The number of ineligible felons depends on state law. Some state
permanently disfranchise felons and a few let even prisoners
vote. Statistics drawn from various Department of Justice report the prison, probation, and parole population of the United
States. Links to these reports are provided on individual election voter turnout data web pages. These statistics are matched against the various state felony disfranchisement laws compiled by the Sentencing Project to estimate the number of
ineligible felons in a given state.
This can be a crude methodology, as some states disenfranchise only persons who commit specific crimes. Unfortunately, the Department of Justice reports do not break down persons in the correctional population to this fine degree.
A word on permanently disfranchised felons
The number of permanently disfranchised felons in the states that have some form of post-correctional voting restriction is not computed, since time-series statistics on recidivism, deaths, and migration of felons are largely unavailable. Chris Uggen and Jeff Manza estimated the number of permanently disfranchised felons in the 2004 election. Because their methodology has not been updated for a longer time period, I have not adopted their adjustment.