What Size Bike Do I Need?

Knowing what size bike you should get is key to getting yourself a comfortable and rewarding ride. And you have to know the measurements in order to get the perfect bike. If you get a bike that’s too short or too long from wheel to wheel, you may end up with a posture you didn’t want to be put through. Things like lower clearance between you and your bike can cause considerable discomfort and even injuries during mounting and dismounting. That just goes to say, whenever you’re buying a bike, there are a few measurements that can drastically impact how the bike will behave.

So let’s look at the factors that decide what size bike will fit you best:

Your own measurements

No surprise here. Your own measurements are the key determinants in deciding what size bike you should get.

What type of bike are you going for?

The anticipated needs that you could have from your future bike is also a primary factor that you need to consider before deciding sizes just based on your own measurements. This is because different types of bikes (like road or mountain bike), for the same person, will differ in basic bike measurements like seat-tube length, stack, and reach.

Specific needs of your daily ride

Even within the same category of bikes, the ideal bike for you will still on what your needs are from the bike. For example, deciding what length racing bike you should get will depend on exactly how aggressive you want to be, what posture you’re comfortable with, what terrain you’ll be facing and your personal tastes in regards to how a bike should be on and off the road.

Measurements: Which ones, and how to take them

There are two domains of measurements to note when you’re looking for a bike. The first one (as already mentioned) is your own body and the second, quite obviously will be the bike’s.

Your body’s measurements

Inner leg length

This is the length between your crotch and the floor when you stand. It helps in seeing what height bike would be ideal for you in mounting, dismounting, and pedaling. This measurement corresponds with the bike’s frame size/ seat-tube measurement as well as the stand-over-height measurement to let you know which frame size is built best for you.

You can measure it by tucking something that can make a mark on a plane surface between your legs while you’re standing against a wall. Once the mark is made, you can measure the length between the mark and the floor.

Note: If you already know your inseam length, don’t be sure it’s the same thing as the length being measured here. Your inseam length may not exactly be the same as your inner leg length. The way some people measure inseam, it may extend between your crotch and your ankle or a little further than the ankle, but not to the ground. The inner leg length is the distance to the floor and in fact, it’ll be measured more accurately if you’re wearing footwear that you’ll usually be wearing when you bike. This will make the difference between ‘inseam’ length and ‘inner leg’ length more significant, especially if you plan to wear extra comfortable, plush soled shoes to push against the hard paddle.

Just so you don’t have to go wandering around trying to find a table to relate inside leg with frame size, here a few tables to help you decide which frame should suit you best for different types of bikes.

Measurements for mountain bikes

(Mountain Bikes Size Measurement Chart)

Measurements for road bikes

(Road Bikes Size Measurement Chart)

Measurement for hybrid bikes

(Hybrid Bikes Size Measurement Chart)

Check our detailed Article on What are Hybrid Bikes

Buying Online and Bike models for women

Something to keep in mind for people looking to buy bikes online or shopping for female bike models is that the above tables are supposed to have been based on measurements (other than inner leg) that aren’t supposed to be different in men and women of the same height and inside leg length.

The things you should keep in consideration are the stack and the reach.

Since women’s shoulders tend to be narrower, the handlebar is conveniently placed a little more in reach i.e. the stem of the handlebar is made a little shorter while also making the overall width of the handlebar narrower.

You should also keep an eye on the reach, whether you’re male or female, if you have particularly short arms, choosing bikes with shorter reach should help you have a more comfortable posture.

The other measurement to consider in deciding reach is your ape index. Now, that may sound like an insult to some but it’s relevant so:

Ape index

This is your fully extended arm span or wingspan minus your height. Mathematically it is:

Ape index= Wingspan – Height

This is relevant to the bike’s ‘reach’ which is explained further down.

If your ape index is positive, you should naturally fit better on a bike with longer reach while if your ape index is negative, a bike with a smaller reach will be closer to what your ideal bike should be.

