What Mountain Bike Seat Tube Length Do I Need?
Bike size is not just a matter of comfort. It plays an important role in preventing injuries. Many riders have gotten used to riding their bicycle without considering the size, but if you ride regularly, it should be a priority. Choosing your bicycle size depends on many factors such as your height and the type of bicycle you want to ride. In this article, we explain everything you need to know about how to determine the right mountain bike seat tube length for you.
To determine the optimal mountain bike seat tube length, check the manufacturer's sizing chart, use a mountain bike seat height calculator, or measure the distance from the ground to the rider’s crotch. For example, a bicyclist with a 28 inch inseam should use a 19 inch long seat tube.
Manufacturers use an inch measurement to describe bike size. This is the same measurement as the seat tube length, meaning that the seat tube length for your height corresponds to the overall size of the bike you need. There are multiple ways you can determine the required seat tube length and bike size for you.
If you are looking for the perfect bike but you’re not sure where to start, our guide on what mountain bike you should buy will help.
What Is Seat Tube Length on a Bike?
Seat tube length is a measurement of the straight line between the top of the seat tube and the center of the bottom bracket, excluding the seat post. This length is commonly used by manufacturers to represent the size of the bike frame in centimeters or inches. A manufacturer may, for example, measure the seat tube length of a bike at 51cm and categorize it as a large frame.
Seat tube length is the distance between the seat and the bottom bracket of a mountain bike, excluding the seat post. Seat tube lengths typically come in 15, 17, 19, and 21-inch configurations.
To learn more about bike sizes, read our mountain bike sizing chart guide, which can help you choose the right bike for your height.
How to Determine Seat Tube Length for Height on an MTB
It is very important that you choose a bike with an appropriate tube length for your height. There are various ways that you can use to calculate this length.
To determine mountain bike seat tube length, check the manufacturer's sizing chart, use a mountain bike seat height calculator, or measure the distance from the ground to the rider’s crotch.
To calculate the correct mountain bike seat tube length, you could choose one of the following options:
Option 1. Check the Manufacturer’s Sizing Chart
One of the easiest ways to determine the seat tube length is to check the manufacturer's sizing chart. The chart below shows riders' heights and bike sizes suitable for them. Do not forget that the chart varies for different types of bikes, so the size chart for a mountain bike is not the same as for a road bike.
When planning to buy a new mountain bike, you should check the manufacturer's sizing chart to avoid injuries, discomfort, and other problems that can result from purchasing the wrong size bike. Our mountain bike reach calculator guide with tips & tricks to alter reach explains the important bike measurements you should consider when purchasing a mountain bike.
Option 2. Use a Mountain Bike Seat Height Calculator
Another easy way to determine seat tube length is to use a mountain bike seat height calculator. This online calculator is a free and easy-to-use option. You simply enter your gender, height, and leg length in inches and the site calculates the bike frame size for you in inches and centimeters. The result shows both the length of the seat tube and the bike size suitable for your body measurements.
Option 3. Measure the Distance From the Ground to Your Crotch
Measuring the distance from the ground to your crotch is another way to determine the suitable seat tube length and bike size for your height. First, stand up straight with your bare feet separated by about 6-8 inches. Then, use a tape measure to measure the distance from the ground to your crotch, also known as your inseam measurement.
Multiply the inseam length you measured by 0.66. Keep in mind that this calculation is for mountain bikes. If you are buying a road bike, you need to multiply your inseam length by 0.7. For example, if your inseam measures 70 centimeters, multiply that by 0.66 and you will see that the suitable mountain bike seat tube length for you is 46cm.
Selecting the correct size bike for your body can reduce non-traumatic injuries to the knees, neck, shoulders, hands, buttocks, and perineum, which, according to this article from the Journal of Sports Medicine, make up as much as 85% of all bicycle-related injuries. Sizing your bike to your unique body shape and size not only improves your comfort but also minimizes the chance of more serious accidents because you are able to better control your bike.
