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Solving the Mysteries of Bike Sizing

By Sheila Ascroft

Victorian_bikeHow do you know what size of bike you need? Some bikes are measured in inches, others in centimetres. Why? Mountain bikes evolved in Californian the early ‘70s and so use the American-favoured inches. Road bikes on the other hand have a long European tradition and are always measured in centimetres. Most of the other bikes designed for the road—racing, touring, recreation, some hybrids and cyclo-cross—also use the metric system.

Measurements for bike sizes can be from the centre of the bottom bracket to the centre of the top tube or to the top of the top tube or to the top of the seat tube. And some top tubes are not horizontal but sloping. Where is the bottom bracket you ask? It is where the bike frame tubes join at the bottom with the chain stay tubes. It’s the circle in the diagram below.

Wait it gets worse. Mountain bikes have 26-inch diameter wheels (except for the 29 inch wheeled bikes created by Gary Fisher), while most road bikes have 700 cm (about 27.5-in.) so a mountain bike will seem shorter. Remember too that the height of the bottom bracket from the ground may also vary; it is higher on a mountain bike to provide extra clearance for getting over logs and lower on touring bikes to provide better stability. Yep, sizing can be confusing.

Frame Dimensions

A1 – Seat tube length (centre-to top) A2 – Seat tube length (centre to centre) B – Top tube length (centre to centre) C – Stem length (centre to centre)

sizing a road bike and mountain bikeYour height can provide a very idea of what size frame you might need, but a lot depends on how long your legs are. Two people may be the same height, but one may have longer legs than the other has and require different size frame. For example, I’m 5’4″ (162 cm) with short legs and prefer a mountain bike in the 14-15 inch range, while a friend who’s 5’3″ rides a 16 inch. In road bikes, I have ridden a 48 cm, but when I stood over the top tube, my crotch was touching it. I now happily ride a 44 cm (measured centre to centre) even though the manufacturer’s label on the frame has it as a 43 cm. Confusing eh?

What is more important to know is your standover height. You need to have at least an inch (2.54 cm) of space between your private parts and the top tube. On a mountain bike, it’s better to have 3-4 inches (7 to 10 cm) of clearance, especially if you plan to do the rough trail stuff.

To find your standover height, you need to measure your inseam or the distance from your crotch to the floor in sock feet. The simplest way is to have a friend help you. Stand close to a wall, feet about 6 in. (15 cm) apart and look straight ahead. Place a book to simulate the bike saddle between your legs and pull up firmly against your crotch. (One end of the book should be touching the wall.) Make sure the book is level then have your friend measure from the book spine to the floor. My inseam is 73.5 cm (29 in.).

For a road frame that is measured—centre to top tube centre (c-c), the general guideline is to multiply your inseam (in centimetres) by .65. For me, 73.5 x .65 = 47.7 cm frame. If I measure my frame centre to centre, it’s 45 cm. According to this formula my current bike should be too small, but it isn’t. Remember this is only a guideline; we are all built slightly different.

For a mountain bike, try a frame that is 4 to 5 inches (10-12 cm) smaller than your road frame size. For example, if you ride a 55 cm c-t road frame, look for a 17-18 inch c-t frame.

The following chart taken from Canadian Cyclist’s 2000 Buyer’s Guide provides a starting point for bike sizing. (This chart was originally adapted from Canadian national coaching certification program.)

Inseam Mtn frame size Road frame size
27-29″ 14″ 48 cm
29-31″ 16″ 50-52 cm
31-32″ 17.25″ 52-54 cm
32-33″ 18.5″ 54-56 cm
33-35″ 19.5″ 56-58 cm
35-36″ 20-21″ 58-60 cm
36-38″ 21-22″ 60-62 cm

You can find other bike fitting tips at www.coloradocyclist.com/BikeFit/index.cfm.

Sheila AscroftI’ve been cycling for 20-some years and writing about it for the last 10. My articles have been published in newspapers and magazines — and now on the women’s cycling website! I’m a member of the Ottawa Bicycle Club and the Canadian Kilometer Achiever Program. www.sheilaascroft.com

7 comments to Solving the Mysteries of Bike Sizing

  • judy alden

    I do not understand cm inches.i am 5 ft tall and weight. 135 pounds. What size in inches in this. I would like a 20 inch bike. I’m not very stable as I have not rode in few years. I want badly. Thank you.

