BMX Size Chart: Which Race Bike Size to Choose

• Size on centimetres, not on years
• The BMX size chart, Micro to Cruiser
• How a BMX frame is measured
• How to measure the rider: standing height and inside leg
• Between two sizes: the four criteria that decide
• Race class and frame size: two different things
• Moving up to the 24 inch Cruiser
• What comes back to the workshop every autumn
• The dimensions that follow frame size
• Checking your size in three steps
• Frequently asked questions

The BMX size chart comes down to three columns — rider height, frame class, top tube length. Here is the short version, with heights in feet and inches for readers who work that way. The detailed table, with indicative ages, cranks and wheel format, is two sections further down.

Size on centimetres, not on years

A BMX race bike is chosen on rider height measured in centimetres. That figure gives the top tube length. Allow roughly 17" for a child of 115 cm, 19" for a teenager of 145 cm, 20.5" for an adult of 170 cm. Age is only a rough guide, never the thing you buy on.

The reason is growth. At the same age, differences in height are enormous. In one club, two riders born the same year can stand a head apart, and the shorter of the two will be riding a Junior while the other is already on an Expert. The second marker, decisive in borderline cases, is inside leg — two riders of 172 cm do not have the same leg length.

The BMX size chart: the full table, Micro to Cruiser

Find the rider's height in the second column and read across. The bands overlap on purpose, because those are the zones where a judgement call is needed. Every class links through to the matching section of the shop.

The same badge does not give the same measurement from one brand to the next

This is the gap the table cannot show, and it is the one that decides the purchase. On a frame badged Expert XL — some brands badge it Expert Pro — Chase and Supercross quote a 20" top tube, Meybo 20.6" on the Holeshot and the Superclass, Inspyre a range of 19.25" to 20" on the Evo. More than half an inch separates two bikes sold under the same name. The Expert does the same thing, starting at 18.5" at Inspyre and finishing close to 19.7" at Meybo. The height bands move in the same proportion, by around five centimetres from one manufacturer to the next. We keep those charts side by side, and it is why we always ask for a rider's height rather than the name of their class.

The wheel format then changes three times. The Micro runs 18 inch wheels on 355 rims. From Mini to Expert XL you race 451 rims, fitted with 20 x 1-1/8" or 20 x 1-3/8" tyres. From Pro upwards it is 406 rims in tyre widths of 20x1.50 to 20x1.75. The Cruiser occupies a fourth format, the 507 rim. Nothing swaps between those standards, and a child of 108 cm already confident at the club will often go straight to a Mini rather than a Micro.

How a BMX frame is measured

The top tube is measured horizontally, from the centre of the head tube to the centre of the seat tube. On frames with a sloping top tube that is a virtual measurement, longer than the tube you would actually put a tape on. Every manufacturer publishes it in inches, and it is the only measurement that lets you compare two frames.

Watch out for two common confusions. The overall length of the bike tells you nothing, because it depends on the fork and the stem fitted to it. And the "20 inch" in the product name refers to the wheel, never to the frame — there is no such thing as a 20 inch frame, only a frame with a 20.5" top tube built with 20 inch wheels.

How to measure the rider: standing height and inside leg

Standing height

Back to the wall, heels together against the wall, buttocks and shoulder blades touching, eyes level. Rest a flat rigid object on the crown of the head, mark the wall in pencil and measure from the floor to the mark. That number is what places you in the "rider height" column.

Inside leg

Still back to the wall, feet about fifteen centimetres apart. Bring a book up flat between the legs, firmly, until it stops — the idea is to mimic the pressure of a saddle. Measure from the floor to the top edge of the book. This measurement drives saddle height and crank length. It also explains why two children of 133 cm do not take the same size: the one with long legs will feel cramped on a BMX Junior, while the other, with a long torso, sits on it perfectly.

The gate position test

In the gate, elbows bent, the rider should be able to bring the shoulders over the front wheel axle without the arms locking out and without feeling squashed, back at roughly 45 degrees. Arms stretched straight means the frame is too long; knees hitting the handlebar means it is too short.

Between two sizes: the four criteria that decide

A rider of 150 cm sits in both Expert and Expert XL, an adult of 172 cm in both Pro and Pro XL. When two lines of the chart both claim the same rider, four things settle it, in this order.

