Maybe you remember back when you outgrew your children's bike: at some point, riding your bike is simply no longer pleasant. Your legs are too long, the handlebars too low - you need a new bike! Apart from the biggest difficulty of what colour the new bike should be, there was certainly also the question which height is the right one. Even as an adult, these two questions arise - or at least the second one. Depending on the type of bike, you need a different frame height!
Translated with www.DeepL.com/Translator (free version)

1. determine inner leg length

The frame height can be determined from your inner leg length. There is a simple trick to measure it. You need a wall, a book and a tape measure. If someone helps you to measure, it makes it easier for you.
To get an accurate measurement, take off your shoes and pants. Now stand with your back against the wall and slide the book between your legs up to your crotch. Make sure that one edge of the book is against the wall and parallel to the floor. Now you (or your helper) measure from the floor to the top of the book - this is your inner leg length. It's best to write it down.

2. choose the type of bicycle

If you have already decided on a bicycle type, you can look for it in a specialized shop or on the bicycle market. If you are still undecided, look around and test, test, test. With your measured stride length, you can get professional advice on frame height, or take matters into your own hands and determine the height with various online calculators or tables.
For example, if you have a stride length of 60cm, your city bike should have a frame height of 40cm. For mountain bikes, the frame height is 36cm. If your calculated frame height lies between two values, you can use this rule of thumb as a guide: if you are a sporty and agile rider, choose the smaller height. If you prefer to ride quietly and relaxed, it may be the larger number.

3. the finetuning

All values are determined, the bike selected, now it's time for the fine tuning. The saddle must be adjusted to fit. Make sure that your toes still touch the ground and that your legs are not stretched too far when riding. If you can comfortably reach the pedals with your heel when it is at the lowest point, the saddle is properly adjusted. Also, the saddle should be completely level to distribute your weight evenly.
The higher you set the bike handlebars, the more upright you sit when riding. On mountain or trekking bikes, the handlebars and saddle should be on the same level. The handlebars on racing bikes are even below the saddle so that the rider can adopt an aerodynamic posture. You can see this very well in the Tour de France. On city and dutch bikes, the handlebars are set high for relaxed and upright riding.
You can do the fine tuning yourself. However, if you are unsure, it is better to get professional help. The adjustment will cost a little, but you can be sure that your new bike is adjusted to you and that you are safe and comfortable on the road. Don't forget the right bicycle helmet!