80億分之一 is a free, browser-based life simulation game. The player is randomly born into a country and family, then makes choices about education, work, relationships, and daily life. The simulation runs from birth to the end of life, based on statistical data from 172 countries, and concludes with a review of the life lived. The game is available in Japanese, English, Traditional Chinese, and Simplified Chinese, and operates on both smartphones and PCs without requiring registration or download. The simulation draws on the UN World Population Prospects 2024 dataset, including annual birth counts of approximately 132 million and population distributions. Two starting modes are available: one determines the country of birth by annual birth rate, and the other by the country's share of the global population. Both begin at age zero, and lifespan is estimated according to the country's average life expectancy. Players can pause or fast-forward the simulation. The game covers topics such as drinking, smoking, romance, and marriage, and is intended for players aged 15 and older. Events involving illness, death, accidents, and violence occur according to statistical likelihood but are not graphically depicted. Completed lives can be preserved in a Life Album, which records every event in sequence. The optional Memory Pass, a one-time purchase, allows players to store life albums on an account and access them from any device. The Book of Life presents a completed life as a readable book that can also be printed. The Memorial is a public space where completed lives are kept, and visitors can light a candle. Shared lives are aggregated into statistics displayed in the Memorial area. The game also includes a daily theme, such as the isolation of the elderly, which appears in all lives born that day, and a feature that compares the generated life with the player's own starting point on the local device.