ZIP Codes of Washington County, MO

Population & People
Population & People
Total Population
20,810
Household Income
Household Income
Median Income
$52,527
Housing
Housing
Total Housing Units
9,230
Business & Economy
Business & Economy
Total Businesses
345
Employment
Employment
Employment Rate
49.66%
Education
Education
Bachelor's Degree+
11.89%
Source: U.S. Census ACS 2024 5-Year Estimates · rolled up across the 8 ZIPs in this county.
Data Last Updated: July 1, 2026
ZIP Codes:8 ZIP Codes (7 Standard, 1 PO Box, 0 Unique) [See All]
Area Codes:235 / 573
Timezone:Central (GMT -06:00)
Local Time:11:54:05 PM
Population:20,810 [See All]
FIPS:29221
Land Area:556 sq mi
Water Area:2 sq mi
Coordinates:37.9206, -90.7856
Quick Link:
https://www.zip-codes.com/county/mo-washington.asp
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Missouri's longest commutes
38-minute average travel time to work.

Standout ZIPs in this County


Most Populous
Potosi · 9,693 people
46.6% of county total
Highest Income
Caledonia · $74,211 median
1.4× county average
Largest Land Area
Potosi · 296.6 sq mi
Spans the most ground
Fastest Growth
Potosi · 20.5% (10-yr)
8,043 → 9,693 residents
Most Educated
Irondale · 14.5% bachelor's+
179 of 1,238 adults 25+

Things worth knowing about Washington County


20,810 residents live across 8 ZIPs covering 556 sq mi, served by 8 distinct USPS-recognized cities.

Population moved from 21,439 to 20,810 from 2014 to 2024 — a -2.9% change. 3 ZIPs gained residents, 5 lost.

The county's wealthiest ZIP 63631 (Caledonia, $74,211 median) and poorest ZIP 63622 (Belgrade, $31,750) are 4.8 miles apart — a 2.3× income gap across local geography.

Median home values vary 1.5× across this county — from $99,500 in 63648 (Irondale) to $149,800 in 63630 (Cadet).

Density varies across the county — from 14 people/sq mi in 63631 (Caledonia) to 80 in 63660 (Mineral Point).

Educational attainment ranges from 5.7% bachelor's+ in 63071 (Richwoods) to 29.4% in 63622 (Belgrade) — the breadth of educational profiles across the area.

Within Missouri, this is the #52 most populous of 116 counties and ranks #95 for median household income .

Weighted by population, the typical household earns about $55,749 across the county — reflecting what a typical resident sees, not a simple ZIP average.

11.9% of adults age 25+ hold a bachelor's degree or higher (1,714 of 14,420 reporting).

7 Standard delivery, 1 PO Box only, 0 single-entity Unique ZIPs serve this county.

Stories about Washington County


Big & sparse

A frontier county — vast land area with only a few residents per square mile. Daily life looks very different here than in the country's denser counties.

Total area
556
square miles
Avg density
37.4
people per sq mi
Population
20,810
across 8 ZIPs
Potosi
296.6 sq mi
9,693 residents
Cadet
72.8 sq mi
4,006 residents
Belgrade
41.6 sq mi
764 residents

All ZIP Codes in Washington County


ZIPCityTypePopulationMedian IncomeArea (sq mi)10-yr Growth
63664PotosiStandard9,693$61,216296.620.5%
63630CadetStandard4,006$52,48272.80.8%
63660Mineral PointStandard2,784$51,41034.7-33.6%
63648IrondaleStandard1,858$51,76531.219.4%
63071RichwoodsStandard1,113$44,85340.4-19.8%
63622BelgradeStandard764$31,75041.6-9.3%
63631CaledoniaStandard544$74,21138.5-56.2%
63674TiffPO Box480.0-75.9%

Adjacent & Nearby Counties