This calculator doesn't seem to support local income taxes, which many states have [1]. For example Maryland has income tax brackets than other states, but each county levies its own income tax on top of that. I usually use smartasset's income tax calculator [2], which also asks for your location so it can factor in any local income taxes as needed.
SmartAsset is only a rough approximation based on standard deductions/exemptions and the rate structure. It excludes tax credits and other provisions that make taxsim much more accurate, especially for low-income households.
But again taxsim is missing local income taxes entirely. That makes it automatically inaccurate for millions of people. Smart Asset is an estimate but it's at least accurate for the cases that it covers and doesn't claim to be a complete tax calculator.
Anyway taxsim is obviously the more complete tool, but without local income taxes it's unusable for many people like me who live in a state that has local income taxes too.
[1] https://www.thebalance.com/cities-that-levy-income-taxes-319... [2] https://smartasset.com/taxes/income-taxes