This ‘back of the napkin math’ shows whether you could have a surprise tax bill, expert says

You can start by finding your total federal taxes paid for 2023, which is listed on line 24 of your tax return. If your gross income and tax situation has not changed from last year, you are likely to owe a similar amount for 2024, Lucas explained.   

Next, you will need to review your pay stubs.

If you have paid roughly 75% of last year’s total taxes by the end of September, “you’re going to be pretty darn close, assuming everything is the same as the prior year,” he said.  

However, “there’s a whole slew of things that can change” from year to year, such as a second job, higher income, divorce, marriage or birth of a child, which makes your tax situation different, Lucas said. 

In those scenarios, you will need a more in-depth analysis to double-check your 2024 withholding, he said.    

If your tax situation changed this year, experts recommend periodically using a free tool from the IRS, known as the “tax withholding estimator.”

The tool factors in your marital status, dependents, number of jobs, other sources of income, most-recent paystub, taxes withheld, estimated tax payments and other details.  

After plugging in your information, the IRS provides a prefilled Form W-4, which you can then provide to your employer to increase or decrease your withholding.

Alternatively, you could make payments directly to the IRS to cover your 2024 tax shortfall, Lucas said.

Either way, “you’ve got to keep an eye on it,” or you could face an unexpected tax bill, along with penalties and interest, said Mark Steber, chief tax information officer at Jackson Hewitt.