For single taxpayers, the top tax rate of 37 percent kicks in for income above $500,000. Yet for married couples, that rate gets applied to income over $600,000.
In other words, two individuals who each have income of $500,000 would pay the second-highest rate, 35 percent, on their income if they filed as a single taxpayer.
However, as a married couple with combined income of $1 million, they would pay 37 percent on $400,000 of that (the difference between their income and the $600,000 threshold for the highest rate). That would mean paying $8,000 more in income taxes.
More from Personal Finance:
Find out if you’re eligible for this tax-savings strategy in 2018
How to get out from under that gift-giving pressure this holiday season
How to get your taxes done for free
There also are other parts of the tax code that can cost married couples more. For instance, while an individual can have up to $200,000 in income before the Medicare surtax of 0.9 percent kicks in, the limit for married couples is $250,000.
Additionally, the new limit on the deduction for state and local taxes — also known as SALT — is not doubled for married couples. The $10,000 cap applies to both single filers and married filers. (Married couples filing separately get $5,000 each for the deduction).
Of course, the deduction only is available to taxpayers who itemize, and fewer people are expected to do so on their 2018 returns due to the doubling of the standard deduction. Itemizers in higher-tax states — including New Jersey, California and New York — who marry could be more affected by it, Westley said.