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A $21k check loses $5,932 to federal withholding. Here is the math your payroll uses.
The most-misunderstood line on any paystub: federal withholding is not your tax rate — it is the IRS aggregate method pretending this one check is what you earn every check, annualizing it through the full bracket stack, then dividing back down. That is why a commission or bonus check gets crushed while your refund later is huge. This calculator runs the exact payroll math with 2026 brackets and shows both the per-check withholding and your true marginal rate.
Anyone staring at a gutted commission or overtime check uses it to learn the annualization rule: payroll pretends this check is every check, pushes the phantom annual salary through the full brackets, and the over-withholding comes back only as a refund. It turns paystub shock into a W-4 decision.
Annualization. Your employer multiplies the check by your pay periods and taxes it as if that is your salary. A single $21,000 biweekly check is treated as $546,000/year of income: after the $16,100 standard deduction, the 2026 brackets produce $5,932 of federal withholding on that one check — 35.9% with Social Security and Medicare. If your real income is lower, the excess comes back as a refund. Verified against a real 2026 paystub: $13,411 gross minus $135.51 pre-tax, biweekly, single → $3,228.51 federal, to the penny.
No — it is withheld higher, not taxed higher. Supplemental wages are withheld at a flat 22% federal rate (37% above $1M), or aggregated with regular pay and annualized like any other check. Either way it is an estimate: your actual tax is settled on your return at your real brackets, and over-withholding becomes refund.
Set your W-4 from your average check, not your biggest one. If commissions or overtime spike some checks, annualization over-withholds on the spikes; you can add a flat dollar adjustment on W-4 line 4(c) tuned to your average, or simply accept the forced savings and take the refund. Pre-tax 401(k) and HSA contributions reduce the annualized wage base directly — they shrink withholding on every check, including the big ones.