I'm 51, and had too high an AGI to make any 2018 Roth IRA contribution. Can I make a $6500 nondeductible Traditional IRA contribution, and convert it all to a Roth?

[Hal_Al]

Not for 2018. Although you can still make a 2018 non deductible (or deductible) contribution for tax year 2018, up to 4-15-10; the deadline to do a conversion was 12-31-18. So, you can still make a 2018 contribution, but the conversion will go on your 2019 return.

Furthermore,  the non-deductible  contributions cannot be converted in isolation from any other existing traditional and rollover IRA(s). It's best explained by example. Let's say you have a $10,000 balance in all your existing traditional IRAs and that balance consist of $3,000 in deductible contributions, $2,000 in previous non-deductible contributions and $5,000 in earnings (interest, dividends & capital gains). Your basis, in all your IRAs, is $2,000. In 2019 you  convert $5000 to a Roth. Only 20% of the $5000 conversion ($1000) will be tax free . TurboTax will divide that $2,000 basis by the $10,000 balance  to arrive at the 20% tax free ratio.  $4,000  of the conversion will be taxable.  This is the way the IRS requires it to be done. The calculations will be shown on form 8606.

See: http://www.bankrate.com/finance/retirement/drawback-one-type-roth-conversion.aspx

http://thefinancebuff.com/how-to-report-backdoor-roth-in-turbotax.html