Calculators that show their working
US tax and retirement questions, worked out from figures transcribed straight from the IRS documents that set them. Every page shows its arithmetic and cites its source. Nothing you enter leaves your browser.
Start with the one most people arrive for. An employer withholds federal tax on an RSU vest at a flat 22%; if your marginal rate is 32% or 35%, the rest is due when you file.
RSU Withholding Shortfall
2026 tax yearGross value on the vest date, before any shares are withheld.
Base salary plus anything else taxed as ordinary income.
A previous job this year, a second job, or a spouse's wages. Counted inside your other income above.
Likely shortfall at filing
$25,330.25
- Federal income tax owed
- $68,430.25
- Withheld at 22%
- −$44,000.00
- Income tax gap
- $24,430.25
- Medicare surtax gap
- $900.00
- Marginal rate
- 35.00%
- Effective rate on the vest
- 34.215%
State tax, Social Security and the base Medicare rate are not included.
Brackets from Rev. Proc. 2025-32, read August 19, 2026
Calculators
- RSU Withholding Shortfall Calculator
What your employer withheld on an RSU vest against what the vest actually costs: federal income tax, and the 0.9% Medicare surtax your employer may never have withheld at all.
- Roth Catch-Up Mandate Calculator
From this year, higher earners must make catch-up contributions as Roth. Whether that reaches you, what the lost deduction costs, and why a plan with no Roth option stops you contributing at all.
- Super Catch-Up Calculator, Ages 60 to 63
For four years your catch-up limit is higher, and then it is not. What the window is worth this year, what it is worth across all four, and the year it shuts.
- 401(k) True-Up Calculator
Reaching the annual 401(k) limit early stops your employer match for the rest of the year. This works out what that costs, whether your plan's true-up pays it back, and which of the three ceilings actually stopped you.
Four so far. Each is built and checked against its source document before the next is started, so this list grows slowly and on purpose. What is coming.
Guides
How this works, and what it is not
Every bracket, rate and threshold here is transcribed from the document that sets it: an IRS Revenue Procedure or Notice, a publication, the tax code itself, or an official Social Security release. A figure nobody has checked against its source cannot reach a page: the build refuses it, and the calculator shows nothing rather than a number that looks confident.
These are educational tools, not advice. They cannot see your circumstances, and nothing here recommends a product or tells you what to do. For a decision that turns on your whole position, that needs a professional who can see it.