By Jason Cook · Updated May 2026
Not tax advice. Substantiation rules for mixed-use expenses depend on your specific facts and have been litigated plenty. Use this as a recordkeeping framework and verify anything that affects your return with a credentialed tax professional.
The IRS lets sole proprietors and farmers deduct what's "ordinary and necessary" for the business. That's a low bar. The harder bar is the second one: you have to be able to show what was business and what was personal. Especially when the same card, phone, or bank account does both.
This is where I see deductions silently disappear. You charge a business meal on the family Visa and forget to flag it. You take a client call on your personal cell, never separate the bill, and at year end you can't substantiate any percentage. You buy office supplies at Target on the same trip as groceries, and three months later the line item just looks like "Target." Each of those is a deduction you earned and didn't claim — or worse, claimed without anything backing it up.
The single best move you can make is putting the business on its own checking account and its own credit card. Sole proprietors don't legally have to. Every accountant alive will tell you to do it anyway. The reason: when the statement contains only business transactions, every line is either a deduction or it's a question — never a guess.
Same logic extends to phones, internet (where feasible), and software subscriptions. If you can run a dedicated business cell line for $20-something a month, the math almost always works: 100% of the bill is deductible, you stop arguing about percentages, and you stop losing client calls because they came in on a phone you weren't watching.
Most people don't start clean. They've had one card, one phone, one bank account for years, and the cost of splitting feels worse than the bookkeeping pain — until the year an audit letter shows up, or until tax time arrives and you're trying to reconstruct twelve months of mixed-use charges in one weekend.
The bookkeeping rule when you can't split is short: every transaction gets categorized at the moment of entry. Not "I'll figure it out at year-end." Not "the bookkeeper can sort it." You see it; you tag it. If that sounds aggressive, it's because every uncategorized transaction is also an unidentified one. Six months later, you don't remember whether that $42 charge was a client lunch or a date night.
Two legal strategies. Pick one and commit to it.
A dedicated business cell phone is fully deductible — Sch C line 25 (Utilities) or 27a (Other), or Sch F line 30 (Utilities). No percentage math. No reconstructing call logs. No risk that a personal call will dilute your deduction. The cost of the second line is usually swamped by the simplicity.
For years, I didn't think about splitting things up. I started splitting things up when I started learning about tax laws with a fellow tax preparer. We take a CPE course every year and it's mandated by the IRS. Once you've sat through a few of those, the case for keeping personal and business in separate lanes stops being a debate.
Cell phones were removed from "listed property" in 2010 (Small Business Jobs Act), so you no longer need a minute-by-minute call log to deduct them. But you still have to substantiate that you used the phone for business and what the business-use share was. A common defensible approach: pick a representative week each quarter, log which calls and data sessions were business, compute a percentage, apply that percentage to the bill all year, and document how you arrived at it. Keep the methodology consistent — picking one quarter where you did all your client calls and applying that to the whole year is the kind of thing that gets reversed in audit.
Internet is the same shape. Dedicated business line: 100% deduct. Home Wi-Fi serving both: pick a percentage, substantiate it, apply it consistently.
This trips people up every single year. The deduction lives on the original purchase, not on the card payment. If you charged $500 of business supplies in November and paid the card in January, the deduction is in November. The card payment itself is just moving money between your bank and the card issuer — not deductible, not income, just a transfer. (For cash-basis taxpayers, IRS Rev. Rul. 78-39 settled this decades ago: a credit card charge is treated as a payment in the year of the charge.)
If you mix business and personal on one card, the answer is the same as for a mixed bank account: every charge gets a category at the moment of entry. The card statement isn't your books. Your transaction log is.
I know I've had charges slip by me over the years. I never thought about it. I always said to myself, "It is too much work." Now, with this budget tracker, I don't think about it — I enter and classify the transaction and I'm done.
Same shape as phones. Either dedicate a vehicle to the business (uncommon for most sole props, very common for farms with a work truck) and deduct everything tied to it, or keep a contemporaneous mileage log and deduct only business miles. There's no honest middle path. "I drive about 30% for business" is not substantiation; the log is. A future article will dig into mileage logs that hold up under scrutiny.
Subscriptions don't usually get split within a charge — they get split between charges. The streaming service for the kids is personal. The trade journal for your industry is business. The accounting software you actually use is business. The household magazine is personal.
Categorize each one at the moment you set up the recurring payment, and put a note on the renewal date so future-you remembers why you signed up. Subscriptions are the easiest category to creep — you start with one for a project, the project ends, the charge keeps showing up, and three years later you're still paying for it.
I design and host websites for people. I never even thought about the domain registrar as being a subscription — I just paid and went on my way. It took writing this article to realize that's exactly what it is, and exactly the kind of charge that should land in Business: Subscriptions rather than getting lost in a generic "Other" line.
If you have a separate business account, you'll occasionally move money from it to your personal account. That transfer is an "owner's draw" — not a business expense (the business doesn't deduct it) and not personal income (you already pay tax on the business's profit, not on what you pull out of it). It's a bookkeeping entry. Period.
The trap to avoid: classifying the draw as an expense. The business pays $1,000 to your personal account and you mark it as "owner salary" or "personal living expenses" — now your Schedule C profit looks $1,000 lower than it really is. Sole proprietors don't get to deduct what they pay themselves. The "salary" of a sole proprietor is whatever's left after legitimate business expenses. That's the number you pay tax on, whether you withdraw it or leave it sitting in the business account.
The category list was built with this exact split in mind. The pairs that matter most for mixed-use:
The category dropdown shows the relevant Schedule line as a small note below the field as you make each selection — handy for the first few months of using the new categories, when the muscle memory hasn't formed yet.
The discipline isn't in the categorizing; it's in the timing. A transaction categorized when you enter it takes ten seconds. The same transaction categorized in March takes ten minutes — if you can do it at all. The goal isn't perfection. It's that when April rolls around, every dollar your business spent has a category and a category has a Schedule line, and you don't have to guess at any of it.
Again — this is not tax advice. Mixed-use substantiation is one of the more litigated corners of small-business tax, and the right answer depends on your facts. Use this as a categorization framework and verify anything that affects your return with a credentialed tax professional.