Lay a long level downhill on the floor with one end of the level on the level part of the floor and the other end at the lowest point.
How to level old hardwood floors.
Hardwood floor installers often use the term level when referring to subfloors when actually they are meaning the term flat while it s desirable to have a level floor most homes.
Remove the existing flooring.
You will have to make sure your joists can handle the addition of considerably more weight from the plywood subfloor and any floor coverings.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
Raise the lower end of the level until the bubble is centered then measure the distance from the bottom of the level to the floor to find the depth of the dip.
On the top side of the floor another fix it idea to lay down new hardwood over the existing floor.
The process of fixing both an out of level and an out of plane floor is simply known as leveling a floor.
Use a pry bar to pull up hardwood floors one section at a time or unfasten carpet or laminate and roll it up from one end.