When buying a mountain bike, choosing the correct frame size is the most important decision. If it is too small for you, it can cause handling problems and be very uncomfortable. In order to avoid this and reduce a chance of injury, you need to pick the one that will fit you perfectly.

So, the question is, how to measure mountain bike frame size?

Well, no need to worry, because it is relatively easy. The method of determining size of it is the measure of the seat tube length. It is crucial to find the right mountain bike for your adventure, so you can be comfortable and satisfied before you buy it.

With these instructions, you will quickly learn how to measure mountain bike frame sizes.

Anatomy of The Bike

It is crucial to know the anatomy of the bike, to understand its parts when measuring them. Mountain bikes come in three different styles: front-suspension, full-suspension, and no suspension. Their frames are made from carbon fiber, aluminum or steel.

  • Bottom bracket – It connects the crankset to the bike’s body.
  • Brake lever – The lever on the handlebar which activates the brakes. Front brake (the left side) and rear brake (the right side).
  • Brake cable – It attaches brake level to its mechanism.
  • Chain – It is a collection of links that attaches back and front gear, allowing you to pedal.
  • Chain ring – Spiky and round bit connected to cranks that pull the chain round.
  • Crank – The lever that connects the bottom bracket spindle to the pedals.
  • Derailleur – The mechanism for moving the chain from one to another cog.
  • Down tube – It connects the head tube to the bottom bracket shell.
  • Front shock – The shock absorber on the front set.
  • Handlebar – The horizontal bar that is connected to the stem, with handgrips on the end.
  • Headset – The mechanism that attaches the front fork to stem and handlebars.
  • Hub – It is the center part of the wheel.
  • Idler pulley – It is a bottom pulley of the rear derailleur, that keeps the chain tight ensuring the spring tension.
  • Nipple – A threaded receptacle that supports the rim to the end of the spoke.
  • Pedal – A platform for the foot to press on, which is connected to the crank.
  • Rim – The metal ring that supports the tire on the outside and the spokes on the inside.
  • Rear shock – it is a shock absorber, on a dual-suspension bike, for a rear tire.
  • Saddle – The seat of the bike.
  • Seatpost – Provides a support for the seat.
  • Skewer – The metal rod that extends through the hub, connecting the dropouts of the frame to the wheel.
  • Spindle –The rotating axle that is a part of the bottom bracket. The crank arms are attached to it.
  • Stem – A bit that connects steering tube to the handlebar.
  • Wheel hub – It is the center of the wheel. The spokes are attached to it.
mountain bike anatomy

Frame Size

Frame size is determined by the length of the seat tube. You will need a tape measure to establish the distance of it. Record your measurements in centimeters (cm) and inches (to the nearest half inch).

Various manufacturers can use different sizing system, but usually, they are measured and sold in 2 inches increments. After measuring the length of the seat tube, from the base to the top, you will have a general frame size that you can compare with the bike you are previewing.

Measure the top tube that connects the head and the seat tube.

Wheel Size

The next thing you want to do is measure the tire size. Generally, bikes have frames which are designed proportionately to the size of the tire. If the tire is larger, the frame will also be.

Final Step

It is best for you to know how bikes sizes are labeled and sold. Mountain bike frames come in different sizes such as small (14 – 16 inches), medium (16 – 18 inches), large (18 – 20 inches), and extra-large (20- 22 inches). If you want to be sure that the bike is the right choice for you, go to the shop and straddle it.

There should be 2-inch of clearance between the top of the tube and your crotch. Your elbows should be slightly bent and shoulders should be relaxed. If you can easily reach the handlebars and the floor with both feet without any difficulty, then you have found the right bike for you.

A proper fit is very important for reaching the breaks and the gears, back and neck comfort and breathing. Don’t forget to protect your head with the right bike helmet.

road bike icon


Measuring the bike frame is very important. If this is done incorrectly, you could end up with an improper fit, which leads to discomfort, pain and poor posture. So, follow this guide, choose your bike wisely and you will avoid these problems.