What you need to know about bike sizes

Have you ever bought a suit? Buying a bike is similar in many ways.

You know that if the suit fits well across your shoulders then you're good to go. You can make adjustments: shorten the sleeves, nip in the waist, etc. Bikes are the same way - once you get close to the right size they're infinitely adjustable.

The fundamental measurement in a bike is the length of the seat tube. This is the tube that goes from the pedals to your seat post (and saddle). The effective length here can be slightly adjusted by moving your saddle up and down, but fundamentally the seat tube is the "size" of the bike. As a convention road bikes list this size in centimeters, mountain bikes and old three-speeds list the size in inches.

A good way to get your bike size is to go to a good bike store and ask for help. However some bike stores want to put you on whatever stock they have on hand, and some cyclists prefer to reduce/reuse/recycle by buying used bikes. In support of that I offer some basic guidance on getting to your size:

Colorado Cyclist offers a calculator to help you find your ideal frame size. I will qualify there are a couple caveats:

1. This is intended for sport cycling, generally a city cyclist will want a slightly smaller bike with a more upright position.
2. It is based on men's measurements. Women tend to have longer legs and shorter torsos so your leg measurement alone will tend to put you on a slightly larger bike.

You might round their measurement down. Their calculator puts me on a 57cm frame, my actual commuter is a 56cm and I would be ok with something even a little bit smaller. So use this to get a measured and calculated idea of your ultimate true size.

The instructions above will be the best way to get your measurement, and the values below should give you some idea of what you'd expect for your result. Actual measurements/calculations are better than this table which I cobbled together from various sources of ambiguous authority from all over the web.

height	inseam	road	mountain	old 3 speeds
5'3"	28"/71cm	49cm	14"	19"
5'5"	30"/76cm	51cm	16"	19"
5'7"	31"/79cm	53cm	17"	21"
5'9"	33"/84cm	56cm	18"	21"
5'11"	34"/89cm	60cm	19"	23"

Once you get the seat tube right you can tailor the bike to fit you perfectly by changing the handlebars and the stem. I'll post on that (with photos of my own bike) soon!

(One more thing: used bike sellers often get confused between TIRE sizes and FRAME sizes - if they claim a bike is 26" or 27" they're probably looking at the tire size)