The British Aerospace 125 has two crew members with eight passengers in a normal layout and fourteen passengers in a high-density configuration. The cabin height is 1.75 meters, the cabin length is 6.5 meters, and the cabin width is 1.8 meters. It has an external length of 15.39 meters, an external height of 2.9 meters, and a fuselage width of 1.85 meters. It is a low-winged monoplane with a wingspan of 14.33 meters and a wing area of 32.8 square meters. The tail height is 5.26 meters and the wheelbase is 6.4 meters.

The aircraft has an empty weight of 5,684 kg, a maximum zero-fuel weight of 7,050 kg, and a maximum payload of 1,700 kg. It has a maximum takeoff and landing weight of 11,340 kg and 10,000 kg, respectively. The fuel tank capacity is 1,418 US gal. The 125 is powered by two Rolls-Royce Viper 601-22 turbojet engines. It has a seven-stage axial compressor, annular combustors with twenty-four burners, a single-stage turbine, and a scavenge oil system. Each engine produces a 3,750 lbf of thrust.

The British Aerospace 125 has a maximum speed of 320 knots. The maximum cruise speed at 28,000 feet is 454 knots and the economic cruise speed at 39,000 feet is 403 knots. It has a stall speed of 83 knots with flaps down, a never exceed speed of 475 knots, and a maximum rough airspeed of 230 knots.

It has a range of 1,650 nautical miles with 454 kg of payload and 45 minutes reserve plus allowances for takeoff and landing and 1,560 nautical miles with maximum fuel and maximum payload. It can fly up to 41,000 feet and can climb at a rate of 4,900 feet per minute. The takeoff run and takeoff balanced field length are 1,341 meters and 1,631 meters, respectively, while the landing run is 1,036 meters, and distance from 15 meters is 649 meters at typical landing weight.