Every ZIP code in the United States, with weather, radar, trivia and the numbers behind it. Five digits since 1963!

ZIP stands for Zone Improvement Plan. The codes went live on July 1, 1963.The lowest ZIP code is 00501 — the IRS in Holtsville, New York.The highest ZIP code is 99950, in Ketchikan, Alaska.Type 07734 into a calculator and turn it upside down: hELLO. It is Keansburg, New Jersey.12345 belongs to one company: General Electric in Schenectady, New York.The first digit is a region: 0 is New England and New Jersey, 9 is the West Coast, Alaska and Hawaii.Mr. ZIP, the cartoon mailman, was the mascot that sold the idea to America.ZIP+4 arrived in 1983 and can narrow a ZIP down to one side of one block.A ZIP code is a mail route, not a place: plenty of them cross county and even state lines.Play ZIP Hunt: guess the secret ZIP code of the day in six tries.
zipcodes.pro — ZIP/21139

21139

ZIP 21139 is Riderwood, Maryland, in Baltimore County; area codes 410, 443 and 667; Eastern time.

Type
No street area
Time zone
America/New_York
County
Baltimore
Coordinates
39.4093, -76.6486

The Census map gives this ZIP code no delivery area of its own, which usually means it serves post office boxes, a single organization or a post office rather than streets of homes.

This ZIP code has no Census tabulation area of its own. Its point falls inside ZCTA 21204, whose figures describe that area — not this ZIP code.

☼ Live from 21139 — Riderwood, MD

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Where 21139 is

This ZIP code has no Census tabulation area, so there is no boundary to draw — the marker is its published point location.

★ Weird and wonderful facts about 21139
  • 🔢 Type 21139 into a pocket calculator and turn it upside down: GEIIZ — not quite a word, sorry.
  • ➕ 21139 is a prime number. It cannot be divided by anything except 1 and itself. Its digits add up to 16, a total 1,922 other ZIP codes share.
  • 💾In binary, the way a computer stores it: 101001010010011
  • 🏠 Its numerical neighbours: 21136 (Reisterstown, MD, 11 mi away) and 21140 (Riva, MD, 32 mi away).
  • 🔄 Backwards, 21139 becomes 93112, which is not a ZIP code at all.
  • 👥 23 other ZIP codes are made of exactly the same digits (11239, 11932, 12139, 12193, 12913, 13219, …). The farthest-flung twin is 93121 in Santa Barbara, CA, 2,380 miles away.
  • 🧭 Farther north than 52% of all ZIP codes, and farther east than 83%.
  • 🎯1,165 miles east of the geographic center of the Lower 48, near Lebanon, Kansas.
  • 🌏 Dig straight through the Earth from 21139 and you would come out at 39.41°S, 103.35°E — 869 miles from Perth, Australia, the nearest big city to that spot.
  • ☀Longest day of the year: 14h 57m of daylight. Shortest: 9h 23m.
🌙 The sky over 21139 today
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Source: USGS.

Land area

ZIP codes near 21139

ZIPCityDistance
21204 Towson, MD 1.2 mi
21022 Brooklandville, MD 1.5 mi
21093 Lutherville Timonium, MD 2.0 mi
21252 Towson, MD 2.2 mi
21285 Towson, MD 2.3 mi
21209 Baltimore, MD 2.5 mi
21284 Towson, MD 2.6 mi
21153 Stevenson, MD 2.8 mi
21212 Baltimore, MD 3.4 mi
21210 Baltimore, MD 3.6 mi

Find every ZIP code within 10 miles of 21139 →

Questions about this ZIP code

Where is ZIP code 21139?

21139 is in Riderwood, Maryland, in Baltimore County, part of the Baltimore-Columbia-Towson, MD metropolitan area. Its published point location is 39.4093, -76.6486.

What county is 21139 in?

21139 lies in Baltimore County.

What area code is 21139?

Riderwood uses area codes 410, 443 and 667, from NANPA's telephone records.

What time zone is 21139 in?

21139 uses America/New_York.

What ZIP codes are near 21139?

The closest are 21204 (1.2 mi), 21022 (1.5 mi), 21093 (2 mi), 21252 (2.2 mi) and 21285 (2.3 mi).

Is 21139 a PO Box ZIP code?

Possibly. The Census map gives this ZIP code no delivery area of its own, which usually means it serves post office boxes, a single organization or a post office rather than streets of homes.

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Cite this page

zipcodes.pro. “21139 ZIP Code — Riderwood, Maryland (Baltimore County).” Data release 2026-10-12. Retrieved 2026-10-20. https://zipcodes.pro/zip/21139

Machine-readable version of these facts:

curl https://zipcodes.pro/zip/21139.json

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