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US ZIP Code Profile

ZIP Code 78666
San Marcos, Texas

ZIP code 78666 is in San Marcos, located in Hays County, Texas, within the Austin-Round Rock metro area. Its 2020 Census population was 86,584. The estimated median adjusted gross income per tax return is $38,820, and about 4% of filers report $200,000 or more.

2020 Population
86,584
Census ZCTA
County
Hays
FIPS 48209
Median income (est.)
$38,820
IRS SOI 2022
Land Area
176.603 mi²
+ 0.93 mi² water

01Location & Geography

Authoritative geographic identifiers from the U.S. Census Bureau, ANSI, and FIPS standards.

Primary CitySan Marcos, TX
CountyHays County (FIPS 48209)
StateTexas (FIPS 48)
Metro Area (MSA)Austin-Round Rock, TX Metro Area (12420)
Census Region / DivisionSouth Region · West South Central Division
Coordinates29.880161, -97.96616
Land / Water Area176.603 mi² land · 0.93 mi² water

02Income & Economics

Distribution of the 36,290 individual tax returns filed in 78666 by adjusted gross income bracket (IRS SOI, 2022).

< $25K
34.3%
$25K–$50K
28.4%
$50K–$75K
14.2%
$75K–$100K
7.7%
$100K–$200K
11.6%
$200K+
3.8%
The $200,000+ bracket accounts for 30% of 78666's total adjusted gross income, from 4% of returns — a concentration that drives the gap between the median ($38,820) and the average.
Estimated median = median adjusted gross income per tax return, interpolated from IRS SOI brackets. Differs from U.S. Census household income.

03Income Over Time

Year-over-year change in 78666's IRS income metrics, 2020–2022.

Median income (2021→2022)
$38,820
▲ 11.1%
Filers earning $200K+
3.8%
▲ 0.4 pt
Total filers
36,290
▲ 2.3%
GET /zipc/v2/zipcodes/78666/soi?year=… $32K 2020 $35K 2021 $39K 2022
Median = adjusted gross income per return, interpolated from IRS SOI brackets. Figures are nominal (not inflation-adjusted).

04Population Trend

Decennial U.S. Census population for the 78666 ZCTA.

2000
51,452
 
2010
63,836
▲ +24.1%
2020
86,584
▲ +35.6%

05USPS Postal Details

Delivery and facility attributes from the monthly USPS dataset.

ZIP ClassificationNon-Unique Zip
Facility TypePost Office
Carrier Route SortD
USPS Finance Number488075
City AliasesSan Marcos

06Frequently Asked Questions

What county is ZIP code 78666 in?

ZIP code 78666 is in Hays County, Texas (county FIPS code 48209).

What city is ZIP code 78666?

The primary USPS city for 78666 is San Marcos, TX.

What metro area (MSA) is 78666 in?

78666 is part of the Austin-Round Rock, TX Metro Area (MSA code 12420).

What is the population of ZIP code 78666?

The 2020 Census population of 78666 was 86,584 residents.

What is the median income in ZIP code 78666?

Based on IRS Statistics of Income data, the estimated median adjusted gross income per tax return in 78666 is about $38,820, with around 4% of filers reporting $200,000 or more. Note that adjusted gross income is a tax measure and differs from U.S. Census household income.