FIT LAB · FIT FINDERFind the right starting size
Tell us your child's age, height, and where they want to ride. We'll suggest a starting size and a path to begin.
Better than guessing or a generic store size: a coach-built starting fit, confirmed in person.
These are starting recommendations, not a final fit. Final sizing should be checked in person.
Fit Finder reference charts
Reference charts
Starting-point tables by sizing system. Each system is separate — numbers do not swap between tables. Balance / kids BMX / BMX Racing use XKids calibration; Freestyle and MTB are public-reference pending XKids calibration.
⚠Starter BMX, BMX Racing, Freestyle BMX, and MTB are separate sizing systems. A Junior race bike is not a Junior MTB; a 20" freestyle is not a 20" kids bike.
Balance bike
XKids calibration: from 85 cm. Inseam is the master measurement — seat at lowest should be about 1 inch below inseam.
| Wheel | Height | Inseam | Age |
|---|---|---|---|
| 12" | 85–105 cm · 33–41 in | 28–46 cm · 11–18 in | 1.5–3 yrs |
| 14" | 95–115 cm · 37–45 in | 35–52 cm · 14–20 in | 2–4 yrs |
| 16" | 105–122 cm · 41–48 in | 42–58 cm · 17–23 in | 3–5 yrs |
Starter BMX / Kids BMX (neighborhood)
XKids calibration for the 12/14/16/18 starter BMX progression. Above 135 cm the neighborhood rider is better served by a regular kids bike / youth MTB-style fit.
| Wheel | Height | System | Notes |
|---|---|---|---|
| 12" | 100–115 cm · 39–45 in | XKids starter BMX | First pedal bike |
| 14" | 110–125 cm · 43–49 in | XKids starter BMX | Bigger kids on first bike |
| 16" | 120–135 cm · 47–53 in | XKids starter BMX | Confident pedalers |
| 18" | 135 cm+ · 53+ in | XKids starter BMX (cap) | Cascade to 24"+ for riders much above 135 cm |
| 20" | ~135–148 cm | Regular kids bike | Neighborhood cascade |
| 24" | ~135–155 cm | Regular kids bike / youth MTB | Neighborhood cascade |
| 26" youth XS | 150 cm+ | Youth XS / adult XS | Coach check required |
BMX Racing
XKids calibration: Nano–Expert (90–150 cm). Above 150 cm is public-reference only and always needs a coach fitting before buying. Race classes are by top-tube length, not by rider age.
| Class | Top tube | Rider height | System |
|---|---|---|---|
| Nano | 15.5–16" | 90–105 cm · 2'11"–3'5" | XKids calibration |
| Micro | 16.25" | 105–115 cm · 3'5"–3'9" | XKids calibration |
| Mini | 17.25–17.5" | 115–125 cm · 3'9"–4'1" | XKids calibration |
| Junior | 18.25–18.75" | 125–140 cm · 4'1"–4'7" | XKids calibration |
| Expert | 19.5–20.0" | 140–150 cm · 4'7"–4'11" | XKids calibration |
| Expert XL | 20.0–20.25" | 150–168 cm · 4'11"–5'6" | Above XKids calibration — coach check |
| Pro | 20.5" | 157–173 cm · 5'2"–5'8" | Above XKids calibration — coach check |
| Pro L | 20.75–21.0" | 168–178 cm · 5'6"–5'10" | Above XKids calibration — coach check |
| Pro XL | 21.25" | 175–183 cm · 5'9"–6'0" | Above XKids calibration — coach check |
| Pro XXL | 21.5–21.75" | 180–200 cm · 5'11"–6'7" | Above XKids calibration — coach check |
Freestyle BMX (public reference, pending XKids calibration)
20" wheel is standard from ~145 cm. Smaller riders use 14"/16"/18" kids freestyle frames. Between sizes, pick the shorter top tube. Currently using public references — XKids calibration is in progress.
| Bike | Top tube | Rider height |
|---|---|---|
| 14" kids freestyle | ~13" | 95–126 cm · 3'1"–4'2" |
| 16" kids freestyle | ~15" | 126–138 cm · 4'2"–4'6" |
| 18" kids freestyle | ~17" | 138–149 cm · 4'6"–4'10" |
| 20" short TT | 19.5–20.25" | 145–168 cm · 4'9"–5'6" |
| 20" mid TT | 20.25–20.75" | 160–178 cm · 5'3"–5'10" |
| 20" long TT | 21.0"+ | 175+ cm · 5'9"+ |
MTB / Trail (public reference, pending XKids calibration)
Wheel size plus standover. Kids MTB frames are deeper than BMX — confirm the child can plant a foot safely. Currently using public references — XKids calibration is in progress.
| Wheel | Height | Inseam | Typical terrain |
|---|---|---|---|
| 12" | 86–102 cm · 34–40 in | 38–46 cm · 15–18 in | First rides |
| 16" | 104–122 cm · 41–48 in | 41–56 cm · 16–22 in | Green trails |
| 20" | 114–137 cm · 45–54 in | 48–64 cm · 19–25 in | Green–blue trails |
| 24" | 124–150 cm · 49–59 in | 58–71 cm · 23–28 in | Blue trails |
| 26" youth XS | 142+ cm · 56+ in | 63+ cm · 25+ in | Blue–black trails |
Setup & fit references
Once you know the frame size, these dial in the details — seat height, crank length, top tube, and wheel size. Starting points, not hard rules.
