Weight, or newtons of force due to gravity, is calculated using the formula: W=mg. Here, W is the weight in newtons (N), m is mass (in kilograms) and g is the acceleration due to gravity (which, on Earth, is always 9.8 m/s2). Solving for mass, then, requires that you divide weight (1,200 N) by gravity (9.8 m/s2). This gives you a mass of about 122 kg.