Free primer: how to fill out a W-4. Calculator: 2026 gross-to-net. A raise that stays one job is the 90-day file. Paid steps are the order: match the stub, pick checkbox or Step 4(c), divide by checks still coming, and take a bonus bump back off. They are not a second tour of the form.
Do this in order
Write the denominator before you open a blank W-4. From last year’s return: filing status, total tax, total withholding, refund or balance due, and how many W-2s. From this year’s latest stub: federal withheld year-to-date, and how many checks are still coming. A $2,800 refund is a clue about last year’s settings. It is not this year’s target. The count of checks still coming is the denominator. It is not 26 just because the stub says biweekly.
Match the paycheck calculator to this job only. Gross, frequency, filing status, traditional 401(k) %, HSA, other pretax, state %. Leave “other household income” at 0 and the extra-withholding box at 0 for this test. If take-home on the page is within about $40 of the stub, the model is close enough to size 4(c). A wider gap is usually local tax, a Roth 401(k), or a benefit the page does not know. Fix the inputs. Do not “correct” a bad input with a made-up 4(c). The page annualizes, then splits. Your employer uses Publication 15-T. The page already says a $20–$40 gap per check is normal.
One W-2, nothing else: stop. Single, $90,000, no other income, no credits, no pretax. Standard deduction $16,100 (Rev. Proc. 2025-32 §4.14). Taxable $73,900. Federal income tax $10,970: 10% on $12,400 = $1,240; 12% on $38,000 = $4,560; 22% on $23,500 = $5,170. The calculator’s “extra to a ~$0 refund” is $0.00. Leave Step 2 blank. Leave 4(a), 4(b), and 4(c) blank unless you itemize above the standard deduction or you owed last year for a reason that is still true. Do not check Step 2 “just in case.” You are done.
Credits and extra deductions go on one form, the higher-paying job. Publication 505: if you or a spouse has another job, complete Steps 3 through 4(b) on only one Form W-4. The paycheck calculator subtracts $2,200 for each child you type into it. That annual figure belongs on Step 3 of the higher job once. Typing it on both forms tells two payrolls to subtract the same credit. Step 4(b) is only deductions above the standard deduction the calculator already uses ($16,100 single or married filing separately, $32,200 joint, $24,150 head of household). Do not type the standard deduction itself into 4(b).
Similar pay: the Step 2 checkbox on the higher job only. Not a second 4(c). Married filing jointly, $78,000 and $72,000, no kids, no pretax, 26 pays. The $78,000 job modeled alone: taxable $45,800, tax $5,000 (10% on $24,800 = $2,480; 12% on $21,000 = $2,520). Both salaries, $150,000: taxable $117,800, tax $15,340 (those two layers, plus 22% on $17,000 = $3,740). Gap $10,340. The calculator will print about $397.69 of 4(c) on the higher job for a full year (10,340 / 26). Publication 15: you either check Step 2(c) or you enter an extra amount in 4(c). These two salaries are close, so check Step 2 on the $78,000 job only, leave 4(c) at $0, and leave the $72,000 form at Step 1 and the signature. The paycheck page does not model the Step 2 checkbox column. Do not add $397.69 on top of a checked box. Checking both boxes is the over-withhold.
Uneven pay: skip the checkbox. Use the other-income field. Divide by checks still coming. Married filing jointly, this job $90,000, the other salary $55,000, no kids, no pretax, state 0%, 26 pays. This job alone: taxable $57,800, tax $6,440 (10% on $24,800 = $2,480; 12% on $33,000 = $3,960). Both, $145,000: taxable $112,800, tax $14,240 (plus 22% on $12,000 = $2,640). Gap $7,800. Full-year 4(c) on the $90,000 job is $300.00 (7,800 / 26). The $55,000 job stays Step 1 only. Do not also check Step 2. That $300 assumes all 26 checks still get it. If only 8 checks remain and year-to-date withholding has only covered the one-job $6,440 path, the same gap is $975 on each of those 8 checks (7,800 / 8). Then file a January W-4 that drops 4(c). Pasting the page’s $300 onto an October form covers $2,400 and leaves $5,400. The page always divides by the full period count. You divide the uncovered gap by checks still coming.
Ordinary income nobody withholds on is a small 4(c). A Schedule C is not that field. Single, $72,000 W-2 plus $8,000 of interest (or any other ordinary amount with no withholding), 26 pays. This job alone: taxable $55,900, tax $7,010. With the $8,000: taxable $63,900, tax $8,770 — the same $8,770 the tax-brackets guide prints for $80,000 of wages. Gap $1,760. Full-year 4(c) is $67.69. That figure is income tax. It is not self-employment tax. Do not type Schedule C net profit into “other household income” and stop. The self-employed calculator owns the 15.3% piece (worked check already on that page: $80,000 Schedule C → SE tax $11,303.64). Add that piece to 4(c), or send 1040-ES. This file does not rebuild it.
