Even if you are unable to deduct your Traditional IRA contribution, you may still contribute up to the limit for the year ($5,500 for 2018, plus an additional $1,000 if you reach age 50 by Dec. 31, 2018) to your Traditional IRA and treat the contribution as a nondeductible contribution. If you choose to make a nondeductible contribution to your Traditional IRA, you should file IRS Form 8606 (available at the IRS website) to report the contribution as nondeductible to the IRS and to keep track of the nondeductible balance (basis) in your Traditional IRA.
If you are not eligible to deduct the contribution to a Traditional IRA, and you are eligible to contribute to a Roth IRA, then the Roth may be the better choice. This is because the earnings on Roth IRA assets accrue tax-deferred and are tax-free if distributions are qualified, whereas the earnings in a Traditional IRA accrue tax-deferred but are taxed when distributed. You are eligible to contribute less than 100% of your compensation or up to the limit for the year ($5,500 for 2018, plus an additional $1,000 if you reach age 50 by Dec. 31, 2018) to a Roth IRA if your modified adjusted gross income (MAGI) is as follows:
For more on retirement savings plans, see Roth or Traditional IRA … Which is the Better Choice?, Traditional IRA Deductibility Limits and A Tour Through The Different Retirement Plans.
This question was answered by Denise Appleby