Which states are covered so far
State income tax is currently calculated for the 15 states that do not tax wage income at all: Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, Wyoming, Colorado, Illinois, Indiana, Kentucky, North Carolina, Virginia. For every other state the calculator shows complete federal, Social Security and Medicare figures and tells you plainly that the state line is missing.
That is deliberate. A state goes live here only after its rate structure has been checked against that state’s own revenue department and covered by tests. Publishing a plausible-looking wrong number would be worse than publishing nothing.
Why 401(k) and health premiums behave differently
A traditional 401(k) contribution lowers your federal income tax but not your Social Security and Medicare — those are still charged on the full amount. Pre-tax health premiums under a Section 125 cafeteria plan lower both. That is why the same $5,000 saves you different amounts depending on which box it goes in.
Why your real paycheck may differ
Your employer withholds based on the Form W-4 you filed, not on a perfect projection of your year. Bonuses, mid-year raises, multiple jobs and dependents all shift the number. Treat this as a close estimate of the annual picture rather than a prediction of any single pay stub.