Start with bar and work your way up. I have no idea if you are a female, male, old, young, fat, skinny. In some cases, the bar might be too heavy, but my point is start light.
As Pemdas said there is no formula. One good way to check your strength would be a push-up test. A push up is considered a bench press with 60% of your body weight. See how many you can do and make your choice base on that.