You may find additional useful information in learning whether or not mountain bike saddles are universal. Be sure to read our related article to find compatibility tips and tricks.
How to Measure Actual Top Tube Length on a Mountain Bike From Center to Center
The top tube is the tube situated between your legs when seated on a bicycle. The actual top tube length is also known as center-to-center top tube length. The method of measuring the actual top tube length has largely been replaced by measuring the effective top tube length; however, it can still be a useful exercise for riders who want to get precise bike measurements.
To measure the actual top tube length on a mountain bike, first place the measuring tape correctly in between the seat tube and head tube, then measure along the top tube.
Step 1. Position the Tape Measure Correctly
First, you should know that the actual top tube length is the measurement of the center of the seat tube to the center point of the head tube. Place your measuring tape against the center of the seat tube and start measuring along the top tube to the center of the head tube.
Step 2. Measure Along the Top Tube to the Center of the Head Tube
Now, check the total distance you get on your measuring tape. This total distance is the top tube’s length. It is important to know that top tube measurements are relatively unimportant for bike fitting. The down tube and reach measurements are crucial for a proper fit.
Consider using the actual top tube measurement method only as an additional data point together with other metrics. If you are comparing frame geometry, it is generally better to use the method that frame manufacturers use. Few manufacturers state actual top tube measurements; most provide the effective top tube length. You may also be interested in learning more about mountain bike saddle rail size. If so, be sure to check out our related article to find out more.
How to Measure Effective Top Tube Length on a Mountain Bike (HT-Top)
Effective top tube length is often referred to as the horizontal or virtual top tube length. On a mountain bike, it is the horizontal distance between the centerline of the head tube and that of the seat post.
To measure the effective top tube length on a mountain bike with a slanted top tube, position a spirit level between the space from the top of the head tube to the top of the line that extends from the seat tube. Measure the horizontal distance between these two points for the effective top tube length.
These are the steps to measure the effective top tube length:
Step 1. Use a Spirit Level to Measure the Length Horizontally
If you have a slanted top tube, you will need to use a long spirit level like this to measure the length horizontally. You should keep one end of the level at the intersection of the top tube and the head tube. Make sure you are holding the level flat so that the air bubble is resting at the center of the liquid and see where it intersects with the seat tube line. If your bike has a flat top tube, you can skip this step.
Step 2. Measure the Length
Use a tape measure like this to start measuring the distance along the top tube to the center of the seat tube while keeping the tip of the tape against the center point of the head tube. This will give you the length of the top tube if you are dealing with a flat tube. If your bike has a slanted top tube, measure to the point you marked with the level.
How to Measure the Effective TT Center Top Tube Length on a Mountain Bike
The effective TT center top tube length is the measurement of the virtual distance between the center of the seat tube axis to the intersection of the top tube and the center of the head tube.
To measure the effective TT center top tube length on a mountain bike, place the tape measure between the center of the seat tube and the center of the head tube where the top tube intersects, then measure the distance between the two points.
Step 1. Position the Tape Measure Correctly
Place the tip of the tape measure at the center point of the seat tube axis and run it along the top tube to the point where the top tube intersects with the center of the head tube, making sure to keep the tape measure parallel to the floor. Make sure you also read about these simple tips to choose a mountain bike extended seat post for some additional information too.
Step 2. Measure the Distance Between the Two points
Measure the distance between the center of the seat tube axis to the center of the head tube. The distance you measure between these two points is the effective TT center top tube length on your mountain bike. As this article from the IOSR Journal of Mechanical and Civil Engineering explains, determining the top tube length of a bike frame affects the rider’s posture and riding comfort, especially when riding on rough terrain for long periods. Your posture is very important to ensure your upper body stays comfortable, so we suggest you check out our article that explains where to hold mountain bike grips to maintain proper posture.
What is a Seat Tube Angle?
The seat tube angle is the measurement of the angle between the seat tube and the horizontal plane. It determines saddle placement and, thus the rider's default position on the bike.
