How Missouri withholds on a bonus
Missouri lets employers withhold a flat 4.7% on supplemental wages paid separately, the state's top rate and the rate that applies to nearly all wage income since the brackets end at $9,436 of taxable income. On a $10,000 bonus the percentage method withholds $2,200 federal, $620 Social Security, $145 Medicare and $470 Missouri, leaving $6,565. A bonus earned in Kansas City or St. Louis also carries the city's 1% earnings tax ($100), not included here.
Because almost everything is at 4.7% anyway, the aggregate method changes Missouri's line only marginally and raises the federal line to about $2,700 for a $60,000 earner paid every two weeks. Missouri allows up to $5,000 of federal income tax paid to be deducted on the state return, so the extra federal withholding on a bonus can slightly reduce Missouri tax for filers below the cap.
What the withholding means for your Missouri return
Missouri's 4.7% supplemental rate matches the rate a bonus is finally taxed at, so the state line settles close to even; the refund comes from the federal side, where 22% withheld against a 12% to 22% marginal rate returns roughly $750 on a $10,000 bonus for a $60,000 earner. Kansas and Illinois residents who earn a bonus in Missouri are withheld for Missouri and credit it at home, since Missouri has no reciprocal agreements. The Missouri paycheck calculator shows the marginal rates for any salary. The filing status on Form MO W-4 does not change the flat 4.7% withheld from a separately paid bonus; it matters only under the aggregate method.