I built a tool that audits already-filed tax returns. After looking at hundreds, the same deductions show up as missed over and over. Here's the list, roughly in order of how often people leave them on the table.

1. Self-employed health insurance premiums

Average missed: $2,500–$8,000 in deductions. If you pay your own health insurance premiums and you're self-employed with net profit, the premium is an above-the-line deduction on Schedule 1, Line 17. Above-the-line means it reduces your AGI, which cascades into every other calculation on your return. People miss this because it's not on Schedule C — it's on the main 1040 schedules.

2. Home office rent (entered as monthly instead of annual)

Average missed: $500–$2,500 in refund. The most common software error I find. Form 8829 Line 19 asks for rent, and most people type in a single month. Full walkthrough here.

3. Cell phone, business-use percentage

Average missed: $200–$500. If you use your personal phone for business, you can deduct the business-use percentage of your bill. Not 100% unless you have a dedicated business phone. 40–70% is a defensible range for most freelancers.

4. Internet, business-use percentage

Average missed: $150–$400. Same logic as phone. If your home internet is used for business, a reasonable percentage is deductible. Most filers skip it because it feels "personal."

5. Software and SaaS subscriptions

Average missed: $300–$2,000. Adobe, Notion, Zoom, ChatGPT, 1Password, Dropbox, every newsletter subscription you use for research. Pull your credit card statement and add them up. Most self-employed people are running $1,000+/year in tools they never deducted.

6. Professional development: books, courses, conferences

Average missed: $200–$1,500. Business books, Udemy courses, industry conferences, even relevant podcasts with paid subscriptions. Directly related to maintaining or improving skills in your current trade.

7. Bank fees and payment processor fees

Average missed: $100–$600. Stripe, PayPal, Square, Wise — all the 2.9% + $0.30 fees add up. Your bank's monthly maintenance fee if you have a business account. Wire transfer fees.

8. Vehicle mileage for business errands

Average missed: $300–$2,500. Drives to the post office, to meet clients, to pick up supplies, to a networking event. 2025 rate is $0.70/mile. If you drove 1,000 business miles and didn't log it, that's $700 in deductions.

9. Self-employment tax deduction (the employer half)

Average missed: $500–$5,000. You owe 15.3% SE tax. But you get to deduct half of it (the "employer half") on Schedule 1, Line 15. Most software does this automatically, but people who file by hand often forget. Worth checking.

10. Retirement contributions (SEP-IRA or Solo 401k)

Average missed: $5,000–$60,000 in deductions. The single biggest tax-saver available to self-employed people. You can open and fund a SEP-IRA as late as the tax filing deadline (including extension). SEP vs Solo 401k here.

11. Startup costs (first-year only)

Average missed: up to $5,000. If this was your first year in business, you can deduct up to $5,000 in pre-launch costs (legal fees for LLC formation, initial marketing, research). Details here.

12. Business insurance

Average missed: $300–$2,000. Professional liability (E&O), general liability, cyber insurance, business property coverage. Not the same as health insurance — these all go on Schedule C Line 15.

13. Office supplies (the actual ones)

Average missed: $100–$600. Printer ink, paper, pens, shipping materials, packing tape. Small but real. Pull your Amazon business history and you'll be surprised.

14. Continuing education: license renewals, certifications

Average missed: $100–$1,200. CPE credits, professional license renewals, industry certifications (AWS, PMP, NASM, etc.). Required-for-your-job training is deductible.

15. Business meals (the 50% ones)

Average missed: $200–$1,500. Coffee meetings with prospects, lunches with collaborators, working dinners during travel. 50% deductible. Full rules here.

16. Qualified business income (QBI) deduction

Average missed: $1,000–$10,000. 20% of your qualified business income, as long as your taxable income is below the threshold. Software usually calculates it — but when it doesn't (some edge cases) or calculates it wrong (phase-out situations), the miss is huge. QBI pitfalls here.

17. Paying your kids (if you have them)

Average missed: $2,000–$14,600 per child. If your kids can legitimately do work for your business (social media, data entry, modeling for your product photos), you can pay them up to the standard deduction ($14,600 in 2024) tax-free to them. The wages are deductible to your business. Requires real work, real hours, real pay.

How to run this check on your own return

Open last year's return. For each of the 17 above, check whether the deduction appears. Anywhere it's missing, either:

You have three years from the original filing date to amend. Any deduction worth more than ~$100 in refund is worth the 30 minutes of paperwork.

We'll run this check for you automatically.

Upload your PDF. We scan for all 17 of these (and more). Free. Launches January 2027.