Choose the bike size closest to this calculation.
Intended use: Are you going to cycle sportily or recreationally? When in doubt, choose the smaller frame for sporty riding and the larger one for long leisurely rides.
If in doubt, consult a Thompson dealer.
Inseam is the distance from your crotch to the ground.
You now know approximately the suitable frame height (seat tube length) for your new bike. What frame size your bike should be is also influenced by other factors:
Arm length and shoulder width: These measurements also play a role in determining the frame size.
Desired sitting position: Sporty or comfortable? This will also influence your choice.