No weight lifted on any machine is the same as using free weights. In all cases, less effort is required to stabilize the weight, making the lift easier. In some cases, the Smith machine itself has a counterweight that reduces the weight of the bar, and often there is no way to know by how much. The bar path itself is different since you do not move in a straight line, despite how it may appear. The Smith machine may allow you to use more weight, but you need to determine if the end result is worth it.
What the Smith Machine Does
The Smith machine is a machine that forces the bar into a vertical path so you do not have to control it. Hooks hold the bar in place and you must slightly elevate then rotate the bar to release it. If you get stuck, you need to quickly rotate the bar back in position, which may not always be feasible, so a spotter is advised for all exercises. There is no built-in spotting feature, nor are there bars that you can set to catch the weight if you need to dump it.
Squatting on the Machine
One of the most common exercises performed on the Smith machine is the squat. Many people start out using the Smith machine because they lack the balance to perform a real squat, and others because they mistakenly feel that it isolates a certain part of their legs. Unfortunately, neither is ideal. The barbell squat recruits far more of the musculature of your legs than any stance in the Smith machine. This is particularly important when squatting, as activation of the hamstring protects the patella tendon during flexion of the quadriceps. When the hamstring is not as actively recruited, such as when using the Smith machine, you are at greater risk of knee injury as your knee joint itself is less stable.
Even though you may be able to move more weight, the mechanics of the lift itself are different. The differences in the mechanical nature of the two lifts is one of the reasons that there is no consistent comparison between the two lifts. This is particularly true of the load on your lower back. The fixed position of the barbell requires you to either push your knees forward while your hamstrings are deactivated, risking your knees, or lean forward increasing the shearing force on your lower back. An increase in shearing force on your spine, particularly the L4-L5 vertebral joint, puts you at extreme risk of injury.
And About the Bench Press ...
While some people can often lift more on the Smith machine when benching, there are other reasons that this does not carry over to your free-weight bench press. When benching, some of the small muscles around the shoulder joint, such as the rotator cuff, are required to stabilize the bar. Since the Smith machine does this for you, your internal rotators do not develop properly and you will have trouble stabilizing the bar when moving to free weights. This is setting yourself up for injury. So instead of trying to figure out how to convert Smith machine weights to free weights, lift free weights and you will never have to wonder. Smith machines vary from machine to machine, but gravity is constant.