Percent of Uninsured in 2008 vs 2015?

The uninsured rate in 2008 was about 15% studies show the current rate at about 12%. Studies are based on real and survey data, so both numbers are estimates. No one has a true count of insured versus uninsured for any year, but looking at census data and recent studies we can get close. To make things a little confusing even the best studies only line up roughly. Those studies include (but are in no means limited to) an April 2015 Gallup study, a May 2015 RAND corporation study, and a number of HHS and CMS studies from 2015.

A few more points of confusion that is going to make everything a little harder is that every day people drop plans, or gain plans, or don't pay, or make an appeal. So the numbers are always fluctuating. Then there is the person who qualified for Medicaid before, but didn't enroll and the person who lost coverage, switched to Marketplace, then to Medicaid, then back again.

There is also the fact that the non-elderly population itself fluctuates.

Given this you are on the right track in your thinking. Here are some facts to help you get a clearer picture:

• About 12 million enrolled in the Marketplace between 2013 and 2015 counting plan drops. As many as 7 million may have not had insurance without the ACA. Some of the 12 million won't pay or will drop their plan.
• About 12 million enrolled in Medicaid. As many as half may not have had insurance. Medicaid enrollment is 365 so this number moves constantly.
• According to the US Census Bureau, before the ACA in 2009 about 48.6 million or 15.7% of the population was uninsured, but Gallup shows an average of around 14.5 in 2008. Was this a problem the ACA was solving, or was the uninsured rate going up due to talks of the ACA?
• About 5.7 million young adults stayed on their parents plan, but only 3.2 million did since 2013.
• A May 2015 RAND corporation study estimated that  22.8 million got coverage and 5.9 million lost plans for a net total of 16.9 million newly insured. 9.6 million people enrolled in employer-sponsored health plans, followed by Medicaid (6.5 million), the individual marketplaces (4.1 million), non-marketplace individual plans (1.2 million) and other insurance sources (1.5 million). To clarify that is 4.1 million newly enrolled in the Marketplace and 7.1 who transitioned to Marketplace coverage for a total of 11.2 million.
• If we take RAND at it's word we actually get something like an 8% uninsured rate (and we see that the employer mandate was actually the most successful part of the ACA for lowering the uninsured rate).

Putting these facts together we don't get round numbers, the best we get is some insight into the fact that the uninsured rate is lowering. Whole numbers make good talking points, and in the case of tallying the uninsured (not just sign ups which are much more concrete) we are simply not left with solid whole numbers.