Jgayman mentioned wishing he knew the lift capacity using his forks on his tractor. This is a difficult question to answer in one sentence, but I can at least try to offer the variables that affect it. Here are some that come to mind, but there may be more.
**This assumes having adequate rear counterbalance, or ballast.**
Attachment weight - Obviously, the heavier the attachment the less potential it will have to lift other things. If you don't need the backrest (aka headache rack, brush guard, etc), take it off and you gain lift capacity.
Distance of the load center of gravity (CG) away from the loader - Obviously, the farther out, the less you will be able to lift. There is a graph here for a point of reference. Always plan your load placement to keep as much weight in close to the loader as safely possible.
Elevation of the load - If you are starting from the back of a pickup truck, your lift capacity will be less than starting from the ground because loaders lose potential as they go higher up.
Curl Angle - If you can curl your load up (and back) it moves the CG closer to the loader, which increases lift capacity.
Pitch angle - If your tractor is sloping uphill, you will be able to lift more than if you are sloping downhill. This is because the CG of a given load will naturally be closer to the loader on an uphill.
Fork tine length - Longer fork tines have higher weight and a CG farther away from the loader. Take two forks, one 48" long and one 24" long. That extra two feet compounds the loss in lift capacity because the weight of that extra length is multiplied by its distance away so a 48" fork that weighs 40 Lbs more than the 24" one might have a 50 or 60 Lb effect on the lift capacity of the whole system.
Fork tine taper - Full taper fork tines have a CG that is closer to the loader than a Standard taper fork tine. See here for an illustration of the difference.
I hope this helps. If more come to mind, I'll add them. Unless you think of them first....