How long could you survive on a desert island?

The romantic version of this question hides a boring hierarchy: water, shelter, fire, food, in that order, and the order almost never changes. Tropical islands are usually warm enough that shelter is about rain and sun rather than cold, which removes one of the fastest killers. Water is the constraint that ends most real cases, because a small island may have no surface fresh water at all and seawater actively makes dehydration worse. Once water is solved, the timeline stretches to months or years, and the failure modes become infection from untreated wounds, injury with no help available, and nutritional gaps rather than outright starvation. Alexander Selkirk, the real castaway behind Robinson Crusoe, lived alone on Más a Tierra for four years and four months, on an island with fresh water, goats and turnips.

What changes the figure

• Any reliable fresh water changes the answer from days to years
• Rainfall capture from a tarpaulin or large leaves is usually the easiest source
• Wound infection with no antibiotics is a leading cause of death in real castaway accounts

Compiled from documented castaway accounts; outcomes vary enormously with island resources and season.

Physiology, not instruction. Every figure here is a population range under particular conditions, and individual variation is large — temperature, exertion, fitness and health move all of them substantially. Nothing on this page is medical advice.

Places

• How long could you survive in deep space? About fifteen seconds of consciousness, and roughly ninety seconds before the damage becomes irreversible.
• Could you survive on Mars? Unprotected, under two minutes. In a pressurised habitat, indefinitely — the limits become supply and radiation.
• How long can you survive in the death zone on Everest? Hours to a couple of days. Above about 8,000 metres the body deteriorates no matter what you do.
• How long can you survive adrift in the open ocean? In warm water with a raft, three to five days without drinking water — but immersion in cold water is measured in hours.
• How long could you survive on the Moon? About fifteen seconds of consciousness without a suit. With one, the record stay is 75 hours.
• How long could you survive in the Mariana Trench? Unprotected, no measurable time at all — the pressure is over a thousand times atmospheric. In a submersible, as long as the life support lasts.
• How long can you survive in Antarctica? Minutes to an hour unprotected in a winter storm — or indefinitely, since around a thousand people overwinter there every year.
• How long can you survive in the Sahara Desert? Around a day without water in summer daytime heat, and several days if you can stay in shade and move only at night.
• How long could you survive on Venus? Seconds. The surface is 465 °C at ninety-two times Earth's atmospheric pressure — the hardest place in the solar system to land on.
• Could you survive on Jupiter? There is no surface to survive on — you would fall inward until pressure and heat ended it, within minutes.
• How close to the Sun could you survive? Nothing solid survives contact — but a shielded spacecraft has flown within about 6 million kilometres of the surface.
• How long can you survive on the International Space Station? Indefinitely while it is resupplied — it has been continuously crewed since November 2000.
• How long can you survive in the Arctic? Hours unprotected in winter — but roughly four million people live above the Arctic Circle permanently.
• How long can you survive in a cave? With water and warmth, months — people have done it deliberately. Without light or a way out, hypothermia is the clock.