The seat tube angle is the measurement of the angle between the seat stay that runs from the head tube to the seat tube and the horizontal plane.
As we explain in our optimal mountain bike seat tube angle guide, a steep seat tube angle means that the seat tube is more forward and a slack seat tube angle means that it is further backward. In general, a slack seat angle will make it harder for you to position your weight over to the front of the bike, this makes it more difficult to pedal especially when climbing. Having a steeper seat angle makes it easier to shift your weight forward and climb. Of course, you can overdo the steepness of the angle, which will limit your movement space and reduce performance.
How to Measure Bike Seat Tube Length
The seat tube length also varies from one bike size to another. Often the bike will have a label at the bottom of the seat tube stating the size of the bike frame.
To measure bike seat tube length measure the distance between the center of the bottom bracket and the top of the seat tube. The distance between the two points is the seat tube length. This measurement is the same as the size of the bike frame.
Tools to Measure Seat Tube Length
| Tool | Function | Example | Price |
| Tape Measure | To measure length | Amazon Basics Self-Locking Tape Measure | ~$15 |
| A caliper | To measure tube width | Electronic Digital Caliper | ~$30 |
| Spirit Level | To adjust the level when measuring top tube length horizontally | 24 inch Magnetic Spirit Level Tool Aluminum | ~$25 |
These are the steps to measure your mountain bike seat tube length:
Step 1. Place the Tape Measure Correctly
Before using your tape measure like this to find the length of the seat tube, consider removing the seat from the top of the tube to get an accurate measurement.
The seat tube length is measured from the center of the pedal crank to the top of the seat tube. Simply place the tape measure at the center of the bottom bracket and run it to the top of the seat tube.
Step 2. Measure The Distance Between The Two Points
The distance between the two points is the seat tube length. Most regular-sized bikes have a seat tube length of around 21–23 inches (53–58 cm). Our article about choosing the correct mountain bike saddle height explains how to adjust the saddle to be at the correct height for your riding style and the type of terrain you will be riding on.
Is Frame Size the Same as Seat Tube Length?
The length of your seat tube is consistent with the bike frame size, and these two measurements can both be used as your bike’s official size.
The size of the bike frame is the same as the seat tube length. Manufacturers use an inch measurement to describe bike frame size, and this inch measurement is the same as the seat tube length. If a mountain bike size is 15", the length of the seat tube is 15-inches.
In the manufacturers’ sizing chart, you can see small, large, and extra large with the equivalent of these sizes in inches, often including the recommended rider's heights for each size.
Signs Your Mountain Bike is too Big
Riding a bicycle that is the wrong size is not only uncomfortable, but it can also increase the risk of having an accident. When the bike is too big for you, controlling your movements will be much harder.
A mountain bike may be too big when the rider is not comfortable while pedaling, struggles in tight turns, or lacks grip on the front wheel.
Read our article about what mountain bike to buy for more information about bike frame types and sizes. These are tell-tale signs that your bike is too big:
Sign 1. If You Struggle With Tight Turns
One sign that your mountain bike is too big is when you find yourself struggling in tight turns. A bike that is too big for the rider will cause excessive stretching and strain to maintain a grip on and control of the handlebars.
Sign 2. If You Lack Grip on the Front Wheel
Another sign that tells you that your bike is too big is when you feel that you are losing grip on the front wheel. This lack of grip is a result of your weight not being appropriately shifted over to the front side of the bike. This might lead to crashes because the front wheel tends to move and skid in turns.
Sign 3. If Your Body Is Not Comfortable
You can also tell that your mountain bike is too big if you are not comfortable while riding. Your neck, shoulders, back, and knees might ache badly. This is a result of you stretching your body to reach the handlebars and keep the bike under control. On long rides, you might also experience discomfort or pain in your arms and wrists.
If your bike is the right size, but you experience hand and wrist discomfort, we suggest that you read through our tips and tricks to choose mountain bike grips for numbness.