From a 2007 Boeing seminar:
Landing Distance and Runway Length Required
The landing distance is defined by FAR/JAR 25.125 to be the horizontal distance to come to a complete stop from a point 50 ft above the landing surface, assuming
• Airspeed at the runway threshold is VREF.
• A level, smooth, dry, and hard surfaced runway. • Airport altitude and standard day temperature. • 50% of the headwind or 150% of the tailwind. • Maximum manual braking.
• No reverse thrust.