Whether you see Salt Lake County’s population is growing or think people are leaving in droves, there’s a set of estimates to back you up.

The U.S. Census Bureau estimates 3.42 million people lived in Utah as of July 1, 2023. That’s about 39,000 people fewer than estimates from the Utah Population Committee at the University of Utah’s Kem C. Gardner Policy Institute.

Most of that difference — one that’s grown larger each year since 2020 — is because of a widening gap in estimates for Salt Lake County.

The Census Bureau estimates there were 1.19 million living in Salt Lake County as of July 1, 2023 — 1,170 fewer people than lived in the county in 2020. However, the Utah Population Committee puts the county’s population in July 2023 at 1.22 million, which is 32,356 more than were in the county in 2020.

There are issues with both estimates, a demographer said, and there’s a time and place to use one over the other.

Why are they different?

Estimates from the Census Bureau and the Utah Population Committee were close in 2020 — within 1,000 people.

That started changing in 2021, when estimates for July 1 differed by 3,259. The gap grew to 19,257 in 2022, and 38,748 in 2023.

That’s despite the two groups starting from basically the same baseline, said Emily Harris, a senior demographer at the Gardner Institute.

Demographers with the U.S. Census Bureau and the state committee use different methodologies, according to a fact sheet Harris emailed.

The two groups look to different agencies for their birth and death data, and the national information the Census Bureau uses is “less up-to-date,” the fact sheet says.

They also use different formulas and sources to calculate net migration.

The Census Bureau looks at tax return data, Medicare enrollment and social security data, among other sources.

The Utah Population Committee’s sources include school enrollment, tax exemptions and building permits.

How far apart are the estimates?

The two entities’ different methods leads to differences in county and state population estimates, though Harris said the large difference in Salt Lake County estimates stands out.

Census Bureau estimates show large and sustained out-migration in Salt Lake County, but none of the Utah Population Committee’s sources indicate more people are leaving than arriving.

That’s led to a growing gap in estimates for Salt Lake County — nearly 35,000 people as of July 1, 2023, making up most of the gap in the state population estimates.

Estimates for other counties have smaller gaps, but none of them are the same. Census Bureau estimates are higher by as much as 3.5% in 13 counties and lower by as much as 4.2% in the other 16.

Those gaps are largely based on differences in estimates for net migration.

The two estimates for natural increases — the number of births minus the number of deaths — are within 365 people for the statewide population.

But the Utah Population Committee estimates about 31,500 more people moved to Utah from out of state, while the Census Bureau puts net migration at about 11,600.

The biggest difference is Salt Lake County, where the Census Bureau shows population loss because of an out-migration of more than 8,000 people — while the UPC shows an in-migration of about 7,000 people and a population gain.

A fact sheet goes into more detail about where estimates differ on natural change and net migration.

Is one more accurate?

Neither estimate is necessarily better than the other, Harris said, as there are always data issues.

“Estimates and projections are always going to be wrong,” Harris said, even though both UPC and the Census Bureau work to make the best estimates possible.

But Harris and other demographers in Utah can explain the UPC estimates and trust them, she said, especially because they’re “only estimating for Utah [and] can address data issues.”

The Census Bureau is creating estimates for many more places, she said, and doesn’t have time to review data issues for all of them.

Still, there’s a time and place for both sets of estimates.

The policy institute recommends using UPC estimates when looking only at Utah because they will be “more closely vetted by local data experts.”

Census Bureau estimates, on the other hand, are better for comparing Utah or any of the state’s 29 counties to other places in the United States, or for using racial and ethnic data.

Megan Banta is The Salt Lake Tribune’s data enterprise reporter, a philanthropically supported position. The Tribune retains control over all editorial decisions.