Find the highest point on your floor and make a mark on the wall next to it with a pencil.
Find the lowest point on your floor and make a mark on the wall next to it with a pencil.
Using a measuring tape with two people, have one person hold the tape at the highest point and the other at the lowest. Hold the measuring tape level, using the highest point as the starting point. Pull it taut between that point and the lowest point. Use a level if necessary. Make a mark above the existing mark for the lowest point, which shows where the corresponding high point is at on the other side of the room. Then, measure the vertical difference between the marks for the low and high point and record it in inches. That is the measurement for the rise.
Look at the measuring tape, which is now stretched the length of the distance between the highest and lowest point. Record the distance in inches. That is the run. Divide the number determined in Step 3 (the rise) by the number determined in Step 4 (the run) to calculate the slope of the floor.