In determining whether your withholding will be sufficient, or whether you will need to make estimated tax payments, your estimated tax for the year is your total expected income tax, self-employment tax, and any other taxes for which you may be liable, minus any tax credits you can take and minus the tax that has been withheld.
Generally, you must make estimated tax payments if you expect that you will owe $1,000 or more when you file your return, and you expect that the tax withheld will be less than the smaller of:
90% of the tax you expect to report on your return for this year, or
100% of the tax you reported on last year’s return.
If your adjusted gross income is over a certain amount, you may have to use 110% instead of 100% of the tax you reported last year.
You should keep in mind, when determining if you need to make estimated tax payments, that part of your social security or railroad retirement benefits may be taxable.