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18- or 20-inch BMX? The choice is often less clear for riders around 145–150 cm tall, when either size may fit.
There’s no rule that reaching 145 cm means moving straight to 20-inch wheels. Experience, body proportions and top-tube length, usually written as TT, matter too.
A beginner around 145–150 cm may find an 18-inch BMX easier to manage. A more experienced rider may already suit a 20-inch BMX with a short top tube.
18- or 20-inch BMX: a quick comparison
| Rider height | 18-inch BMX | 20-inch BMX | What to consider |
|---|---|---|---|
| 135–145 cm | Yes | Usually still too big | Usually 18-inch |
| 145–150 cm | Yes | Yes, with a short TT | Depends on riding experience |
| 150–160 cm | May be getting small | Yes | Usually a 20-inch BMX with a short TT |
These ranges are approximate. Around 145–150 cm, take riding experience, body proportions and the particular model’s top-tube length into account. Check its manufacturer’s sizing guidance too.
Who is an 18-inch BMX for?
An 18-inch freestyle BMX is a smaller bike for shorter riders who want to learn tricks and ride skateparks, pump tracks or street spots.
For riders around 135–150 cm, it offers a more compact position than most 20-inch BMX bikes. That can make the bike easier to control and lift as a young rider starts learning bunny hops and manuals.
How tall should you be for a 20-inch BMX?
You can consider a 20-inch BMX from around 145 cm tall, but top-tube length is particularly important for shorter riders.
At around 145–160 cm, start by comparing shorter top tubes, roughly 19.5–20.25 inches.
“20-inch wheels” doesn’t describe the size of the whole bike. A BMX with a 19.5-inch TT and one with a 21-inch TT share a wheel size, but fit quite different riders.
145–150 cm tall: which size should you choose?
This is where a chart alone won’t settle the choice.
An 18-inch BMX will often be easier to manage if the rider:
- is new to BMX;
- has a smaller build;
- is close to 145 cm tall;
- needs a bike that is easy to lift and control.
A 20-inch BMX with a short top tube is worth considering if the rider:
- is closer to 150 cm;
- already has riding experience;
- can control a BMX well;
- chooses a model with a suitably short TT.
Is an 18-inch BMX less capable than a 20-inch?
No. An 18-inch freestyle BMX isn’t a toy version of a 20-inch bike. It’s a different size.
A well-designed 18-inch freestyle bike lets shorter riders learn tricks, jumps and skatepark riding. Its smaller proportions give them room to make the same movements a taller rider makes on a 20-inch BMX.
There’s no need to move to 20-inch wheels just because they seem more grown-up.
Should you buy a 20-inch BMX to grow into?
A bigger BMX isn’t automatically a better choice for the future.
A frame that is too long can make it harder to lift the bars, bunny hop, manual or turn the bike. The rider ends up struggling with the size while trying to learn the basics.
At 145 cm tall, don’t choose a 20.75- or 21-inch TT simply because your child might grow into it later. Choose a bike they can control at their current height.
Does height tell you everything?
Height is the starting point. Between sizes, also consider riding experience, body proportions, arm and leg length, bike weight and the top tube of the particular model.
Two riders of the same height, around 145–150 cm, won’t necessarily choose the same BMX.
For wider height ranges, wheel sizes and top-tube lengths, read our BMX size guide.
What to remember when choosing
135–145 cm: an 18-inch BMX is often the starting point.
145–150 cm: either size may fit. Beginners often find an 18-inch BMX easier; a more experienced rider may suit a 20-inch bike with a short TT.
150–160 cm: a 20-inch BMX with a short top tube will often be worth considering.
Still unsure? Tell us the rider’s height, experience and the model you’re considering. We’ll help you compare the fit of an 18- and 20-inch BMX.
Browse all BMX bikes at RMDBike
Questions about 18- and 20-inch BMX bikes
Can you do tricks on an 18-inch BMX?
Yes. An 18-inch freestyle BMX is designed for shorter riders to learn bunny hops, manuals, jumps and skatepark tricks. Check the intended use of the model you choose.
What rider height suits an 18-inch BMX?
Our approximate guide puts 18-inch BMX bikes around 135–150 cm. Check the particular bike’s sizing guidance and fit before choosing.
When can you consider a 20-inch BMX?
From around 145 cm, depending on the rider and model. At 145–160 cm, start by comparing short top tubes around 19.5–20.25 inches.
At 145–150 cm, is an 18- or 20-inch BMX better?
Either may fit. Beginners often find an 18-inch bike easier to manage. A more experienced rider may suit a 20-inch BMX with a short TT, particularly closer to 150 cm.
Should you buy a 20-inch BMX to grow into?
Avoid choosing a bike that is clearly too big. An overly long frame can make controlling the bike and learning basic tricks harder.
Source article updated: 17 September 2026.