1. Build, inside leg first

Long-limbed, long legs, short torso: take the upper figure. Compact, long torso: stay on the lower one, and you keep the bike quick enough to place in a berm. At 150 cm, a rangy rider is already well set on a BMX Expert XL at 19.5", while a stockier build rides better on a long BMX Expert.

2. Ability

A beginner takes the smaller size, every time. A short frame lifts more easily, comes back into line faster after an untidy landing and forgives mistakes in a line. An experienced rider, by contrast, makes use of the extra reach.

3. Growth still to come

On a young rider in a growth spurt, the margin is counted in months, not years. A child of 133 cm gaining six centimetres a year will be out of the Junior band before the end of the season, and the upper figure is justified. A rider who has finished growing has no reason to take it.

4. Type of track

Fast tracks, long straights, big hills: the longer wheelbase settles the bike and the upper figure makes sense. Technical tracks with tight berms: the short frame stays the more manageable of the two.

The special case of the Expert XL to Pro step

It is the only step where the wheel format changes. Above 160 cm the rider moves to a BMX Pro on 406 rims. The rim, the tyre, the inner tube and the spokes all have to change; the hub, on the other hand, is kept and relaced onto a 406 rim. In practice: do not invest in a high-end pair of wheels at 158 cm if the move to Pro is due within the year. Drivetrain, stem, handlebars and saddle almost always carry over.

Race class and frame size: two different things

This is the most common confusion of all, and it produces genuinely wrong purchases. A BMX race rider has two "categories" that have nothing to do with each other.

The race class is the one on the number plate and on the gate. Under UCI rules it is set by the age reached during the calendar year, independently of the rider's height and of the bike being used. The rule book is revised each season, so check the current version with your club. Frame size is a manufacturing measurement — top tube length in inches — decided by the manufacturer and chosen on body proportions.

Why the confusion is so persistent

Because the same words get used twice over. "Micro", "Mini", "Junior", "Expert" and "Pro" are frame sizes set by manufacturers. Under UCI rules your race class is set by the age you reach during the calendar year — and, confusingly, "Junior" in that rule book means 17 to 18, nothing to do with an 18.25" frame. In USA BMX the classes work differently again: Novice, Intermediate and Expert are proficiency classes there, not a model that applies anywhere else.

The federation decides who you race against. Your height decides what you race on.

A 165 cm adult beginner therefore buys a "Pro" frame without being a professional, because that name describes a top tube of 20.5" to 20.75" and nothing else. A 175 cm adult new to the sport enters the age class the rules put them in, and rides a BMX Pro XL because they are 175 cm tall. Entering, joining a club and getting a licence are covered in our guide on how to start BMX racing.

One case comes back to the workshop every autumn. A parent is after "a BMX for the 10 year old class", a class that gathers children spread over more than twenty centimetres of height. Depending on the rider, the answer is a Junior at 18.25", a short Expert at 18.5" or a long Expert at 19": three bikes for one number plate.

Moving up to the 24 inch Cruiser

The Cruiser is not a cruising bike. It is a competition class, run on the same track and from the same gate as the 20 inch, on 24 inch wheels with 507 rims, with its own motos and its own results sheet, contested in age brackets.

Who it is for

You mostly see three kinds of rider move to the Cruiser. Riders coming back to competition after years away, who find a bike that forgives more on landings. Very tall riders, for whom the big wheel is easier to find than a 22" top tube. And those who enter both formats on the same race day.

The Cruiser size chart

It maps across directly. Take the size you already use on 20 inch, or one step above. A rider settled on a 20.75" Pro most often lands on a 21.25" Cruiser.

What the big wheel changes

Inertia goes up. The bike is slower to wind up, but the rhythm holds better at the end of a straight. Jumps are taken higher, and the stretched wheelbase improves grip through a berm. Against that, allow a few hundred grams more and a rollout 17 to 20% longer for the same gearing, depending on the rim you are coming from. Rear triangle clearance, the fork and the position of the brake mounts are specific to 24 inch, so a Cruiser wheel never goes onto a 20 inch frame.

What comes back to the workshop every autumn

The bike bought too big "so it lasts two seasons"

This is mistake number one. The reasoning looks economical — take one size up to spread the cost — but the rider spends a whole season on a bike they cannot control.

