Although most people only file taxes once a year -- unless they file quarterly estimated taxes -- income taxes are actually paid on a rolling basis through employer withholding. Your employer withholds a portion of your earnings from each paycheck based upon information you provide about your marital status and number of dependents. If you claim married status on your W-4, you can expect to have slightly less withheld from your individual paycheck each pay cycle.
When you fill out your W-4, if you are married or a head of household, you can take an additional allowance. This allowance lowers your payroll tax burden. Some people opt to take no allowances so that they don't have to pay taxes at the end of the year or to increase their likelihood of getting a refund. You're not legally required to take all of the allowances to which you're entitled.
The Internal Revenue Service establishes withholding rates each year. Single people generally have a greater percentage of their income withheld for the simple reason that married people are allowed to jointly make more money before they fall into a higher tax bracket. For example, in 2017, single people who make between $37,951 and $91,900 are in the 25 percent tax bracket, while married people filing jointly don't fall into the 25 percent bracket unless their income is between $75,901 and $153,100.
Starting in 2018, the tax brackets are different after tax law changes, but the principle still holds. A 22 percent tax bracket includes single people with income between $38,701 and $82,500, or married people filing jointly with income between $77,401 and $165,000.
Standard Tax Deduction
The IRS offers a standard tax deduction for basic living expenses upon which tax filers do not pay taxes. As of 2017, single people and married people filing separately get a deduction of $6,350, while heads of household get $9,350 and married people filing jointly get $12,700. As of 2018, under the new tax law, the deductions are $12,000 for individuals, $18,000 for heads of household and $24,000 for married couples filing jointly.
This standard deduction is factored into payroll tax withholding, and is another reason single people tend to pay more. The standard deduction can also reduce your tax liability when you file.
Married couples sometimes find that filing jointly pushes them into a higher tax bracket or otherwise increases their shared tax burden. For example, if one spouse is near the high end of a tax bracket, the second spouse's earnings could push them both into a higher bracket. If you are married, but choose to file separately, claiming the marriage allowance on your W-4 could result in your owing taxes at the end of the year because you did not have enough income withheld from your paycheck.