In Japan, mattress sizes can vary slightly from international standards, and there are some unique sizes specific to the country. Here are the common mattress sizes in Japan:

• Small Single: 97 cm x 195 cm (38" x 77")
• Single: 105 cm x 195 cm (41" x 77")
• Semi-Double: 122 cm x 195 cm (48" x 77")
• Double: 140 cm x 195 cm (55" x 77")
• Queen: 152 cm x 195 cm (60" x 77")
• King: 182 cm x 195 cm (72" x 77")

These sizes are approximate and can vary slightly between manufacturers. It's always a good idea to check the specific dimensions provided by the manufacturer or retailer when purchasing a mattress. Additionally, some retailers may offer custom sizes to fit specific needs or bed frames.

Japan also has traditional futon mattresses, which are often used on tatami mats and have different sizing standards. The size of a futon mattress typically corresponds to the size of the tatami mats being used.