How Big Is An 18 Inch Bike?

An 18-inch bike can refer to a wheel bike for kids or a frame bike for adults.

Kids’ bikes are measured by the size of the wheels whereas adults’ bikes are measured by the frame size along with the wheel size.

However, knowing how big the wheels are will not provide you with an understanding of how big an 18-inch bike is going to be.

For that, you need to also have a grasp on two other aspects of the bicycle which we discuss below.

Marin donky jr 18 inch wheel bike for kids

How Big Or Tall Is An 18 Inch Wheel Bike:

An 18-inch bike is a size that straddles a 16-inch bike on one end and a 20-inch bike on the other.

An 18-inch wheel bike is not a common size in the kid’s bike space.

It’s a new entrant trying to bridge the gap between a 16-inch and a 20-inch bike.

So, not many brands do carry this size in their portfolio.

The 18-inch is a measurement of the size of the wheels or specifically the diameter of the wheels.

It’s a recommended bike size for 5-year-olds and 4-year-olds who are tall and 6-year-olds and 7-year-olds who are shorter than average height.

However, the size of the wheels alone will not give you a clear picture of how big the bike is.

For this purpose, you need to look at two other aspects called the Stack and Reach of the bike and the Standover height.

Stack and Reach: In broad terms, this refers to the space available to the rider when seated on the bike.

This is roughly the triangular area between the saddle, the cranks, and the handlebars.

This space has to be optimum for the rider to feel confident while biking.

Too small a space and the rider can feel cramped and will have to overexert while pedaling.

Too big a space and the rider will be positioned way back on the saddle and will have to overextend to grab the handlebars.

This can lead to less confidence in handling and control of the bike.

Standover Height: This is the distance from the floor to a point on the top tube (the tube that connects the handlebars in the front to the seat post in the rear).

Generally, an 18-inch bike will have a standover height between 18’ and 18.5’.

So, the inseam height of the kid needs to be a couple of inches taller than this height when standing over the bike when the bike is stationary.

This would make the kid feel comfortable and at ease with the bike.

What Size Is An 18 Inch Bike Frame:

The 18-inch frame denotes a bike for adults or for tall teens.

This is the measurement of the size of the frame and this is how adult bikes are sized.

But, frame sizes are a bit more complicated than wheel sizes where sizing is standard across.

With frames, we also have to take into account the type of bike we are talking about such as a road bike, off-roader gravel bike, mountain bike, BMX bike, etc., as each of these bike types will be sized differently.

Below is how the sizing of an 18-inch frame bike is done:

  • Mountain: Medium
  • Road: XX-Small
  • Hybrid/Gravel: Medium

As you can see the same frame lengths are categorized differently for different types of bikes.

This is because of the purpose for which the bikes are built.

For a mountain bike, control of the bike on the trails is extremely crucial and so the frame would be built for that purpose.

The head tube angle of a mountain bike would be slacker resulting in a shorter top tube compared to that of a road bike.

A road racing bike on the other hand focus on aerodynamics and so the head tube angle would be steep with a longer frame.

The below chart provides a good picture regarding frame size classification.

Frame Size (Mountain & Hybrid Bikes)

Frame Size

Frame Size (inches)

Frame Size Centimeters

X-Small (XS)

13″ – 14″

33 – 36

Small (S)

15″ – 16″

38 – 41

Medium (M)

17″ – 18″

43 – 46

Large (L)

19″ – 20″

48 – 51

X-Large (XL)

21″ – 22″

53 – 56

XX-Large (XXL)

23″ – 24″

58 – 61

Frame Size (Road Bike)

Frame Size

Frame Size (inches)


XX-Small (XXS)


47 – 48cm

X-Small (XS)


49 – 50cm

Small (S)


51 – 53cm

Medium (M)


54 – 55cm

Large (L)


56 – 58cm

X-Large (XL)


58 – 60cm

XX-Large (XXL)


61 – 63cm

What Size Is An 18 Inch Mountain Bike:

An 18-inch wheel mountain bike for kids will measure 18 inches in diameter at the wheels.

The 18 inch mountain bike is a new entrant into the kid’s bike size arena and is trying to bridge the gap between 16-inch and 20-inch wheels.

You may not find many brands having this size in their portfolio though.

An 18” frame mountain bike for adults on the other hand will be sized as a size Medium bike.

The 18-inch here refers to the size of the frame and this is how adults’ bikes are categorized.

An 18-inch frame mountain bike will come in 26”, 27.5”, and 29” wheel sizes.

What Size Is An 18 Inch BMX Bike:

When it comes to BMX bikes one has to consider both the wheel size as well as the top tube length to figure out how big the bike is.

The 18 inch here refers to the diameter of the wheels.

An 18” BMX is a size between 16” and 20”.

An 18-inch BMX with a top tube length of 16.9” can accommodate a rider who is between 3’8” and 4’8” tall.

Is An 18 Inch Bike Large:

An 18-inch frame bike is classified as size Medium for a mountain/gravel bike and size XX small for a road bike.

These are bikes that cater to the adult segment.

An 18-inch wheel bike on the other hand refers to a bike for a kid with a standover height of around 18 inches.

The wheel measurement denotes the diameter of the wheels.

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