  • I would say this is a good article on bike fitting.
    But it’s NOT. Read Sheldon Brown:


    The thing that matters MOST in cycling is STAND OVER HEIGHT. You MUST have a few inches of clearance over the tope tube to be a safe cycling. But what after that?:

    I cannot say it enough, and the great Sheldon Brown said it:

    EFFECTIVE TOP TUBE LENGTH is critical to a correcrly sized bike.

    EFFECTIVE TOP TUBE length (ETT) is the line horizontal to the Earth that is drawn from the headset, across the top tube to the seatpost.. But Giant changed this years ago when they introduced a bike with a slopinmg top tube. It was almost disqaulified it was so “radical”. I think Jan Uhlrick won the Tour de France on this sloping top tube frame. Why a sloping top tube?: It SUPPOSEDLY increases the lalteral rigidity of the bike frame.

    So, we now talk in terms of ETT length as an IMAGINARY line drawn from the headset, horizontal to the Earth, to the seatpost.

    Many Europen high bikes have long EETT lengths. Most riders are male so this works well for them.

    To apply the same calculator to men and women to size their bikes is wrong. Using this calculator (and there are many of them in the cycling fitting universe….and ALL are silly) will ALWAYS put women on bikes that are TOO BIG for them. Then the “expert” bike fitter keeps putting on a shorter and shorter stem and movine the saddle closer and closer to tbe headset. I’ve seen this on Youtube.

    Males have longer torosos than females….usually. I have very long legs and a short torso and I’m a guy. Arnold Schwarrzenegger had a shorter toro and longer legs than most bodybuilders and no one would say he’s a girl…at least not to his face.

    Here’s an example: I am 6 feet tall. I used to ride a 56cm bike with an ETT length of 56.5cm. I kept moving back and forth in the saddle, changed stem lenth, moved my saddle forward. All the things riders do wben a bike has an ETT length that is too long for them.

    I bought a sweet 54cm seattube length Diamond Back Podium 6. It has an ETT length of 54.5cm(It has a sloping top tube) That doesn’t seem like much of difference: 2cm. But on a bike it makes ALL the difference in comfort and handling. I sit absolutely still on tbe bike. No moving back and forth to find comfort on a bike. The bike simply FITS. Because the smaller Diamondcback has a shorter top tube length (ETT).

    In summation: If you’re buying a bike you have to have a few inches of room between the top tube and yor crotch. This is stand over beight.

    Then your torso must allow your hands to rest comfortablly on the bike’s hooods without feeling “stretched out”-as women often feel because the ETT length is too long. Some manufacturers build women specific bikes. These bikes are usually colorful AND have a shorter ETT length.

    If you do NOT feel comfortable on a bike the moment you get on it, then it’s not sized correctly for YOU. You shouldn’t have to fit the bike.

    I’m a bike enthusiast. I have seen so many people out, trying to get/stay in shape. And their bike is poorly sized for THEM and they are fighting the bike as much or more as they are working on their workout.

    I feel sorry for riders who put in the effort and are

  • Doris Geith

    I am purchasing a mountain bike. I have road several and for some reason I am very comfortable on a 19.5″ frame. I am only about 5’4 but have a long inseam of around 29″ . I measured myself so may be a little off. I know for my height I should probably be more in the 16″ frame, I just want to make sure I get the right size frame. I am in a small rural area and the person that has the bike shop really doesnt know much.

  • Alison

    Great article, thanks for the helpful information! I’ve been bike shopping and have had an absolutely difficult time trying to find a road bike small enough. I’m a female about 5’4 but have a very short inseam of 24″. What sizes do road bikes typically come in beyond 48cm? It’d be great to know what to look for in my search, or do I even need to result to a child size..?
    Thanks again!

  • New to cycling

    For this bike: http://www.ghost-bikes.com/bikes-2014/bike-detail/cross-1300-lady/

    Should a 5′ 0.5″ tall female with a barely 27″ inseam go for the 40 or 44 (framesize)? I can’t try the bike before purchasing because it would have to be ordered to where I am…

    According to the company’s own frame sizing guide: http://www.ghost-bikes.com/service/ (which says to multiply inseam in cm x .61 for the cross bikes), I seem to be between sizes, so which one might be best and why?


  • Chris Coan

    I am an American Male 55 years old.
    I have been riding bikes all my life (usually 25″ frames)
    I now live in Germany and want to bike.
    I understand I need a 55cm road bike (touring) frame.
    but all the newspaper ads here read 28″
    What are they referring to, wheel size?

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