METHOD 1: THE 109% METHOD
BikeRadar says that the 109% Method was developed in 1967, after some experiments done by Hamley and Thomas. It was concluded that the ideal seat position was a distance of 109% of the length of your inseam from the top of the seat to the lower paddle axle. Of course, this needs some further explanation.
Let’s start with your inseam length. To find this, stand up facing a wall and put a thick book between your legs. Pretend it’s a mountain bike seat. Make sure your heels are firmly on the floor and your legs straight. Then, draw a line where the top of the book touches the wall. The distance from that line to the floor is the length of your inseam.
Now, your ideal mountain bike seat height is 109% of that length. Measure that distance from the top of your seat to the lower pedal axle. Make sure that the crank aligns with the seat tube and your seat post when doing this.