Right Onsen › How we know

How we know this

Every fact on this site says where it came from: the facility's own website, its public listing, or the number of Japanese visitors who stated it and the years they said it. Where the evidence is thin, the page says so. This page explains exactly how that works, in numbers.

Three kinds of source, in a fixed order

When two sources disagree, the higher one wins the value and the lower one stays visible underneath.

1. Official — the facility's own website, read in Japanese. We quote the sentence, link the page and print the month we read it. Today: 2682 statements from 659 websites, mostly on tattoos, day-use and prices.
2. Measured — the facility's public listing (hours, closing days, its own yes/no on things like an outdoor bath) and government statistics. When a listing is a sento or a day-use onsen, admitting day visitors is what the category means; 510 pages say so with that source.
3. Verified third parties — where another site verifies a policy by contacting the facility, we cite its value with a link and its verification date, never its text. Today that is tattoofriendlyonsen.com for tattoo policy on 28 baths.
4. Visitors — Japanese-language reviews, read in full (49,317 so far) and distilled into yes / no / unknown facts with a count and a date range. We never republish review text: a sentence belongs to the person who wrote it, and “stated by 6 visitors · Nov 2025 – Aug 2026” is more useful than one stranger's sentence.

What has to be true before we print a visitor fact

• The quote is real. Every fact carries the Japanese sentences it rests on, and a program checks each one against the reviews, character for character. A count we cannot back with quotes is not a count.
• Enough people said it. Policies (tattoos, day visitors, price) need two different visitors; physical facts a single visitor can see (an outdoor bath, a sauna) need one. “Mentions” is the number of distinct reviews with a verified quote, not a number the model reports about itself.
• The door gets a second reader. Every published tattoo policy and every “guests only” or “residents only” is re-read by a second model whose only job is to find a reason it is wrong. So far it has rejected 21 of 269 — including three “tattoos allowed” that rested on bathers complaining about tattooed people. Those never reached a page.
• Old policies are marked. A price or rule whose newest report is more than two and a half years old is shown with its year and kept out of the finder and the lists.

Today: 15,067 facts on 1384 bath pages, out of 19,442 extracted. The difference is the gate doing its job.

We check a sample by hand, after the gate

After every extraction round a stronger model re-reads a random sample of published facts with the full reviews beside them. In the latest round, 100.0% of 122 judged facts held up. A field that scores under 90% is switched off entirely — not shown thinner, not shown.

• Last entry: switched off — in the last precision round, closing times were being reported as last admission
• Parking: switched off — “free” was being inferred from “large lot”, never stated

Why we so often say “not known”

This is the one fact where being wrong hurts you at the door: undressed, at the counter, being turned away. So it never rests on a single visitor, it always gets the second reader, and where the facility's own website states a policy that wins. Today 139 pages carry an official tattoo policy and 171 a visitor-reported one; the rest say “Not known · call ahead” with the number of reviews we read and whether we read the website too. That is not laziness: it means nobody stated a policy, and we will not guess one.

What we cover, and what we know we miss

• Gunma: 170 baths described — the Ministry of the Environment counts about 250 onsen public baths in the prefecture, so we are close but not complete, and small unlisted neighbourhood baths are the ones most likely missing
• Tokyo: 538 baths described
• Kanagawa: 225 baths described
• Kyoto: 156 baths described
• Oita: 296 baths described

1385 baths have a page; 298 listings were kept out: duplicates (they redirect to the surviving page), closed facilities, listings that are not baths, and inns whose bath is for staying guests only. We also check the list against OpenStreetMap for baths the listings missed.

• Visitor reviews in Japanese — Google Business listings and reviews
• Facility websites — read for tattoo policy, day-use rules and prices, quoted and linked
• Ministry of the Environment, Report on Hot Spring Use — prefecture-level counts
• Ministry of Health, Labour and Welfare, Report on Public Health Administration — licensed bath counts
• OpenStreetMap — railway stations, distances and the completeness check

How often this is updated

Opening hours, closing days and whether a bath has closed permanently are refreshed monthly. The review-derived facts are re-read once a year, before the autumn and winter bathing season, because prices and rules change with the season rather than the month — and every fact carries its year range so you can see how fresh it is.

Something wrong? Tell us

If a price has changed, a bath has closed, or a tattoo policy is not what a page says, we want to know — you are standing in front of it and we are not. Use the form at the bottom of that bath's page, or the site-wide form below. Corrections from people who were actually there beat anything we can distil.

What this site is not

• Not a booking site. We do not sell or reserve anything.
• Not a ryokan review site. We describe the bath, not the stay.
• Not a substitute for calling ahead when a rule matters to you — especially tattoos, private baths, and whether day visitors are admitted at all.