The most standard capacity for mezzanines is 125 psf. This equates to 1,500 lbs. on a 40" x 48" pallet. This is the bare minimum that you can buy but you need to take into consideration how much weight the mezzanine will be holding and design accordingly. Another thing to consider is point loads. If you are going to be putting machinery with feet especially, this needs to be taken into account during the design process so the mezzanine structure can be designed accordingly.