Measure your inside leg:

The step length is the length of the inside of the leg. The best and most accurate measure of your stride length is by using a spirit level and a tape measure. If you have no spirit level to hand a book or similar works as well. Now, position yourself barefoot (!) on a flat surface and clamp the level (or the book) as level as possible between the legs and pull it upwards until it becomes gradually unpleasant. Now measure the distance from the top of the level to the ground. The reading corresponds to your stride length.

Determine the right frame size:

For a Road bike

Multiply your inside leg by a factor of 0.665. The value that you get is the theoretical frame height in cm. For a Triathlon bike subduct 3cm of your value. For a Mountain bike: Multiply your inside leg by a factor of 0.226. The value that you get is the theoretical frame height in inches. Multiplied by 2.54, you get the frame height in cm.

For trekking bikes

Multiply your inside leg by a factor of 0.66. The value that you get is the theoretical frame height in centimeters. Multiply by 0.259 to get the frame height in inches. With suspension seat posts deduct about 5 cm (or 2 inches) from the theoretical size.

If your value is between two offered heights, apply the following rule of thumb: Choose the smaller frame size for sporty driving style. A larger frame results in a more comfortable seating position

Determine the framesize by your body size:

Alternatively you can determine your framesize also about your body size. However, the method of the step length is more accurate.