We present a method to determine precisely the absolute momentum of the external proton beam from the Jülich Cooler Synchrotron COSY near 1930 MeV /c. In the pp → d π + reaction at 1930.477 MeV /c incident beam momentum, the forward going pions ( θ c.m. =0° ) and the backward going deuterons ( θ c.m. =180° ) have the same laboratory momentum. Such coincident pion–deuteron events are detected in the focal plane of the magnetic spectrometer BIG KARL located at θ lab =0° . Using the nearly linear dependence of the difference between the measured pion and deuteron momenta as a function of the proton beam momentum, the absolute momentum of the external proton beam from COSY near 1930 MeV /c was determined with a precision of 5.2×10 −5 .