Withholding check.

You avoid an underpayment penalty if your total federal tax payments (withholding + estimated payments) by year-end equal or exceed the smaller of:

• 90% of your current-year total tax, OR
• 100% of last year's total tax (or 110% if your prior-year AGI was over $150,000 — $75,000 if married filing separately).

For high-income filers, the 110% rule almost always applies — and it's the easiest to plan around because last year's number is fixed and known.

Employers default to 22% supplemental federal withholding on RSU vesting, bonuses, and equity-related compensation (37% applies only above $1M in supplemental wages per year). If your marginal bracket is 24%, 32%, or 35%, the 22% withholding leaves a gap.

Example: $50,000 of RSU vesting at the 32% federal marginal rate. Withholding takes $11,000 (22%). Actual tax owed: $16,000 (32%). Gap: $5,000 — multiplied across multiple vests per year, this is how clients end up owing $20-50K at filing.

• Two-earner households where each spouse fills out W-4 as if they're the only earner
• Year-end bonuses withheld at supplemental rate when your marginal bracket is higher
• Multi-state work creating gaps in residency-state withholding
• Self-employment income with no withholding mechanism — must be covered via quarterly estimates
• Stock option exercises (ISO/NSO) where the spread isn't covered
• Retirement plan distributions with default 10% (or 20% on lump sums) withholding instead of marginal rate
• Roth conversions where the converted amount is taxable but rarely has matching withholding
• K-1 income from S-corps and partnerships with no entity-level withholding
• Significant capital gains from a sale or exit event

Quarterly federal estimated tax payments (Form 1040-ES) are due:

• Q1: April 15
• Q2: June 15
• Q3: September 15
• Q4: January 15 of the following year

If a due date falls on a weekend or federal holiday, it shifts to the next business day. Late or skipped payments accrue underpayment penalties even if the year-end balance is paid.

W-4 adjustment changes how much your employer withholds going forward. Use this when the under-withholding is structural (you'll have the same gap each pay period) and you have several pay periods left in the year.

Estimated tax payment is a direct payment to the IRS for amounts not covered by withholding. Use this for one-time events (RSU vest, bonus, property sale, K-1 distribution) and for self-employment income.

Both count toward your total federal tax payments for safe harbor. Withholding is treated as if paid evenly across the year; estimated payments count when made.

Submit a new Form W-4 to your employer's HR or payroll system. The most direct way to add withholding is Step 4(c) — Extra withholding per pay period. Divide the additional amount you need by the number of pay periods left in the year.

Example: You need $4,800 of additional withholding and have 8 pay periods left. Enter $600 in Step 4(c). Your employer withholds an extra $600 per check until further notice.

The IRS Tax Withholding Estimator at irs.gov/individuals/tax-withholding-estimator can help you compute the exact W-4 entries for your situation.

The IRS underpayment penalty is calculated as interest on the shortfall, accrued from the date each quarterly estimate was due. The rate adjusts quarterly and is currently around 8% annualized.

On a $10,000 underpayment, this typically translates to $400-700 of penalty depending on when the shortfall accrued. Not catastrophic, but unnecessary — and the calculation itself (Form 2210) is annoying to fill out at filing time. Meeting safe harbor avoids it entirely.