Oxygen Generation Calculator

Human oxygen consumption isn't constant — it rises with physical exertion, which is why this calculator scales the per-crew-member daily O2 rate with activity level: about 0.84 kg/day at rest, 1.0 kg/day for normal activity, and 1.4 kg/day for heavy work. Spacewalks (EVA) add a separate, substantial demand on top of that baseline — roughly 0.6 kg of O2 per hour to cover suit leakage and the elevated metabolic rate of working in a pressurized suit — averaged here across the week and added to daily crew consumption. To resupply that oxygen without resupply missions, the calculator models water electrolysis, splitting water into hydrogen and oxygen (2H2O → 2H2 + O2).

Because water's molecular mass exceeds oxygen's, generating 1 kg of O2 actually consumes about 1.125 kg of water (the 18:16 mass ratio used here), which is why closing the water loop elsewhere in the life support system directly reduces the water burden of oxygen generation. Electrolysis isn't free: at roughly 3.6 kWh of electrical power per kilogram of O2 produced (typical of proton exchange membrane electrolyzers), oxygen generation is a meaningful continuous power draw, not an afterthought. The calculator also sizes a 7-day emergency O2 reserve as a standard safety margin and reports 21.3 kPa as the target oxygen partial pressure, the ISS design standard for a shirtsleeve, Earth-like breathing environment — closely matching the 21.2 kPa nominal "no impairment" value NASA's Life Support Baseline Values and Assumptions Document lists for crew oxygen partial pressure, itself keyed to the Earth-normal atmosphere the ISS approximates — note this last figure is a fixed reference value, not something the calculator derives from your other inputs.