When you receive a bonus from your employer, have restricted stock vest, have taxes collected on the cashless exercise of a stock option, or receive any other form of supplemental wages, you may have too much or too little tax withheld depending on your marginal tax bracket and the method your employer uses for tax withholding. On a normal (non-bonus paycheck), payroll withholding tables take the amount of taxable income you earn for the pay period and translate that to the amount of tax that should be withheld, using the marginal tax brackets for your filing status (from your W-4), the number of allowances you claim (from your W-4), and number of pay periods in a year. For example, if you claim “Single, with 0 allowances”, you earn $8k in a pay period, and you are paid bi-weekly (26 x per year), the withholding tables will determine the projected annual tax liability based on the Single tax brackets, $8,000 * 26 = $208,000 of projected taxable income for the year. Dividing by 26 gives the Federal withholding amount for the pay period.
From the example above, you should be able to see how wildly the withholding rate can vary if your paycheck varies from period to period, which is why it is so hard to accurately set your withholding and why you never seem to get the same refund or owe the same amount year after year. (Add in exemptions, deductions, and credits, and it gets even more difficult). If you receive bonus pay as part of your regular pay, your employer can combine the two and determine the withholding on that paycheck based on the extrapolated annual income if you earned that amount each pay period. In this case, your projected annual income and the withholding tax rate will be very high because 26 * your combined wage and bonus is a very large number. You’d therefore have more withholding than is necessary for the period and would accumulate that amount toward a tax refund when you file. The more typical scenario is that your bonus would be paid either as a separate paycheck, or as a separate line item on your regular paycheck but considered as supplemental wages. In both of these cases, a statutory 25% withholding rate is used for the supplemental wages. If your marginal tax bracket is actually higher than 25% (taxable income over about $90k as a single filer, or over $150k as a married couple), then you’d have less withholding than is necessary for the period. That would accumulate toward an amount you’d owe when you file your taxes. In an extreme example, let’s say that you earn $250k per year as a single filer (33% tax bracket), but that you have an windfall of an additional $250k (bonus, stock, whatever). That $250k windfall is taxed at 25% when it should be taxed at ~35%, meaning you’d stand to owe $25k in tax when you file for that tax year.
The moral of the story is to be careful whenever you receive a bonus (or earn some other form of supplemental income like vesting equity). If the following conditions exist, it may cause you owe a substantial amount of tax at the end of the year:
1) It is paid in a separate paycheck, as supplemental wages on your normal paycheck, or withheld automatically as part of an equity transaction
2) You earn more than $90k per year (single) or $150k per year married, including the bonus payment.
Note that the tax is the same whether it is appropriately withheld at the time of the bonus payment or if you pay it at the end of the year. The problem isn’t that you pay additional tax. The potential problem is that you may owe a lot of tax and may not have been prepared for it (e.g. you used the bonus for a downpayment on a house or to payoff debt, and don’t have the cash remaining to pay your tax). It’s always a good idea to keep 10-15% of your gross bonus tucked away to make sure you have it available for taxes if needed. If you need a more detailed estimate of the potential tax impact of a large bonus payment, contact your financial advisor.