-deferred until retirement
• Takes about 10 minutes to set up at Fidelity or Vanguard

If your freelance income is higher — say $100,000 net — the SEP-IRA math starts looking transformative. The limit is $69,000, and you can get there at roughly $276,000 in net SE income (where 25% × $276k × adjustment ≈ $69k). At that point, you’re sheltering an amount that would take a typical person years to accumulate in a 401k.

Solo 401k: More Levers, More Power

A Solo 401k (also called an Individual 401k or Self-Employed 401k) is an alternative that can actually outperform a SEP-IRA at lower income levels.

Here’s why: Solo 401k contributions have two components.

Employee contributions: Up to $23,500 in 2026 (same as a regular 401k). If you’re 50+, add $7,500 catch-up. These contributions are not limited to a percentage of income — you can contribute up to 100% of your net SE income up to the cap. If your freelance income is $25,000, you can contribute all $25,000 as the employee portion.

Employer contributions: Up to 25% of net SE income (after the SE tax deduction), same as the SEP-IRA formula.

Total limit: $69,000 in 2026 (same cap as SEP-IRA, shared between employee and employer contributions).

Why Solo 401k wins at lower incomes: With a SEP-IRA, you’re capped at 25% of net SE income from dollar one. On $30,000 of net SE income, your max is about $6,970. With a Solo 401k, you can put in the full $23,500 employee contribution if you have at least that much in SE income — a dramatically higher contribution.

The tradeoff: a Solo 401k requires more paperwork, must be opened by December 31st of the tax year, and requires annual filings (Form 5500-EZ) once the balance exceeds $250,000. For most freelancers just starting out, the SEP-IRA is easier. For anyone with meaningful side income who wants to maximize tax deferral, the Solo 401k is worth the setup.

Quarterly Estimated Taxes: Don’t Wait Until April

One more thing that trips up engineers turning freelancers: side income creates a quarterly estimated tax obligation.

The IRS wants taxes paid as income is earned, not just once a year in April. If you’ll owe more than $1,000 in taxes after withholding credits, you’re supposed to be making quarterly payments. Miss them and you’ll owe an underpayment penalty on top of the actual tax bill.

The 2026 deadlines: April 15, June 16, September 15, and January 15, 2027. Pay at IRS Direct Pay (free, instant, no account required) or through EFTPS.

For the full breakdown on safe harbor rules, how to calculate payments, and how to avoid the penalty even if your income is lumpy, see the Quarterly Estimated Taxes article.

The Summary Math: What Side Income Actually Costs

Let’s close with a clean summary using $50,000 of gross freelance income, after $5,000 in legitimate Schedule C deductions:

Without the SEP-IRA contribution, that effective rate jumps to around 32%. With it, you’ve sheltered over $10,000 in a retirement account that will compound tax-free until you need it.

This is why “just track your expenses and open a SEP-IRA” is genuinely useful advice, not just accountant boilerplate. The savings are real and the math is not complicated.

The Bottom Line

Side income is taxed harder than W-2 income because you pay both halves of the FICA taxes that your employer normally splits with you. The headline 15.3% SE tax rate on top of your regular marginal rate means the effective cost of freelance income is higher than most people expect.

But you have levers your W-2 self doesn’t: deductible business expenses, an above-the-line SE tax deduction, and access to retirement accounts with contribution limits that dwarf a standard 401k.

Track everything that touches your freelance work. Open the retirement account — SEP-IRA if you want simple, Solo 401k if you want to maximize. Make the quarterly payments so April doesn’t turn into a four-figure surprise.

The IRS is going to get their cut no matter what. The question is how much of what’s left you keep.