How can I estimate the taxes to be withheld for next year on a withdrawal from a non-spouse inherited ira?

[Bsch4477]

You will be taxed at your marginal tax rate. 

[SouthShore]

I'm unclear about "inherited" IRAs rules, but for a regular IRA taxable withdrawal (and/or Roth Conversion) , if you use the TT desktop version (I use Premier), there is a "well hidden" form (switch to Forms View) called "what if" worksheet.  It shows your most recent year summary and allows you to create next year "planning versions" with the ability to modify the inputs fields.  

You might be able to add in the distribution and have it show the added taxable income and tax due impacts.

I use this feature each year for multiple current year and next year planning scenarios such that I end up *owing* the IRS only a few $100 every April.  Doesn't cover State scenarios though.  A few MS Excel manipulations can get that pretty exact too.

I don't use the Online Versions so don't know if they have similar capabilities.

Good luck!

/ss

[fanfare]

What is your tax bracket on an additional dollar of income.?

That's what you will pay.