So, how much will be taxable?
It may best be explained by example. Let's say you have a $4,000 balance in all your existing traditional IRAs on 12-31-20 and earlier in 2020 you converted $6000 to Roth. Your balance for the year was $10,000 (the 4000 on hand at year end plus the 6000 you converted). 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. Only 20% of the $6000 conversion ($1200) will be tax free . TurboTax will divide that $2,000 basis by the $10,000 balance to arrive at the 20% tax free ratio. $4,800 of the conversion will be taxable.
