G-31 ・ Explore ・ 1–2 min

Postcode Guesser — Name the prefecture from a Japanese postcode

A five-question daily browser quiz about Japanese postcodes. A sign at the top shows the first three digits (for example 〒900), and below it sits a tile map with one tile per prefecture. Tap a tile and it sinks at once; then the correct tile lights up, a line is drawn from your pick, and the correct prefecture and distance appear in large type in the corner of the map. Distance is the straight line between the two prefectural capitals: the same prefecture is 🟩, under 300 km is 🟨, anything farther is 🟥. You also learn a representative town for that number, such as “〒900 is Naha, Okinawa”. The five postcodes are set by the date in Japan time and drawn from five of Japan's eight regions, so no area dominates. Landmark postcodes (Sapporo, Tokyo, Osaka and more) are printed on the map, so you can get close even with numbers you don't know. The result shows five colored squares, your total in km and your personal best, and you can share it as “Postcode Guesser 10/26 🟩🟨🟩🟥🟩 total 612 km”. The question table is built from Japan Post's postcode data, leaving out any three-digit prefix shared by two or more prefectures.

How to play

  1. The sign at the top shows the first three digits of a postcode (e.g. 〒900). Tap the prefecture it belongs to on the tile map below. There are five questions, and the lower your total distance off, the better
  2. Hold your finger to see the prefecture's name; release to answer. On phones a magnified window of nearby tiles appears above your finger, and sliding moves the selection inside it. Your tile sinks at once, then the correct tile lights up, a line joins the two, and the correct prefecture and distance appear in large type
  3. Same prefecture is 🟩, under 300 km is 🟨, farther is 🟥. Distance is measured in a straight line between prefectural capitals
  4. Small numbers on the map (060, 100, 530…) are landmark postcodes. Prefectures you answer keep their number too
  5. After five questions you get five colored squares, your total in km and your personal best. Today's set is played once; after that you get random practice rounds

Keyboard: ←→↑↓ choose a prefecture / Enter or Space answer, next question / R practice from the result

What the first three digits mean

According to Japan Post's postcode and barcode manual, a postcode has seven digits, and the old postcodes generally live on inside the current ones: four digits were added to the old three-digit codes and two to the old five-digit codes, each pointing to a town area. The manual's examples are old 154 (Wakabayashi, Setagaya) becoming 154-0023 and old 350-11 (Komuro, Kawagoe) becoming 350-1106. That is why the first three digits tell you the rough area, and this game shows only those three digits.

How the question table is built

Grouping the 124,841 rows of Japan Post's nationwide postcode data (updated 30 October 2026) by their first three digits gives 948 prefixes. 22 of them span two or more prefectures and are left out, so questions come from the remaining 926. The representative town shown for each prefix is the one on its XXX-0000 row (or the town with the most rows if there is none).

Distance examples (demo numbers)

Distance is the straight line between the city halls of the two prefectural capitals. In the demo (?demo=1), picking Yamagata for 〒980 is 46 km from the answer, Miyagi (🟨), and picking Osaka for 〒810 is 485 km from the answer, Fukuoka (🟥). The city hall coordinates come from the Geospatial Information Authority of Japan's address search, and the distances are this game's calculation (great-circle distance rounded to whole km). In large prefectures the town in the question can be far from the capital.

Sources

Japan Post, postcode and barcode manual (in Japanese) https://www.post.japanpost.jp/service/search/zipcode/zipmanual/p04.html / Japan Post, postcode data download (updated 30 October 2026) https://www.post.japanpost.jp/service/search/zipcode/download/kogaki-zip.html / Japan Post, about the postcode data https://www.post.japanpost.jp/service/search/zipcode/download/readme.html / GSI address search https://msearch.gsi.go.jp/ . Japan Post pages checked on 5 November 2026; coordinates retrieved on 29 October 2026.

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