The way I know how to measure frame size is by measuring from the center of the cranks to to top of the seat tube.

I ordered a 16" bike from Halfords but the seat tube is only 9". On the box it says 16" x 9" what???? sso where does the 16" come from?

So my neice is 115cm tall. According to halfords size guide a 16" will be fine for her since her daddy has got her a 14" but thats in london.

Is it me thats order the wrong size or has the oem packed in the wrong bike?

  • Measuring seat tube length has not worked in about 20 years, since frame design went bonkers. Commented Sep 29, 2017 at 22:02
  • (And mail-ordering a bike is a dumb idea, unless you live in Timbuktu.) Commented Sep 29, 2017 at 22:04

2 Answers 2


Youth bike sizing is traditionally done by the wheel size. Check your tires they may say 16".

Other possibility is the 16" is top tube length (from center of seat post to center of headset traditional measurement)

  • Gotcha. I didn't know it was done by wheel size which idiot thought of that. So for a given inseam how do I know the size then? Her inseam is 50cm which is greater than the top tube at 15"
    – Ageis
    Commented Sep 29, 2017 at 16:04
  • So is there a way I can work out if the bike is going to fit without driving down to london lool. I dug myself a hole here. This is what i get for being nice/bike fanatic uncle god damn it
    – Ageis
    Commented Sep 29, 2017 at 16:06
  • things are bit different here in the US (where i am) standard kids sizing is 12,16,20,24 then we go to frame sizing (traditionally) if you could find out what bike they have in london that would probably make it easier. Good Luck
    – emoney
    Commented Sep 29, 2017 at 18:42
  • US does have 14, 18, and 22" wheels as well, but they are less common. Commented Sep 29, 2017 at 20:14
  • Re ‘what idiot though of [sizing bikes by wheel diameter]’. Kids bikes come in a sequence of wheel sizes because they have to scale the wheels with the frame. 20” wheels won’t work on a bike for a 3 year old for instance. There is not much variation in frame size for a given wheel diameter, and kids bikes are designed to have a very large range of adjustability of sear height and handlebar position, so wheel diameter serves as a good overall size indicator. Commented Sep 30, 2017 at 15:02

Here's a generic sizing chart: http://www.ibike.org/education/buying.htm

From that, it looks like they would recommend an 18"-wheel bike. But given that, there will still be size variation in the frame. Always best to have the kid try it out. 20"-wheel bikes are more common than 18" and they would probably give some room to grow into. I'm just about to put my 5-year old son on a 20" and he's shorter than your niece.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.