Seat height by inseam
| Inseam | Seat height range |
|---|---|
| 28–30 cm | 26–29 cm |
| 30–35 cm | 29–34 cm |
| 35–40 cm | 34–39 cm |
| 40–46 cm | 39–45 cm |
| 46–55 cm | 45–54 cm |
Recommended crank length by class
| Class | Rider height | Starting crank |
|---|---|---|
| Nano | 90–100 cm | 125–130 mm |
| Micro | 100–115 cm | 130–140 mm |
| Mini | 115–125 cm | 140–145 mm |
| Junior | 125–140 cm | 145–155 mm |
| Expert | 140–155 cm | 155–165 mm |
Top tube length by class
| Class | Rider height | Top tube length |
|---|---|---|
| Nano | 90–100 cm | 14.5"–15.5" (37–39 cm) |
| Micro | 100–115 cm | 15.5"–17" (39–43 cm) |
| Mini | 115–125 cm | 17"–18.5" (43–47 cm) |
| Junior | 125–140 cm | 18.5"–20" (47–51 cm) |
| Expert | 140–155 cm | 20"–21.5" (51–55 cm) |
Standover: 1–3 cm clearance when straddling the frame flat-footed.
Wheel size by rider height
| Rider height | Typical age | Wheel size |
|---|---|---|
| 80–94 cm | 2–3 | 12" |
| 90–104 cm | 3–4 | 14" |
| 100–115 cm | 4–6 | 16" |
| 110–125 cm | 5–8 | 20" |
| 118–135 cm | 7–10 | 24" |
| 130–145 cm | 9–12 | 26" |
| 140–155 cm | 11–14 | 27.5" |
| 150–175 cm | 13+ | 29" |
When to move up
- Balance → first starter BMX: inseam ≥ ~40 cm and confident feet-up glide.
- 16" → 18" starter BMX: height ~135 cm and comfortable braking on the 16.
- 18" starter BMX → regular kids bike / youth MTB: height above ~135 cm on neighborhood riding.
- Kids BMX → BMX Racing: when the family chooses racing — match by height, get a coach fitting.
- 24" kids MTB → 26" youth XS MTB: inseam 63+ cm and riding blue trails confidently.
Common parent questions
Short answers to the questions we hear most. If your child is between sizes or you're not sure, send us their height and we'll help.
What size bike does my child need?
Height first, then riding style. For starter BMX / neighborhood riding: 12" at 100–115 cm, 14" at 110–125 cm, 16" at 120–135 cm, 18" at 135 cm+. BMX Racing uses different classes (Nano–Expert). Tell us height + riding style and we'll pick.
Balance bike or pedal bike?
Inseam is the gate, not age. A balance bike from ~85 cm (seat ~1 inch below inseam). Move to a starter BMX / pedal bike when inseam is ~40 cm and the child can glide feet-up.
12 inch vs 14 inch — how do I choose?
There's an overlap band (110–115 cm). Brand-new rider → 12". Confident pedaler at 110 cm → 14". When in doubt, size down for first bikes — control beats room-to-grow.
16 inch vs 18 inch kids BMX?
16" covers 120–135 cm. At 135 cm+ move to 18". If your child is well above 135 cm and just rides around the neighborhood, a regular 24" kids bike is often a better fit than staying on an 18" starter BMX.
My child is 110 cm — can they race BMX?
Yes — that's the Micro class in XKids calibration (105–115 cm). Always start with a coach fitting on a real gate before buying a race frame.
What's the difference between starter BMX and BMX Racing?
Starter BMX (12/14/16/18) is a beginner kids bike. BMX Racing frames have gate-ready geometry and are sized by class (Nano / Micro / Mini / Junior / Expert) by top-tube length. The sizing tables don't swap.
What about mountain bikes and freestyle BMX?
Different sizing systems. Kids MTB goes by wheel size + standover. Freestyle BMX goes by height + top tube. Never use race classes for freestyle or MTB.
Is BMX Racing sizing just about height?
Height gives the race-class starting point. BMX Racing fit is more sensitive than normal kids-bike sizing — inseam, crank length, cockpit width, arm length, and track use can change the final setup. That's why we treat Racing recommendations as a starting class, not a final purchase decision.
Plain-language glossary
Technical terms we use on this page, in one line each.
- BMX Racing
- Racing on a BMX track with gates, jumps, and berms. Uses specific race frames sized by top-tube length (Nano → Pro).
- Freestyle BMX
- Tricks at skateparks and street. Heavier frames, thicker tires, different geometry than a race frame.
- MTB (mountain bike)
- Off-road / trail riding. Kids MTB frames have deeper standover and gears. Sized by wheel size first.
- Top tube
- Horizontal tube between the handlebars and the seat. The main number for race frame sizing.
- Inseam (inside leg)
- Inside of the leg, crotch to floor, measured with a book held flat against the wall. More reliable than age for kids bike sizing.
- Standover
- Vertical height from ground to the top tube. Child should be able to plant a foot on the ground safely when standing over the frame.
Ready to go further?
Book a Bike Service check
Full fit + tune at Bike Service & Fit Check before your next session.
See Bike ServiceStarter BMX (12/14/16/18) and BMX Racing (Nano/Micro/Mini/Junior/Expert) use XKids internal calibration. Balance is XKids calibrated with an 85 cm floor. Above the XKids race range (>150 cm) and for Freestyle / MTB we currently use public references from Trek, Two Wheeling Tots, Rascal Rides, Supercross BMX, SkatePro, and Early Rider, with XKids coaching review on top. Numbers are starting points; real fit is confirmed in person.