This International Standard gives requirements for estimating the mass of the remaining usable propellant of an unmanned spacecraft in low Earth orbit (LEO) or geostationary Earth orbit (GEO), and for designing propellant measurement systems. It is applicable to spacecraft with either mono- or bi-propellant propulsion systems using liquid or gaseous chemical propellants, and is limited to such systems because they are the most common for spacecraft in LEOs and GEOs.