For example, if someone has contributed $10,000 post-tax and $30,000 pre-tax to a traditional IRA and wants to convert the post tax $10,000 into a Roth, the pro-rated amount (ratio of taxable contributions to total contributions) is taxable. In this example, $7500 of the post tax contribution is considered taxable when converting it to a Roth IRA. The pro-rata calculation is made based on all traditional IRA contributions across all the individual's traditional IRA accounts (even if they are in different institutions).