What is Step 4(c) on Form W-4? Extra withholding explained

Step 4(c) is one line on IRS Form W-4: "Extra withholding. Enter any additional tax you want withheld each pay period." It is the most flexible and under-used lever on the whole form — the quickest way to fix under-withholding without refiling the rest of the W-4 or adjusting dependent credits. This guide explains what the line does, who should touch it, and the arithmetic that tells you what number to write there.

What the line does mechanically

Your employer's payroll already computes federal income tax withholding from each paycheck based on your filing status (Step 1) and your claimed dependents and adjustments (Steps 2–4(b)). Whatever you enter on Step 4(c) is added on top of that base amount, flat, every pay period. The IRS spells this out in Publication 15-T (Worksheet 1A, line 4a): after the baseline per-period withholding is computed, Step 4(c) is added verbatim to produce the final tax to withhold.

A few mechanical facts:

• It is per-paycheck, not annual. If you write $50 on a biweekly W-4, payroll adds $50 to each of your 26 paychecks — $1,300 a year.
• It only affects federal income tax withholding. It does not change Social Security, Medicare, state income tax, or state disability withholding.
• It cannot go negative. If you are over-withholding, the fix is to reduce Step 3, remove the Step 4(a) other-income amount, or uncheck Step 2 — not to write a negative Step 4(c).
• It takes effect on the next payroll run. Usually one pay cycle after your employer processes the new W-4.

When to use Step 4(c)

The short answer: whenever your projected annual withholding is going to fall short of your projected annual liability. A few specific scenarios where Step 4(c) is the right tool:

• Under-withholding in a two-income household. A working spouse, a second job, or both spouses leaving Step 2 unchecked are the most common causes of a balance-due return. Step 4(c) is the cleanest fix — enter the gap per paycheck and move on. For the full diagnostic, see why you might owe taxes this year.
• A bonus withheld at 22% when your real rate is higher. Supplemental income (bonuses, RSU vests, commissions) is withheld at a flat 22% up to $1 million. If your marginal bracket is 32% or 37%, Step 4(c) is where you make up the 10- to 15-point gap for the rest of the year.
• Side income without estimated payments.If you have 1099 income and have not been filing Form 1040-ES quarterly, Step 4(c) on your day job's W-4 can cover the gap — converting a future balance due into spread-out withholding.
• A mid-year life change that raised your tax bill. A spouse who started a job, a Roth conversion, a dependent who aged out of the $2,000 credit — any event that tilts the gap can be patched without rewriting Steps 1–3.

How to calculate the right amount

The arithmetic is simple:

• Project the gap — what you will owe minus what will be withheld under the W-4 you have today.
• Divide by the number of pay periods left in the year.
• Round up to the nearest dollar (or nearest five).

If your employer pays biweekly, you have 16 paychecks left in the year, and you are projecting a $2,000 balance due, Step 4(c) needs to be $125 per paycheck to close the gap. The Breakeven calculator does this math from your year-to-date paychecks and current W-4 and shows the exact number to write on the form.

Two practical notes:

• The per-paycheck amount grows as the year progresses. Catching under-withholding in March means spreading the gap over 20-plus paychecks; catching it in October means spreading over three. Sooner is cheaper per paycheck.
• Small gaps often do not need Step 4(c) at all. Under the IRS safe-harbor rule in Publication 505, you avoid an underpayment penalty if your withholding covers the smaller of 90% of the current year's tax or 100% of the prior year's (110% if prior-year AGI was over $150,000). A small balance due inside the safe harbor can be paid in April without a penalty and without touching your W-4.

How to file just a Step 4(c) change

You do not need to rewrite the whole W-4 from scratch. Fill out a fresh form, copy your existing Step 1 (name, SSN, filing status) and Steps 2–4(b), put the new Step 4(c) amount on line 4(c), sign and date. Hand it to HR or payroll. Most employers accept the PDF directly; some require re-entry through a payroll portal like ADP or Workday.

Breakeven's calculator will generate a pre-filled W-4 PDF with the recommended Step 4(c) amount stamped in — print, sign, and submit.

Common mistakes

• Entering an annual number instead of per-paycheck. If you mean to withhold $2,000 extra over the year and you write $2,000 on Step 4(c), payroll withholds $2,000 per paycheck. Always divide by pay periods first.
• Forgetting to reset it next year. Step 4(c) stays on your W-4 until you file a new one. If you set it to catch up a 2026 gap, file a fresh W-4 at the start of 2027 to reset it — otherwise you will over-withhold through the next year.
• Using Step 4(c) to hit a target refund. Mechanically it works, but you are giving the federal government an interest-free loan. Zero (or a small balance due inside the safe harbor) is a more efficient target than a large refund.

Bottom line

Step 4(c) is a scalpel, not a hammer. It targets one problem — per-paycheck under-withholding — and solves it without disturbing the rest of your W-4. When you know the gap, the math is trivial. For the full model of how Breakeven computes the gap, see the methodology.