What is lost is very concrete. In the gate, arms stretched to reach the handlebar, they can no longer push against it when the gate drops, and a missed start is rarely recovered. Weight sits too far forward and the rear wheel breaks traction the first time they get back on the power. On landing, the hips can no longer travel back behind the bottom bracket and every jump finishes on the arms. The economics are wrong too, because a bike that is too big does not make anyone faster, whereas a correctly sized frame resells very well.

The giveaway is the seat post slammed all the way down with the saddle still touching the top tube.

Keeping the old cranks after a frame change

A bare frame built up with the crankset from the previous bike very often ends up with cranks 10 mm too short. Cadence is thrown out, and so is power out of the gate.

Going up a size because of rider weight

A heavy rider does not need a longer frame but a stiffer one with oversized components. On a BMX Pro XXL chosen for build rather than for height, the right answer is a 36 hole hub, a 14 mm rear axle and a thick-walled handlebar, not half an inch more top tube. Above 193 cm, on the other hand, height genuinely does call for a BMX Pro XXXL.

The part bought in the wrong format

The second case that comes round every autumn is the rider who moved to Pro last year being given a brand new 451 wheel. It does not fit, and it only sells on at half price. We also see Expert frames wearing a Pro-size number plate, which catches the knees every time the rider gets back on the power.

The dimensions that follow frame size

The top tube never stands on its own. A correctly sized frame surrounded by the wrong parts behaves like a badly sized frame.

Crank length

It is chosen on leg length, not on age and not on top tube. The "cranks" column of the chart gives the full progression, from 127 to 140 mm in Micro up to 180 mm in Pro XXXL. Too long, and they break cadence and ground out in a berm; too short, and they cost torque out of the gate. On a slight rider, better to drop 5 mm than to force it. Our BMX race cranks cover every one of those lengths.

Handlebar width

It is set on shoulder width, never on frame size. Too wide and you lose control of the leverage; too narrow and you lose the leverage you need to lift the front wheel. In the adult classes, widths run from 720 to 737 mm (28" to 29"), article 6.1.075 of the UCI rule book capping handlebar width at 73.7 cm (29"). Check the figure before you buy, because a bar wider than that limit puts the rider outside the rules. A few millimetres can be cut off an aluminium handlebar — but on aluminium only, since a carbon bar is never cut down and can fail in your hands if it is. Rise, meaning the height of the bar above the stem, is chosen at the same time: the higher it is, the more upright the torso sits. On the youth classes the rule is absolute, a narrow bar, or the child controls nothing.

Stem length

This is the fine adjustment once the frame is settled, over a range of about a centimetre, 45 mm to bring the torso back and 55 mm to open the reach out. Beware the reverse reasoning. A long stem does not rescue a frame that is too short, it shifts weight forward and makes the steering heavy.

Rollout and gearing

Rollout follows wheel size as much as gearing, which makes comparisons between classes misleading. Two formulas are enough.

• Rollout (m) = (chainring ÷ rear cog) × wheel circumference (m)
• Gear inches = (chainring ÷ rear cog) × wheel diameter in inches

Wheel diameter is obtained by adding twice the tyre width to the rim diameter, a method that overestimates the real diameter by 2 to 3% because the tyre squashes under the rider. A 406 in 20x1.75 gives 494.9 mm, that is 19.48 inches and 1.55 m of circumference. A 44x16 on that wheel, the common adult ratio, is therefore worth 53.6 gear inches and 4.28 m of rollout. The same 44x16 on a 451 rim in 20 x 1-1/8" gives 55.0 gear inches and 4.39 m, which is why you should always state the wheel format when you quote a gearing. You go to 44x17 on short tracks and 44x15 when the hill is high, adjusting at the rear cog rather than at the chainring, because one rear tooth is worth about three at the front. The full workings are in our BMX gear ratio chart.

What that changes in practice:

• A Cruiser on 507 rims running 24x1.75 measures 595.9 mm of developed diameter, 20% more than a 406 in 20x1.75 and 17% more than a 451. For the same gearing, rollout goes from 4.28 to 5.15 m. A cog on its own cannot make that up, since it would take 19 teeth and that size is not made; you correct at both ends, a 44x16 on 20 inch landing around 42x18 on a Cruiser.
• A 451 rim in 20 x 1-3/8" measures 520.85 mm, so more than a 406 despite both being sold as "20 inch". Moving from Expert XL to Pro, a rider who keeps the same gearing loses rollout.

Run the calculation again with your own tyre widths, because a change of width moves the result faster than a tooth on the rear cog.

