Bicycle Mechanics - Can someone explain bolt sizing please?

I have never been a handy guy so the last place you would find me at is a hardware store. Because of this, I am not familiar with the sizing of bolts. I'd like to order some titanium replacement bolts for my stem, seat post clamp...etc but not sure what size to order.

For example:

M5x16
M5x20
M6x16
M4x10

ETC...

I would imagine the second number is in regards to the length? Is there an easy way to tell which size bolts I will need besides taking them off and going to a hardware store and inserting them into their testing holes?

thanks

M for metric.

The first number is outside diameter. For example, an M5 screw is 5mm in diameter.

The next number is length. An M5x20 bolt is 20mm long.

There may also be multiple thread options for larger sizes. M8 threads come with 1.25mm between threads and 1.0mm.

And aditional, coarse vs fine thread,, proportional to diameter

common, a 5mm , 0.8 thread per mm is common, 6mm x 1 tpmm..

An inexpensive digital caliper will help as well as being generally useful. One of these http://www.amazon.com/Metric-Thread-Detective-Screw-Gauge/dp/B002MMIK1U/ref=sr_1_6?s=hi&ie=UTF8&qid=1358383621&sr=1-6&keywords=Nut+%26+Bolt+Thread+Checker+metric is nice for checking nuts, threaded holes AND bolts

I was under the impression that the "M" in the spec meant a specific mm per thread. M3 would be .5, M5 is .8, M6 is 1. Also the bolt length is measured as only the portion that is the diameter listed. Anotherwords the M5x20 cap screw is a 5mmx.8mmpt with 20mm of 5mm diameter length (usually fully threaded at this length) with a head that is an Allen wrench fitting. Andy.

Bolt sizes follow the following formula (M)D(xP)xL.

M If present specifies that all dimensions that follow are metric
D Diameter in inches or millemeters
P If present specifies the pitch, if not present the industry standard pitch for that diameter is used. (You need to be careful here because course thread is sometimes refered to as standard as opposed to fine thread.)
L Length measured fron the tip to the flat portion of the head

As always, Sheldon explains. http://www.sheldonbrown.com/gloss_m.html#metric