Standing flat-footed on the ground, you should have a minimum of 2.5cm clearance between yourself and the top tube of the frame
If you are not able to physically measure the size yourself, the simple calculation below can be used to estimate your required frame size.
Take your inside leg measurement to the floor
Subtract 5cm for mountain bikes, or 2.5cm for road bikes
Subtract 23cm, the average distance from the bottom bracket to the ground
Using these rules will give you the maximum frame size that you will need.
Keep in mind, buying a bike of the correct size is not just about the correct height measurement. As bikes get taller height-wise, they also get longer. The distance from the seat to the handlebars increases, on average, by 2.5cm for every 5cm increment in frame size. Therefore, while you may be able to stand astride a taller framed bike, you must also ensure that you can still comfortably reach the handlebars and manage all the controls. Remember that the seats on all bikes are adjustable by between 10cm and 25cm so the bike can be tailored to fit you.
As we are all built to different shapes, finding the right size bike and getting the right position set up on the bike can be quite difficult. The above information should start you off in the right direction, however advice from a professional who can physically see you on the bike and adjust your position is best.
Having the right size bike and the right position is more important if you are doing long rides (30ks plus) and if you are buying a racing bike. Problems created from a poor position on the bike may be amplified the longer a person is on the bike.
As mountain bikes have a more upright, comfortable position, the ideal position is not as critical. This is why three or four adult sizes can cater for the majority of the population. In contrast, as race bikes demand a less comfortable and more efficient position, the perfect size and position is much more important and is the reason why the size of race bikes can vary by centimetres.
Lastly, buy a good brand. Although what brand you buy often comes down to your budget, the better brand bikes generally have increased performance, are safer and are more durable.
It is recommended that someone who is doing long rides or riding race bikes be fitted by a professional. To find a bike shop in your area click here