How Bike Sizes Work & How to Find the Right Size for a Kid's Bike

by Jacob Rheuban July 05, 2017

How Bike Sizes Work & How to Find the Right Size for a Kid's Bike

One of the most common questions for parents searching for a bike for the kids is "how do I make sure I get the right size bike for my kid?"

At Prevelo, we try to keep bike sizing simple and predictable.  We have a size guide to help you find the right bike.  We are are going to break things down in a bit more detail here.

Navigating Bike Sizing Language

If you are confused about how bikes are sized, it's not your fault.  The bike industry has different sizing conventions for different types of bikes.  Mountain bikes, road bikes and kids bikes are all measured differently.  Mountain bike frame sizes typically indicate the length of the seat tube (the tube that runs vertically down from the seat).  Road bike frame sizes typically indicate the length of the top tube (the tube that connects the seat to the handlebar) in centimeters.  And kids bike frames typically indicate the diameter of the wheel in inches.

how mountain bike size is measured

Mountain Bikes

how road bike size is measured

Road Bikes

how kid bike size is measured

Kids Bikes

Kids Bikes Sizing

So when we talk about kids bike sizes in terms of 14, 16, 20 or 24, those numbers indicate the diameter of the wheels on the bike in inches.  Let's layout some of these bikes next to each other:

14 inch kids bike size

14 Inch Bike

16 inch kids bike size

16 Inch Bike

20 inch kids bike size

20 Inch Bike

 24 inch kids bike size

24 Inch Bike

 

The problem is that wheel size is a pretty vague indication of size.  As evidence of this, check out sizing guides for adult bikes.  Many adult bike manufacturers will size a bike for an adult that is less than 5 feet tall and an adult that's over 6 feet tall using the same wheel size, but changing the frame geometry.  So, while kids bike sizes are indicated by wheel size, wheel size alone is not a reliable indication of whether a bike will fit your young rider.

The solution here is to use a size chart.  Most reputable kids bike brands provide size charts.  But before we get to the size chart, let's talk about which factors about the rider are important.

Fit Factor #1:  Rider Inseam

At Prevelo, we think the most important measurement is inseam.  If the bike is too big for the riders inseam, then the rider will have difficulty getting their feet on the ground - a fit which we don't recommend for many young riders.  If the bike is too small for the rider's inseam, then the rider will be cramped, uncomfortable and inefficient.  So this is really a critical measurement for children.

There are indeed other measurements that are significant.  For example, the rider's torso length and arm length impact reach.  However, if the bike fits the rider's inseam, reach is seldom outside of an acceptable range.  Further, a child can ride a bike with suboptimal reach, but if height is not within an acceptable range, the bike is likely to be unrideable.

Fit Factor #2:  Rider Experience

The second element we look at is rider experience.  There's a progression that occurs with children on bikes.  

  • When children are first learning to ride they are most confident and safest if they can get their feet flat on the ground while seated in the saddle.  We call this the Learner Fit.  
  • Once a child is comfortable riding, they can be fitted so they are on their tip toes while seated in the saddle.  We call this the Pedaler Fit.
kids bicycle fit learner

Learner Fit

kids bicycle fit pedaler

Pedaler Fit

 

Properly Sizing a Kids Bike

Now that you have both rider inseam and experience handy you are equipped to size the bike.  Many kid focused bike manufacturers will have a guide to help you find the correct size.  Here is the Prevelo Size Guide:

Prevelo Learner Size Guide

Model Wheel
Size
Approx. Rider Age
(years)
Learner Inseam
(inches)
Alpha Zero 12" 1½ + 9.5 to 13.5
Alpha One 14" 2½ to 4½ 12.5 to 16
Alpha Two 16" 3½ to 6 16 to 18 with short seat post
(21.5 to 24 with long seat post)
Alpha Three 20" 5 to 9 18 to 23
Alpha Four 24" 7 to 11 24 to 29
Zulu Three 20" 5½ to 9 21 to 26
Zulu Four 24" 8 to 11 24 to 29

Prevelo Pedaler Size Guide

Model Wheel
Size
Approx. Rider Age
(years)
Pedaler Inseam
(inches)
Alpha Zero 12" 1½ + n/a
Alpha One 14" 2½ to 4½ 12.5 to 16
Alpha Two 16" 3½ to 6 15 to 17.5 with short seat post
(20.5 to 23.5 with long seat post)
Alpha Three 20" 5 to 9 17.5 to 22.5
Alpha Four 24" 7 to 11 23 to 28
Zulu Three 20" 5½ to 9 20 to 25
Zulu Four 24" 8 to 11 23 to 29

Talk with the Pros

If you have any doubt, make a quick phone call or email.  Even if you choose to buy elsewhere, most reputable children's bike sellers are more than happy to help you find the right size.  And since getting the right size helps minimize returns, most sellers have gotten pretty good at it!

Jacob Rheuban
Jacob Rheuban


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