Learn how to run a bonus payroll and accurately calculate bonus pay and taxes with our comprehensive small business guide. The IRS views any bonus you receive as wages.This means that almost all bonuses are taxed. Who must file Form IL-941? This Illinois bonus tax calculator uses supplemental tax rates to calculate withholding on special wage payments such as bonuses. To do so, you'll need to fill out a new Form W4 and submit it to your employer. Profit-sharing and stock bonus plan, or IRA payments. This method results in high sample rates, higher than typically experienced when sending out a survey. DO NOT fill out if you have already sent in a form for your new provider. If you did not receive a letter in the mail, please verify your address on file with your Ethics Officer.