When buying kids' bike, wheel size is just one starting point to consider as you look for a kids' bike.
When shopping for kids' bikes, remember that they are measured by wheel size, not frame size, unlike adult bikes. Kids' bikes usually range from 12" to 24" wheels, and then bigger kids can switch to adult bikes with 26" wheels or more.
But wheel size is not the only factor to consider when choosing a kids' bike. The chart below is a rough guide, but you should always try the bike in person before buying. Age is not a reliable indicator of bike size, because kids vary in height, torso and leg length. A bike that fits one 7-year-old may be too big for another. Also, different bike brands may have different wheel sizes for the same bike.
Kids' Bike Sizing Chart
Bike Wheel Child Height (in.) Approx. Age Inseam (in.)
12-inch 30-39 2-4 12-17
14-inch 37-44 3-5 16-20
16-inch 39-48 5-8 18-22
20-inch 42-52 6-10 21-25
24-inch 50-58 8-12 24-28
26-inch 56+ 10+
The most important thing is to buy a bike that fits well now, not one that is too big for your child to grow into. A bike that is too big will be hard to control, unsafe and less fun to ride.
Once you have an idea of the wheel size you need, let your child try out several bikes and pick the one that feels the best.