♨️ Japan Onsen Lookup (Spring Quality & Sources)

A Japanese onsen (温泉) is legally defined by the Onsen Law of 1948: natural water from underground that is at least 25 °C at the source, or contains one of the specified components (such as lithium, radon or sulfur). Hot-spring water is classified into 10 spring-quality types based on its dissolved components, and the type determines the feel of the water and its traditional indications.

Beyond the type, baths are classified by source temperature: cold spring under 25 °C, lukewarm 25–34 °C, hot 34–42 °C, and high-temperature over 42 °C.

This lookup has two modes. Choose spring quality to read the character, temperature feel and example sources of that type. Choose famous source to see the spring quality, source temperature and character of a well-known onsen area:

The temperature and quality let you choose: want the classic sulfur smell and high heat? Go acidic or sulfur (Kusatsu, Noboribetsu). Want skin-softening water? Choose bicarbonate or alkaline simple (Dogo, Gero). The lookup is a travel aid, not a medical reference.

1. Choose a mode: browse by spring quality or by famous source.
2. Pick the type or source you are curious about.
3. Read the character, temperature feel and components.
4. Check the pH and source temperature before deciding to visit.
5. At the bath, compare with the official onsen analysis card posted on site.

What makes water legally an onsen in Japan?

The Onsen Law (1948) defines an onsen as natural underground water at least 25 C at the source, or water containing any of the listed components (e.g. lithium, radon, sulfur, iron) above set levels.

What are the 10 spring-quality types?

Simple, chloride, bicarbonate, sulfate, carbon dioxide, iron, iodine, sulfur, acidic, and radioactive springs. Some areas also show sub-classifications by pH or salt content.

Which onsen is good for skin?

Bicarbonate springs (美肌の湯) and alkaline simple springs (pH 8.5+) are traditionally associated with smooth skin - Dogo, Gero and many Yufuin baths are examples.

Why does Kusatsu burn?

Kusatsu's spring is strongly acidic (pH around 1.2) and very hot (51-95 C). The traditional yumomi cooling ritual lowers the temperature before guests enter.

What is the difference between onsen and sento?

An onsen is natural hot spring water meeting the Onsen Law; a sento is a public bath usually using heated tap water. Many inns have both or mix them.

Is hot spring water safe to drink?

Only if the source is designated as drinkable (飲泉) - most are not. Bathing water may contain metals or minerals unsafe to consume, so never drink from a tub.

Read the onsen analysis card (温泉分析書) posted at every bath - it shows source temperature, pH, and spring quality. A pH over 8.5 is called alkaline simple and is the famous "beauty water". Sulfur springs are strong: people with sensitive skin should limit their soak. High-temperature springs (over 42 °C) are often cooled at the bath, so check the bath temperature, not just the source. When visiting, follow local etiquette - wash before entering, keep towels out of the water, and try a short soak (10-15 min) with rests. For a full trip, pair the onsen with a local specialty: rice country springs (Gero, Yufuin) go perfectly with rice-based meals, and hot-spring towns make classic gift-giving destinations for omiyage.