If this election has not already made your head hurt, allow us to finish the job.

The Texas secretary of state announced Wednesday that about 78 percent of the state’s voting-age population is registered to cast ballots in the Nov. 8 election. It’s a simple computation: A record-breaking 15,100,824 people are registered. The population of Texans old enough to vote is 19,307,355. Divide this, multiply by that and you get 78 percent, give or take.

Or is it? It turns out the state’s top election counters may be off the mark, and incidentally understating the number of Texans who don’t vote.

The mathematical muddle lies with the presumed number of people living in Texas who are 18 years or older.

The voting-age population figure being used by the secretary of state for this year’s elections is actually a 2015 projection, and it’s a guess based on state population growth that specifically does not account for adults moving into the state.

Had the secretary of state factored in migration, the 2015 voting-age population would have totaled 20,383,361 adult Texans. But there are already 2016 estimates available. The 2016 voting age population projection — accounting for migration — totals 20,832,609.

Against those voting-age population figures, estimates of the number of Texans that didn’t vote would have been between 1 and 1.5 million higher during the March primary election. Put more simply: the already depressing 22 percent turnout rate among adult Texans in the primaries was probably even lower.

A spokeswoman for the secretary of state’s office said it used the state demographer’s one-year-old projection of voting-age population because that’s the way it’s always been done.

“The idea is to continue with the methodology that has been used for quite some time in order for the numbers to be consistent,” Alicia Pierce, the spokeswoman, said.

The 2016 projection, including migration to the state since 2010, has been out for a couple of years, said state demographer Lloyd Potter. But under the scenario the state is using, only the growth in voting-age population caused by people turning 18 is counted, he added.

In recent years, people moving to Texas from other states — rather than from other countries — have played a key role in the state’s population growth.

Texas is already home to some of the lowest turnout rates among adults. More than four — and fewer than five — of every 10 adults has voted in each of the past 10 presidential runs.

The calculations would be even murkier if you considered the share of adult Texans that aren’t citizens. But the secretary of state doesn’t distinguish between eligible and ineligible adults in choosing its voting-age population number.