How to measure the bike?

Fit should be the main consideration when buying a new bike. After all, even the most expensive bike won't seem that great if it's too big or too small. Make sure you get a bike with a frame that fits you properly for maximum comfort when going on bike rides.

Sit on the bike

When you are sitting on the bike, and the pedal is at the lowest position, is your leg completely extended? You should be able to touch the ground with the tips of your toes when you're sitting on the bike. An extended leg will give full power when riding the bike. You're more efficient (why waste energy?) and protected from strain on your knees. If you pedal the bike and your knees are parallel to the ground at the highest point, this bike is too small. If you can't touch the floor when you're sitting on the seat, the bike is too high. Keep in mind that most bikes allow for an adjustable seat post, and many models, such as folding bikes, do not have frame sizes. You can simply move the seat to achieve the best position. 

How to measure the bike

Instructions

1. Measure your inseam. This is best done barefoot. Stand against the wall and put a book between your legs so it pressed right up against your pelvic bone. Make a mark with a pencil on the wall along the top of the book. Measure the distance from the floor to the mark in centimeters.
   2
   Use your inseam measurement to get a rough idea of your road bike size. Multiply your inseam by .65. This will give you a good estimate of your road bike size for bikes measured center to center. Thus if your inseam is 86 cm, you will fit a 56 cm road bike (86 x .65 = 55.9). Note that many road bikes are measured center to top. To determine how to fit these bikes, multiply your inseam by .67.
   3
   Subtract an additional 10 cm and convert this measurement to inches to get your mountain bike frame size, roughly. Thus, if you fit a 56 cm road bike (c-c), you will fit a 46 cm -- or an 18 inch -- mountain bike.
   4
   Consider top tube length. In many ways, this is the most important aspect of sizing a bike. Two 18-inch mountain bikes may have different length top tubes. Or a 54 cm and an 56 cm road bike may have the same length top tube. Given the same top tube length, the bigger bike may be more comfortable because it will allow you to get the bars up a bit higher.
   5
   Know that women have longer legs and shorter top tubes than men. Unfortunately, most bikes are designed for men, and women often have a hard time finding a bike that will fit them well. Luckily, some manufacturers have begun to make women-specific models.
   6
   Test ride some bikes once you have gotten a rough idea of what will fit you. This will help you determine what is most comfortable for you.