Write down the mass of the hanging load. This will either be provided to you in the problem statement, or it would have been previously determined from prior calculations. The mass should be in units of kilograms. If given in grams, divide the mass by 1,000 to obtain a unit of kilograms.
Ignore the length of the bar or the string from which the load is hanging. These are irrelevant to calculating the weight of the load. A difference in bar length will only change the distribution of the force along the bar, not the total force itself.
Multiply the mass in step 1 by gravitational acceleration, g. Gravitational acceleration is a constant, which means that it never changes from problem to problem. Gravitational acceleration equals 9.81 meters per second squared, or 9.1 m/s^2.
Verify that your units are correct. No units will cancel out in the multiplication procedure. The resulting value will be your force, or weight, in kilogram meters per second squared (kg-m/s^2), which is also known as a "Newton."