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You’ve been planning for a while to buy your child their first “big kid” bike, but there’s a problem: you don’t know what size of bike would be best for your child.

With all the information available, we don’t blame you! 

The way we used to recommend bicycle sizes (based on age) is a little bit outdated. 

Bike sizes used to be recommended based on how old your child is, and this can be useful to get a ballpark estimate of what size bike to buy. 

However, using age alone might not be the best way to find the correct bike size for your child if you want them to have the best bicycling experience. 

Think of it this way: did you ever notice that your child was not the right size for their baby clothes based on age? 

It’s not uncommon for an 18 month old baby to wear clothes 2T or above, or for a 3 year old to still fit into their 24m clothes. This sizing system is based on the average size of children for that age, and as you may have already guessed, not every child is going to fit the average for their age group!

For this reason, we have started using a more accurate way to match your child to the perfect fitting bike, no matter what their age recommendation or what other sizing guide suggests: inseam length.

The reason we recommend you use your child’s inseam length as a reference is that your child’s bike size should be based on their leg length at total extension.

This ensures that your child is pedaling comfortably and riding their bike in an efficient manner. 

1. Measure the Inseam

Now that we know why you should be using inseam length for your child’s bike, let’s get into the how-to portion of this guide.

  1. Have your child stand up straight against a wall with their shoes on and their feet slightly spread apart 
  2. Place a hardcover book between their legs and slide it up towards the crotch of their pants. This simulates your child sitting on a bike seat. 
  3. Measure the distance from the top of the book down to the floor
  4. This will be the inseam length that your child will use for their bicycle

2. Choose the Wheel Size According to the Inseam

Now that you have their inseam, it is time to choose the wheel size of the bike for your child. If your child’s inseam lies in 2 different size areas, we recommend that you choose the larger wheel size so your child can grow into the bike and use it for a longer period of time before they need the next size up.


Wheel Size Approx Age Seat Height / Approx Inseam Length
Belsize 12" 18-36 months 14-19"
Belsize 16” 3-7 years 18-22"
Belsize 20” 7-10 years

3. Adjust the Seat Height For Best Fit

Now that you have figured out the size of bike to ride, it is time to adjust the seat height. To do this, 

  1. Begin with the bike seat in the lowest position. 
  2. Have your child stand straddling the bike with 1 leg on each side as they stand over the seat. 
  3. Slide the seat up under their bottom and lock the seat into place using the seat clip. 
    1. If this is your child’s first pedal bike, their feet should be flat on the ground when they are seated on the bike. 
    2. As your child gains more experience and gets more comfortable on the bike, they should be on their tippy-toes when they are seated on the bike. This will allow greater leg extension for faster, more efficient pedaling over longer distances. 
  4. Your child should be ready to start riding their Belsize kids bike!

If you have any questions about what size bike is right for your child, please send an email to and a member of our team will be happy to help you pick out the right size for your child!