Contract income tax has two layers: regular federal income tax and a 15.3% self-employment tax that covers Social Security and Medicare. Because no employer withholds anything from your 1099 payments, you calculate both yourself, reduce the taxable figure with legitimate business deductions, and pay the result in four estimated installments across the year. The math starts with your net profit on Schedule C and flows through Schedule SE and Form 1040.
How Self-Employment Tax Works
Self-employment tax is how the IRS collects Social Security and Medicare contributions from people who don’t have an employer splitting the bill. In a W-2 job, your employer pays 7.65% and withholds another 7.65% from your check. As a contractor, both sides fall on you: 12.4% for Social Security and 2.9% for Medicare, for a combined 15.3%.1Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates
The rate doesn’t apply to every dollar of net profit. The IRS first multiplies your net self-employment earnings by 92.35% before applying the tax, which accounts for the fact that an employer’s share of FICA isn’t taxed in a W-2 setting.2Internal Revenue Service. Schedule SE (Form 1040) – Self-Employment Tax If your Schedule C shows $100,000 in net profit, you multiply by 0.9235 to get $92,350, then apply 15.3% to that figure.
The 12.4% Social Security portion only applies to earnings up to $184,500 in 2026.3Social Security Administration. Contribution and Benefit Base Earnings above that cap still get the 2.9% Medicare portion but not the Social Security piece. If your net self-employment earnings fall below $400 for the year, you generally don’t owe self-employment tax at all.4Internal Revenue Service. Self-Employment Tax (Social Security and Medicare Taxes)
You get a partial break: half of your self-employment tax is deductible when calculating your adjusted gross income on Form 1040. It won’t reduce the self-employment tax itself, but it lowers the income subject to regular income tax.4Internal Revenue Service. Self-Employment Tax (Social Security and Medicare Taxes)
Additional Medicare Tax for Higher Earners
If your self-employment income exceeds $200,000 for single filers or $250,000 for married filing jointly, an extra 0.9% Medicare tax applies to the amount above that threshold. That brings the Medicare portion to 3.8% on earnings past the line. You don’t split this surcharge with a hypothetical employer, and you can’t deduct any part of it.5Internal Revenue Service. Questions and Answers for the Additional Medicare Tax
Deductions That Lower Your Taxable Income
Every dollar you deduct from gross contract income reduces both your income tax and your self-employment tax, because both taxes flow from the same Schedule C net profit. The IRS standard is that an expense be ordinary (common in your line of work) and necessary (helpful and appropriate).6Office of the Law Revision Counsel. 26 USC 162 – Trade or Business Expenses
Home Office
If you use part of your home exclusively and regularly for business, you can deduct a portion of your housing costs. The simplified method allows $5 per square foot up to 300 square feet, for a maximum of $1,500. The regular method involves calculating the actual percentage of your home expenses (rent or mortgage interest, utilities, insurance, repairs) that correspond to the office space. The regular method usually yields a larger deduction but requires more detailed records.
Vehicle Expenses
Business use of a personal vehicle is deductible either by mileage or actual costs. The standard mileage rate for 2026 is 72.5 cents per mile driven for business.7Internal Revenue Service. IRS Sets 2026 Business Standard Mileage Rate at 72.5 Cents Per Mile, Up 2.5 Cents The alternative is tracking actual expenses like gas, repairs, insurance, and depreciation, then multiplying by your business-use percentage. If you choose the standard mileage rate, you have to start with it in the first year the vehicle is available for business. For leased vehicles, once you pick the standard rate, you’re locked in for the entire lease.
Health Insurance Premiums
Self-employed individuals can deduct 100% of health, dental, and vision insurance premiums paid for themselves, their spouse, and their dependents. This is an above-the-line deduction on Schedule 1, so it reduces your adjusted gross income even if you take the standard deduction. Medicare premiums (Parts A through D) and qualified long-term care premiums also count. The deduction can’t exceed your net self-employment income from the business that established the plan, and you can’t claim it for any month you were eligible for employer-subsidized coverage.
Retirement Contributions
Contributions to self-employed retirement accounts reduce current taxable income while building long-term savings. A SEP IRA allows contributions up to 25% of net self-employment earnings, with a maximum of $72,000 for 2026.8Internal Revenue Service. SEP Contribution Limits A Solo 401(k) can offer even higher limits because it combines an employee elective deferral with an employer profit-sharing contribution.
Other Common Deductions
- Fees paid to accountants, bookkeepers, and attorneys for business-related work.
- Business insurance, including liability and professional indemnity coverage.
- Software subscriptions, tools, supplies, postage, and advertising tied to your contract work.
- Professional development: courses, certifications, and conferences relevant to your trade.
In an audit, the IRS expects proof of the amount, date, and business purpose for every deduction. Keep receipts, invoices, bank statements, and mileage logs.
