W-4 Step 4(c): How to Add Extra Withholding and Close a Tax Gap

If our calculator shows you'll owe at tax time, the easiest fix is often the one already on your pay stub: ask your employer to withhold more. Line 4(c) of Form W-4 is labeled Extra withholding. Whatever dollar amount you write there comes out of every paycheck, on top of normal withholding.

The method in three steps

1. Find your gap. Your projected federal tax for the year, minus everything already withheld and paid. The calculator's federal balance is this number.
2. Count your remaining paychecks. Paid every two weeks with 8 checks left this year? Your number is 8.
3. Divide. Gap ÷ remaining paychecks = the amount for line 4(c). Round up a little for cushion.

Example: You're $12,000 short federally with 8 paychecks left. $12,000 ÷ 8 = $1,500. Enter $1,500 on line 4(c), give the new W-4 to payroll, and the gap is closed by December.

You don't redo the rest of the form. Steps 1 through 4(b) can stay exactly as they are.

Why withholding beats a late estimated payment

This is the part most people miss. The IRS treats withholding as if it were paid evenly across the whole year, no matter when it actually came out of your check (IRC §6654(g)). Estimated payments count only on the day you make them.

So if you came up short in the spring, a big December estimated payment still leaves the spring quarters underpaid, and the underpayment penalty is charged on those quarters. The same dollars withheld from your December paychecks are spread back over all four quarters, and the penalty for the early quarters can disappear.

That makes extra withholding the best late-year fix, especially after a big fall vest or commission.

Which situation are you in?

• RSUs vested and only 22% was withheld
• Commission or bonus checks come up short
• You're planning a stock sale

RSUs vested and only 22% was withheld

Employers withhold a flat 22% federal on RSU income (37% above $1 million). If your real bracket is 32% to 37%, the difference is your gap. Enter each vest in the calculator, read the federal balance, and divide by your remaining paychecks. If more vests are coming this year, include them now so you set the right amount once. See too little tax withheld for why this happens.

Commission or bonus checks come up short

Commissions withheld at a flat 22% can fall short month after month. Because the pattern repeats, a standing 4(c) amount works well: set it once based on your expected yearly commissions, then check your year-to-date withholding after each big month and adjust. Checks withheld under the aggregate method may already be over-withheld, so check your pay stub first. See how commissions are withheld.

You're planning a stock sale

Brokers withhold nothing on a sale. If you know a sale is coming, model it in the calculator, then spread the extra tax over the paychecks left in the year. Because withholding counts as paid evenly all year, you can raise it after the sale and still be treated as having paid on time, which an estimated payment can't do.

State withholding

Most states have their own withholding form with an extra-amount line, such as California's DE 4. The same math applies using your state balance. A few states don't take a state form and use your federal W-4 instead, so ask payroll.

When 4(c) isn't the right tool

• Few paychecks left. With one or two checks remaining, the per-check amount may be larger than your pay. Make an estimated payment for the rest.
• You're changing jobs. A new employer starts fresh with its own W-4. Estimated payments may be simpler.
• The gap is one-time. Remember to reset line 4(c) in January, or you'll over-withhold next year.

Not sure why you're short? Start with too little tax taken out, how commission withholding works or penalties for paying late.

Model it in the calculator

Enter your grants, sales and year-to-date withholding, then read the federal and state balances. Those are the numbers to divide by your remaining paychecks.

Try it: open the calculator and check your balance.