Bike’s measurements:

Frame size/ Seat-tube length

This is the length between the bottom bracket and the center of the joint where the seat-tube meets the top tube. The standards of this measurement can vary and so it can be the length between the bottom bracket and the top of the joint or the bottom of the joint but usually with trim bikes, the difference shouldn’t be humungous. If you’re one for numbers though, you can check out the manufacturer’s website (most, if not all mainstream bike manufacturers have websites) for how they measure the frame size. Although usually, manufacturers don’t measure the exact length of the frame size but classify sizes as anywhere between XS and XL.

Stack and reach

 ‘Stack’ is the vertical distance between the bottom bracket up to where the head tube meets the handlebar. It’s a measure of how much you’ll be leaning down onto the handlebar.

The reach is the horizontal distance between these two points i.e. center of the bottom bracket and the center of the head tube. This too is a measure of how recumbent your posture is going to tend to be on the bike.

For reference, mountain bikes have long stacks and short reaches for upright posture while race bikes have short stacks and long reaches.

Stand over height

This is the vertical distance between the floor and the top-tube. The top tube is directly in front of and beneath the bike’s saddle. It’s mostly erect in modern bikes although utility bikes, comfort bikes, and size S (Small) or XS (Extra Small) models may still have the bent top tube design for easier mounting and dismounting, especially in female bike models.

Wheelbase

This is the horizontal distance between the bike’s front and rear wheel hubs. It helps in guessing how nimble or how sturdy, how grounded, or how lively your bike’s handling and ride quality is going to be. The length should be equal to the sum of the chainstay’s length and the front-center.

1. Chainstay: The chainstay is the connection between the bottom bracket and the rear wheel hub.

This provides a measure of how quickly the bike can cut through corners or how easy it is to navigate. Shorter chainstays help you navigate through tight and technical terrains better while longer chainstays should provide a more stable and grounded feel at higher speeds.

2. Front-center: This is the front wheel equivalent of the chainstay. It’s the horizontal distance between the bottom bracket (or the bolt on the bottom bracket) and the front wheel hub.

This should help in guessing how quickly the bike will turn or how nimble it will be and how the handling will react to lighter, less intense commands. It’s also worth measuring if you’re getting a bike with a shorter reach because a shorter reach is generally supposed to mean shorter front-center distance and if this becomes too short, it can become detrimental to ride quality as it may ultimately interfere with the pedaling motion or make it much more likely for you to fly over the handlebars if you have to stop quickly on a rocky descent ( generally a mountain bike scenario).

Bottom bracket drop

Another factor that tells you about the stability of the bike is the bottom bracket drop. It’s the vertical distance between the axle or hub of the front wheel and the bottom bracket. Think of it this way, the lower the bottom bracket is, the lower the bike’s center of mass will be.

A bike whose bottom bracket is higher than the front axle will be dramatically more responsive, lively and nimble as compared to a bike that has a higher front wheel axle than the bottom bracket.

An example of how nimble the bike can get due to this factor can be seen in BMX bikes. Since their bottom brackets are extremely close or just a little lower than the level of the front wheel hub, their handling and nimbleness goes uphill dramatically.

Longer bikes usually have lower bottom brackets drops and as mentioned multiple times, longer bikes have a more grounded feel so that’s a manifestation of how bottom bracket height contributes to stability and handling.

A couple of other geometric terms

These measurements might be a little less obvious in their impacts to the riders that are just starting out but their impacts are significant and obvious for people that are serious about their bikes and understand their requirements.

Heat tube angle

This is the angle between the head tube’s axis or the steering axis and the ground. Geometry tells us that lower or slacker head angles result in longer wheelbases or to be specific, longer front-center distances. That in turn should lower the bottom bracket and as was just mentioned above, that grounds your bike better.

So if you’re looking for a bike that’s more grounded, more stable at high speeds then a slacker head tube angle is probably the direction you should be looking towards.

Seat tube angle

This is the angle between the seat tube’s axis and the ground.

Slacker seat-tube angles provide more recumbent postures but are a hindrance in climbing so the more steep the seat-tube angle is, the better the bike should perform on steep climbs.