How the District withholds on a bonus
DC's Office of Tax and Revenue publishes no separate supplemental rate; a bonus is withheld on the combined amount with regular wages under the District's graduated tables. Because DC's 8.5% bracket begins at $60,000 of taxable income for every filing status, a bonus paid to a middle earner is withheld largely at 8.5%. For a $60,000 single filer paid every two weeks, that is about $780 of DC withholding on a $10,000 bonus, alongside $2,200 federal, $620 Social Security and $145 Medicare under the percentage method, leaving roughly $6,255.
The District cannot tax non-residents, so a Maryland or Virginia resident who earns a bonus in DC pays their home state's withholding on it instead, not DC's. A DC resident working in Maryland or Virginia is withheld for DC under the reciprocal arrangements. There is no separate city layer; the District's tax is the only one.
What the withholding means for your DC return
DC withholds under its actual brackets, so the state line on a bonus is close to the tax owed; the 8.5% is what a $60,000 earner's marginal income really faces in the District. The federal 22% flat rate is above the 12% to 22% federal marginal rate at that salary, so roughly $750 of federal withholding on a $10,000 bonus returns at filing. The DC paycheck calculator shows the marginal rates for any salary. Allowances on DC Form D-4 feed the combined-wage calculation, so a bonus is withheld less heavily for an employee claiming several allowances than for one claiming none.