Why a tax estimate you cannot see is a liability, not a feature

Every app shows a quarterly number. Almost none show the rule that produced it. Here is what that hides, and what a calculation you can check looks like.

ExactSplit · Updated May 18, 2026

The black box problem

Open any tax app for freelancers and you will get a quarterly figure. It looks authoritative. It has a dollar sign and two decimal places. What you almost never get is the calculation. Which brackets, which safe harbor test, whether it knew about the 400 dollar self-employment floor, whether it applied the 110 percent rule because last year's AGI was over 150,000 dollars, whether it clawed back the 199A deduction when you entered a retirement contribution.

That matters because the number is what you are setting money aside against. If it is wrong by 15 percent, you are either over-reserving thousands of dollars that sit idle all year, or under-reserving and meeting a penalty notice in the spring. And you have no way to know which, because you cannot see inside.

What a checkable calculation looks like

ExactSplit's dashboard shows the estimate as a ledger. Business income. Less deductible expenses. Net earnings. Self-employment tax, with the 92.35 percent base and the statutory floor shown. The half SE tax deduction. Retirement contributions, split by account. Health insurance. Adjusted gross income. The standard deduction. The section 199A deduction, with the basis it was computed on. Taxable income. Federal tax and the marginal rate. State at the rate you set. Total. Then the safe harbor: both tests, which one won, and why in a sentence.

Each constant in that ledger carries its source in the code. The 2026 brackets and standard deduction come from Revenue Procedure 2025-32. The retirement limits come from Notice 2025-67. The mileage rates come from Notice 2026-10 and Announcement 2026-11. The self-employment floor is section 1402(b)(2). If you or your CPA want to check a figure, the citation is there.

Tested against hand-computed answers

The engine runs against several hundred automated checks, and the expected values in those checks are worked out by hand from the source documents and written into the test comments. When a check fails, it says which rule broke rather than just that a number moved. That is how we found and fixed a real defect: the engine was charging self-employment tax on profits below the 400 dollar floor, which is wrong in the direction that costs trust, because it told someone with a 300 dollar side business to reserve money against a liability of zero.

We are telling you that because a product that claims its numbers are always right is a product you should not trust. The right claim is that the numbers are checkable and the errors get found.

Approximations are labeled

Above the section 199A threshold, the W-2 wage and SSTB rules get genuinely intricate, and ExactSplit approximates them. It says so. The result is flagged for a CPA and the dashboard tells you the figure is indicative. The same flag appears on the S-corp comparison, always, because electing is a structural decision and no calculation should present it as settled.

The export carries the caveats too. A bank feed cannot see cash income, unlinked cards, unpaid invoices, cost of goods sold, or the home office deduction, and the CSV says so in its own header rows, because the person opening it in March never saw the app.

Why this is worth paying for

A number you can check is a number you can act on. You can set aside against it without wondering whether it is padded. You can hand it to a CPA without a shrug. You can see that a retirement contribution saves 18.8 percent per dollar rather than the 24 percent a bracket multiplication would claim, and make the decision on the real figure.

That is the whole design position. The model never touches the numbers. The code that does is cited and tested. The approximations are labeled. Everything else in the product sits on top of that.