unless you use the annualized income method, under the safe harbor 100/110% prior year tax or 90% this year's tax rules 1/4 must be paid in each quarter 4/18, 6/15, 9/15, and 1/16/2024. it can be through withholding or estimated tax payments or a combination of the two.
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for withholding the default is 1/4 is considered paid each period but you can use the actual amount (requires that form 2210 be filed). the one year I used actual, which was many years ago, I got a notice requesting proof. I used paystubs showing YTD withholding. I think this is unusual, but you never know.
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from my experience knowing what the 90% will be is difficult if not impossible to determine before year-end (estimating higher than the actual 90% is ok), so the 110% is generally the safer option. I say generally because some taxpayers do not get all their tax info by the original filing date so must estimate what their taxes will be. too low and the payments in the early quarters may be short subjecting the taxpayer to penalties. Meeting either safe harbor makes you safe from penalties.
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