Your design pressure is the amount of pressure put on your windows during a storm. The pressure is caused by wind blowing on and around your house. As required by the Florida Building Code every new window installed must meet a minimum design load. Usually an engineer does this for you, but local building departments often have "cheat sheets" like the one below that they will allow you to use instead of an engineer. You can get the "Tributary Area SqFt" by the opening size, for example a 5 foot wide by 4 foot tall opening is 20 SqFt which is what you use as the tributary area; then simply determine if your window is located in a zone 4 or 5 and you are done.
