The difference between the amount of income tax withheld for single versus married persons in the United States depends on the amount of income earned. However, in general, those who take who take the married withholding allowance have less of their income withheld. This is based on the assumption that a couple qualifies for a greater number of exemptions at tax time than a single person.
The U.S. withholding tax policy uses a tiered rate system based on the amount of income earned. Tax rates grow incrementally as a person's income increases. For the 2018 tax year, the tax rate tiers are 10%, 12%, 22%, 24%, 32%, 35% and 37%. However, the minimum and maximum income for each tier varies depending on whether the taxpayer is married or single. To determine the amount of pay that should be withheld, each individual must file a Form W-4 with his or her employer.
For example, single individuals that earn less than $308 per month are not subject to any withholding in 2018. Any income above $308 but below $1,102 is taxable at the 10% rate. Income above $1,102 but below $3,533 is taxed at the 12% rate. However, individuals who claim the married allowance may earn up to $963 monthly without incurring a withholding tax. The 10% tier applies to income up to $2,550 for married individuals, while the 12% tier applies on income over $2,550 but below $7,413.
The withholding tax is implemented as a means of spreading out each person's tax burden over the course of the year. Depending on the number of allowances, it is possible to have too much or too little income withheld. However, any such discrepancies are rectified at tax time.
The Internal Revenue Service (IRS) can adjust rates at any time, so the exact amount of income tax due for a single or married individual should be verified either by a tax professional or by using the withholding calculator on IRS.gov.