Bicycle frame size?

[Cider Andy]

How do you measure the size of a bike frame? Is it from the pedal shaft to the seat mounting?

[Chiswickboy]

From the centre of the bottom bracket to the top of the seat tube. (Which I think is what you were saying).

[soda]

Centre of bottom bracket to centre of top tube AFAIK

[shalmaneser]

Either of the above two answers are correct, depending on frame.

Some manufacturers measure to the top of the seat tube, some to the centre of the top top (still measuring the distance along the seat tube though)

Mountain bikes are measured in inches, and road bikes in centimeters.

[b2hbm]

^^^^^^ exactly wot he said.

Tradition is for seat tube measurements but you ought to think about top tube or what's sometimes referred to as "effective top tube" length. It's usually measured from the centers of the head tube along a horizontal line to the seat tube, or the seat pin if the top tube slopes downwards. That governs the reach of the frame and relates to your torso size rather than traditional leg length/seat tube approach. If you're "long body/short legs" for example, you'd probably favour a longer top tube.

You can alter it by the h/bar stem length but it's obviously not as easy or as much adjustment as you can get by simply raising the seat pin. And adjustment means buying a new stem if you get it wrong....

[Dibblington]

Instead of going off sizes, sit on the bike and take it for a test spin.

You can buy the same size frame from Kona and Trek for example and find they handle totally differently, short and chuckable vs strteched and long. It's your back and knees that will suffer if you get it wrong.