The 20% Qualified Business Income Deduction
Beyond Schedule C expenses, contractors filing as sole proprietors may qualify for the Qualified Business Income (QBI) deduction under Section 199A, which allows eligible taxpayers to subtract up to 20% of their qualified business income from taxable income. It’s calculated on your personal return, not on Schedule C, and it reduces only income tax, not self-employment tax.
For 2026, the deduction begins to phase out when taxable income exceeds $200,000 for single filers or $400,000 for married couples filing jointly. Above those thresholds the rules get more complicated, and the deduction may be reduced or eliminated depending on the type of business and the wages it pays. Below the phase-out floors, the calculation is straightforward: take 20% of your net qualified business income or 20% of taxable income (before the QBI deduction), whichever is less.
This deduction was made permanent by the One Big Beautiful Bill Act and applies to income from sole proprietorships, partnerships, S corporations, and single-member LLCs. It does not apply to W-2 wages, investment income, or C corporation income.
Paying Estimated Quarterly Taxes
Because nobody withholds from contract payments, the IRS expects you to pay as you go. Owing more than $1,000 at filing time, after accounting for any withholding and refundable credits, triggers an underpayment penalty.9Internal Revenue Service. Estimated Tax for Individuals (Form 1040-ES) The penalty is calculated as interest on the underpaid amount, with the rate adjusted quarterly.10Internal Revenue Service. Quarterly Interest Rates
The four payment deadlines are April 15, June 15, September 15, and January 15 of the following year. When a deadline falls on a weekend or holiday, it shifts to the next business day.
Safe Harbor Rules
You can avoid the underpayment penalty entirely by meeting one of the IRS safe harbors, even if you still owe additional tax when you file:
- Pay at least 90% of the total tax that will appear on your current return. This works if you can predict your income accurately.
- Pay at least 100% of the total tax shown on your previous year’s return. This is the most reliable safe harbor because the number is already known.9Internal Revenue Service. Estimated Tax for Individuals (Form 1040-ES)
- If your prior-year adjusted gross income exceeded $150,000 ($75,000 if married filing separately), the 100% safe harbor bumps to 110%.9Internal Revenue Service. Estimated Tax for Individuals (Form 1040-ES)
To use the prior-year safe harbor, take last year’s total tax, divide it into four equal installments, and pay each one by the quarterly deadline. If your income is climbing, hitting the safe harbor protects you from penalties but leaves a gap you’ll still owe in April. Setting aside a cushion prevents a surprise.
The IRS accepts estimated payments through IRS Direct Pay, the Electronic Federal Tax Payment System (EFTPS), or by mailing a check with the Form 1040-ES voucher. Most states with an income tax impose their own separate estimated payment requirements.
Penalties for Late Filing and Late Payment
The IRS penalizes late filing far more harshly than late payment. If you can’t pay the full amount, filing on time and paying what you can is almost always the better move.
- Failure to file runs 5% of the unpaid tax per month (or partial month) the return is late, capped at 25%. If a return is more than 60 days overdue, the minimum penalty for 2026 is $525 or the full tax owed, whichever is smaller.
- Failure to pay is 0.5% per month on the unpaid balance, also capped at 25%. When both penalties apply in the same month, the IRS reduces the late-filing penalty by the late-payment amount, so the combined rate is 5% rather than 5.5%.
Filing an extension on Form 4868 gives you six extra months to submit the return, but it does not extend the payment deadline. You’re still expected to estimate and pay your tax by the original due date to avoid the failure-to-pay penalty and interest.
The Forms That Tie It Together
Every client who pays you $600 or more during the calendar year should send you a Form 1099-NEC by January 31. The IRS gets a copy too. You report all contract income whether or not a client sends a 1099; the threshold is the payer’s obligation, not yours.11Internal Revenue Service. Independent Contractor (Self-Employed) or Employee?
Schedule C (Profit or Loss from Business) is where the core calculation happens. You enter gross contract income at the top and list deductible business expenses below. The bottom line is your net profit or loss, and that figure drives everything else.12Internal Revenue Service. Instructions for Schedule C (Form 1040)
Schedule SE takes your net profit from Schedule C, applies the 92.35% multiplier, and calculates self-employment tax at 15.3%. The result goes on Form 1040, and half of it flows to Schedule 1 as an above-the-line deduction.2Internal Revenue Service. Schedule SE (Form 1040) – Self-Employment Tax
Form 1040 pulls everything together. Your Schedule C net profit, the deductible half of self-employment tax, any QBI deduction, the health insurance deduction, and retirement contributions all feed into this return. The final 1040 reconciles your total tax liability against the estimated quarterly payments you made using Form 1040-ES, and the difference is what you owe or what comes back as a refund.