Checking your size in three steps

1. Measure standing height and inside leg barefoot, back to the wall.
2. Place the height in the chart and read off the top tube, the cranks and the wheel format on that line.
3. If the rider falls in an overlap, settle it with the four criteria, in order — build, ability, growth still to come, type of track. When in doubt, the smaller size stays the lower-risk choice.

Every size, Micro to Cruiser, is gathered in the BMX race bikes and frames section, and our complete builds arrive set up to the size quoted. Send us the rider's height and inside leg and we will confirm the top tube and the crank length before the bike leaves, with the Meybo, Chase, Inspyre and Supercross charts open side by side. Orders are dispatched within 24 to 72 hours, and you can spread the cost over 2 or 3 payments.

Frequently asked questions

How is a BMX frame measured?

The top tube is measured horizontally, from the centre of the head tube to the centre of the seat tube. On a frame with a sloping top tube that figure is virtual, and longer than the tube you would measure with a tape. It is the measurement every manufacturer publishes, in inches, and the only one that lets you compare two frames.

Is a 20 inch BMX the wheel or the frame?

The wheel. "20 inch" describes the outside diameter of the wheels, never the length of the frame. A frame is described by its top tube, from 15" in Micro to 22" and over in Pro XXXL, and almost all of those frames are built with 20 inch wheels. A Cruiser runs 24 inch.

What size BMX for a 10 year old?

It depends on height, not on age. At 10, a child usually sits between Junior and Expert. Junior covers 122 to 134 cm with a top tube of 17.75" to 18.25", Expert 135 to 154 cm with a top tube of 18.5" to 19.5". Measure barefoot, back to the wall, and read the matching line of the chart.

What size BMX race bike for 160 cm or 170 cm?

At 160 cm you are on the Expert XL to Pro step, so a 20" or 20.5" top tube. A rider still growing, or a beginner, stays on Expert XL and 451 rims; an adult moves to Pro and 406 rims. At 170 cm the answer is the Pro, top tube of 20.5" to 20.75", cranks of 170 to 175 mm.

What size BMX race bike for an adult of 180 cm?

A rider of 180 cm belongs in Pro XL: top tube of 21" to 21.25", cranks of 175 to 177.5 mm, 20 inch wheels on 406 rims. A short torso and long legs can justify the upper figure of the band. If the saddle has to be slammed all the way back to feel comfortable, the Pro XXL is the right answer.

How do I know if a BMX is too big?

Put the rider in the gate position, elbows bent. If the arms stretch straight to reach the handlebar and the shoulders can no longer come over the front wheel axle, the frame is too long. Another sign: on landing a jump, the hips cannot travel back behind the bottom bracket and the rider comes down on the arms.

What is the difference between a 451 and a 406 BMX?

They are two rim diameters, both sold as "20 inch". The 451 equips the Mini to Expert XL classes, with narrow tyres in 20 x 1-1/8" or 20 x 1-3/8". The 406 equips the Pro classes, with tyre widths from 20x1.50 to 20x1.75. The rim, the tyre, the inner tube and the spokes all change at that step; only the hub is kept, relaced onto the new rim.

What height do you need for a Pro BMX?

From 161 cm. The BMX Pro covers riders of 161 to 172 cm, with a top tube of 20.5" to 20.75", cranks of 170 to 175 mm and 20 inch wheels on 406 rims. Below 160 cm you stay in Expert XL, the last size in the 451 format. Above 172 cm, look at the Pro XL.

Should I buy a size up so the bike lasts longer?

No. A frame that is too long stops the rider pushing on the arms out of the gate, throws weight forward and prevents the hips travelling back on landing. The rider loses places all season. On a growing child, take the upper figure of the correct band, never the class above.

What is the size difference between a race BMX and a freestyle BMX?

A race BMX runs from 15" to 22" and over of top tube, chosen on the rider's height. A freestyle BMX sits between 20.5" and 21.25" whatever the rider's height, with 406 wheels only, a more compact geometry and a much higher weight. The two charts do not transpose.

BMX Cruiser or 20 inch: which should I choose?

The 20 inch remains the benchmark for progressing and for racing the deepest classes. The 24 inch Cruiser suits riders coming back after a break, very tall riders, and anyone wanting to race both formats on the same day. Take the size of your 20 inch or one step above: a 20.75" Pro corresponds to a 21.25" Cruiser.