You didn't specified what you mean by "need". Need physically or need as defined by safety regulations and used in real life? Length of necessary runway by regulation, depends above all from landing speed, which by regulations for safety reasons is predetermined to be specific percentage value above stall speed, which depends on total weight of the plane, which in turn depends on weight of fuel, passengers and cargo plane is carrying (where speed varies considerably between minimum and maximum), from temperature and pressure of air (which defines its density), as well from flaps setting and runway condition as wet, iced etc. In real life pilots enter so called ZFW value (Zero Fuel Wight) provided on special document called "Load Sheet" provided by ground services responsible for loading and balancing the plane before take off, into FMC (Flight Management Computer) - on board computer in cockpit, specify airport temperature and pressure, runway surface condition (dry, wet, iced), as well desired flap settings which allows FMC to calculate landing speed (V AT). As FMC tracks fuel consumption and remaining fuel during flight, it knows and can predict precisely in the future total weight of the plane, exactly at the time of landing. In theory FMC also knows what is safe minimum length of the runway and will alert pilot if runway with insufficient length is chosen for landing, still, believe it or not, for safety reasons, in order to create redundancy, pilots use paper tables and graphs, than are required to enter calculated manually values into flight documentation. Additionally, different air carriers use different safety margins. Halo, as the ONLY one, summarized (to all likelihood with only one unintended purely accidental typo), all most important variables. Wiki reference provided by Jono on other hand are completely unusable, as they only give runway lengths at MTOW (minimum take off weight). Safe runway lengths for take off and landings represent completely two different values.