A large gig: pick the safe harbor, then prefer withholding over a late voucher. Publication 505 (2026): generally no underpayment penalty if you will owe under $1,000 after withholding and credits, or you pay 90% of this year’s tax, or 100% of last year’s tax — 110% if last year’s AGI was more than $150,000 ($75,000 if married filing separately). Labeled example, not your return: 2025 tax $8,400 and 2025 AGI over $150,000 → 110% is $9,240. Withholding that reaches $9,240 avoids the penalty. It does not pay a larger 2026 bill. The four 1040-ES dates for a 2026 calendar year are April 15, June 15, September 15, and January 15, 2027. Publication 505 treats wage withholding as paid in four equal parts on those dates unless you elect to count it when it was actually withheld (Form 2210). A 4(c) increase in October can still be credited across the year. A check on January 15, 2027 cannot reach backward the same way. If the gap does not fit on the remaining checks, pay the voucher. Do not pretend the W-4 replaced EFTPS. Dates and the safe-harbor primer already live on the self-employed guide.
A bonus is a temporary 4(c), then you take it off. Publication 15 (2026): if income tax was withheld from your regular wages this year or last year, and the bonus is identified separately, the employer may withhold a flat 22% (no other percentage). Supplemental wages from that employer above $1,000,000 in the calendar year are withheld at 37% on the excess, without regard to the W-4. Worked flat-method bonus: single, $220,000 of wages and a $20,000 bonus. Taxable income before the bonus is $203,900, already $2,125 into the 32% bracket that starts at $201,775. The whole $20,000 is taxed at 32% = $6,400. Flat 22% withheld is $4,400. Gap $2,000. If 8 biweekly checks remain, 4(c) of $250 on those 8 checks only (2,000 / 8), then a new W-4 that removes the $250. Do not leave it on next year. If the stub used the aggregate method — bonus added to regular wages and run through the normal tables — this $2,000 gap is not yours. If your marginal rate is already 22%, flat 22% matches and you do nothing. One check is not a reason to change Step 1. Social security wage base for 2026 is $184,500 (Publication 15). A bonus that crosses it changes FICA, not this 4(c) line.
File it the same week. The remaining-check count starts when payroll changes, not when you click. Ask which check the new W-4 hits. Often the next cycle, not this one. Screenshot the confirmation. If the first stub after the date they named still shows the old federal amount, you filed into a void. Recount checks from the first stub that moved.
After two changed stubs, compare once, then stop touching it. Federal withheld so far, plus the new per-check federal amount times checks still coming, should land near the household tax you already computed: $14,240 in the step-6 pair, $8,770 in the step-7 interest example, or the step-9 job’s ordinary tax plus $6,400 on that bonus. If you are still short, raise 4(c) one time, by the shortfall divided by checks left. Do not add $25 every Friday. Recheck when you marry, divorce, add a dependent, or add a job. A raise that stays one job is the 90-day file, and it often leaves 4(c) at $0.
A refund is not a savings plan, and the exempt box is not a strategy. Once the gap is covered, cut 4(c) and move that dollar to a HYSA on payday. That order is paycheck to investing. Publication 505: the “Exempt from withholding” box under Step 4(c) on the 2026 form is only if you had zero tax liability for 2025 and you expect zero for 2026. A refund is not zero liability. Publication 15: the exemption lasts for that calendar year; a new W-4 is due by February 15 if you still qualify. Checking the box to enlarge a paycheck is how April becomes the whole bill. State withholding is a separate certificate in many states. A correct federal form does not fix a state that still uses allowances. Hard stops: Step 2 on both jobs, Step 3 on both jobs, checkbox plus 4(c) for the same second salary, a Schedule C typed into “other income” with SE tax ignored, the page’s full-year 4(c) pasted onto checks you no longer have, a bonus bump left on next year’s form, and exempt claimed off a refund.
One-page decision
Match this job on the paycheck calculator before you type 4(c). One W-2 at $90,000 single is $10,970 of tax and $0 extra. Similar joint salaries ($78,000 and $72,000) use the Step 2 checkbox on the higher job only — the $10,340 gap is real, and you do not also enter the $397.69 the page prints. Uneven joint salaries ($90,000 and $55,000) skip the checkbox: $300 per check for a full year, or $975 if eight checks remain and nothing extra has been withheld yet. An $8,000 ordinary amount with no withholding is $67.69 a check. A Schedule C is the self-employed calculator, then 4(c) or 1040-ES. A $20,000 bonus withheld at 22% on a $220,000 single wage can still owe $2,000; $250 on the remaining eight checks, then take it off. Safe harbor at 110% of an $8,400 prior-year tax is $9,240 of withholding, not a paid-in-full return. Cut 4(c) once April is boring. Educational, not a withholding order and not advice.