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. [1] https://www.thebalance.com/cities-that-levy-income-taxes-319...…
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.
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.