Estimated Tax Payments
Quarterly tax payments made to the IRS and state tax authorities to cover income tax liability on income not subject to withholding, such as gains from exercising stock options.
What Are Estimated Tax Payments?
Estimated tax payments are quarterly payments you make directly to the IRS (and your state tax authority) to cover income tax on income that is not subject to employer withholding. The U.S. tax system is pay-as-you-go — you are expected to pay taxes throughout the year as you earn income, not just at filing time. When you have income without automatic withholding (such as AMT from ISO exercises, capital gains from stock sales, or investment income), estimated payments are how you stay current.
The four quarterly deadlines for federal estimated tax payments are April 15, June 15, September 15, and January 15 of the following year. Missing these deadlines or paying too little can result in underpayment penalties.
How Estimated Tax Payments Work
When You Need to Make Them
You generally need to make estimated tax payments if you expect to owe $1,000 or more in federal income tax after subtracting withholding and credits. Common scenarios for equity compensation include:
- ISO exercise: AMT on the spread is not withheld by your employer
- Stock sales: Capital gains from selling exercised shares (if not automatically withheld)
- Large NSO exercises: Supplemental withholding at 22% (or 37% above $1M) may not cover your actual marginal rate
- RSU vesting at higher brackets: Flat-rate withholding may fall short of your actual tax rate
The Safe Harbor Rules
The IRS provides two safe harbors to avoid underpayment penalties:
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100% of prior year tax: If your total payments (withholding + estimated) equal or exceed 100% of last year's tax liability, no penalty applies. This threshold rises to 110% if your AGI exceeded $150,000 ($75,000 if married filing separately).
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90% of current year tax: If your total payments cover at least 90% of your current year tax liability, no penalty applies.
Meeting either safe harbor protects you from penalties, even if you owe a large balance at filing. Most tax advisors recommend the prior-year safe harbor because it is a known number.
How to Pay
Federal estimated taxes are paid using IRS Form 1040-ES or through the IRS Direct Pay or EFTPS (Electronic Federal Tax Payment System) websites. State estimated taxes use the equivalent state form and payment system. You can also increase your W-2 withholding to cover the shortfall — extra withholding is treated as paid evenly throughout the year, which can be simpler than quarterly estimates.
Practical Implications for Startup Employees
ISO Exercises and AMT
When you exercise ISOs, your employer does not withhold any tax. If the spread creates a significant AMT liability, you are responsible for paying that amount through estimated tax payments or by the April 15 filing deadline. Many employees are caught off guard by a five- or six-figure AMT bill from a large ISO exercise because they did not make estimated payments.
Strategy: If you exercise ISOs early in the year, you have time to make estimated payments across the remaining quarters. If you exercise late in the year (October-December), consider increasing your W-2 withholding for the remaining paychecks to avoid an underpayment penalty.
Large NSO Exercises
For NSO exercises, your employer withholds tax at the supplemental income rate (22% federal, or 37% above $1 million in supplemental wages). If your marginal tax rate is higher (32%, 35%, or 37%), the withholding is insufficient. The difference must be covered through estimated payments or additional withholding.
Capital Gains from Stock Sales
When you sell stock and realize capital gains, no withholding occurs unless you instruct your broker to withhold. If you sell a large position and realize significant gains, make an estimated tax payment for the quarter in which the sale occurred.
State Estimated Taxes
Most states with income taxes have their own estimated tax payment requirements, often mirroring the federal schedule. California, New York, and other high-tax states can impose significant state tax on equity compensation income. Make sure your state estimated payments are also sufficient.
The Annualized Income Installment Method
If your income is concentrated in one quarter (for example, a large stock sale in Q3), you can use the annualized income installment method to reduce earlier quarterly payments and increase later ones. This avoids overpaying estimates in quarters before the income event.
How It Relates to Exercising Stock Options
Estimated tax payments are an essential part of exercise planning. Before exercising stock options, calculate not just the exercise cost but also the tax due — and determine how you will pay it. For ISOs, model the AMT and plan your estimated payments. For NSOs, calculate whether employer withholding covers your marginal rate. For stock sales following exercise, estimate the capital gains tax and make timely payments. Failing to plan for estimated taxes can result in both cash flow surprises and IRS penalties, undermining the financial benefit of your equity compensation.