Withholding, W-4 and estimated tax: avoiding the April surprise

The US system is pay-as-you-go. Tax is due through the year, either by withholding from pay or by estimated payments, and an underpayment attracts a penalty even if you pay in full with the return.

Why NRI households under-withhold

• Two incomes. Each employer withholds as if that job were your only income. The combined income is taxed at higher rates than either payslip assumes.
• Indian income. Bank interest, rent and capital gains from India carry no US withholding.
• Stock pay. Flat supplemental withholding is too low for higher earners (equity compensation).
• Large gains. A property or share sale has no withholding in the US at all.

The safe harbours

You are protected from the underpayment penalty if your payments through the year equal at least the smaller of:

• 90% of this year's tax
• 100% of last year's tax (110% if last year's adjusted gross income was over $150,000)

The second is the practical one. If your income is going up, you can pay based on last year and settle the difference in April without penalty.

How to fix it

1. Use the IRS Tax Withholding Estimator (on irs.gov) with your latest pay stub, and fill in a new Form W-4 for each job. Step 2 (multiple jobs or a working spouse) is the most commonly skipped item.
2. Make estimated payments with Form 1040-ES on 15 April, 15 June, 15 September and 15 January, via IRS Direct Pay or your online account. Estimates are the right tool for Indian rent, interest, and capital gains.
3. Count foreign tax correctly. Tax paid to India does not reduce US withholding obligations directly. It reduces your final US tax through the foreign tax credit (see credit article), and the credit can be less than the Indian tax paid.

State estimates

Most states with an income tax have their own estimated tax schedule and safe harbour. California, New York and New Jersey are particularly strict. A state notice is often the first sign that estimates were missed.

Penalties

The underpayment penalty is interest-like and calculated on Form 2210 by quarter, so paying everything in January does not remove it for earlier quarters, unless you use the annualised income method, which helps when income (like a stock sale) came late in the year.

A rough rule for a two-income household: if your combined salary is over about $200,000, assume the paystubs under-withhold and check.