i am ordering a bell and carlson stock and was wondering how do you measure your lenght of pull to determine what fits you best.

Bend your elbow at 90°. Extend your fingers, palm up. Measure from the crease in your skin on the inside of your bent elbow to the middle of the tip of the index finger. That's the standard method.

To customize, pick up a long gun that you know fits you well and that you like the fit of. Check that it is empty. Lay the stock of the gun in that same position (crook of the elbow) and see where the side of the trigger lands on your outstretched index finger? If the gun fits the way you want, that becomes your personal pull measuring point.

What Nick said.

Bend your elbow at 90°. Extend your fingers, palm up. Measure from the crease in your skin on the inside of your bent elbow to the middle of the tip of the index finger. That's the standard method.

To customize, pick up a long gun that you know fits you well and that you like the fit of. Check that it is empty. Lay the stock of the gun in that same position (crook of the elbow) and see where the side of the trigger lands on your outstretched index finger? If the gun fits the way you want, that becomes your personal pull measuring point.

i keep gettin a measurment of about 18". i don't think that sounds right. i measured the lenght from were my elbow bend to the middle of the tip of the index finger(my arm is straight out) is 15 and a half inches. am i doing something wrong.

The "standard" bend-your-elbow-and-measure technique is the standard half-a***d way of doing it. Perhaps most of the time it'll get you reasonably close, but a large percentage of the time it's worthless. The only real way to measure proper length-of-pull is with a try-gun equipped with an adjustable stock and a stock fitter who knows what he's doing -- and LOP for your shotgun will not necessarily be the same as for your rifle. Lacking access to a well-equipped stock fitter (or $$ to pay one) the best alternative is to try rifles with various-length stocks and see what works for you. Keep in mind -- the clothing you'll be wearing when you shoot will have a significant effect on length. If you usually hunt deer, say, in frigid weather and you choose your stock length during high summer while wearing a T-shirt, it's going to be too long -- and an inch too short is better than a quarter-inch too long. Keep in mind, too -- the "one size fits all" lengths found on mass-produced rifles don't really fit all, but they're perfectly fine for probably 80% of the adult shooters out there. It's the 10% of them at either extreme end of the range that really need fitting.

Pisgah is right in that the crook of the elbow trick only gets you close. Your measurement of 15 1/2" is close to mine. A 15" length of pull is OK for me on a shotgun with a fairly straight stock and an open pistol grip. This is the way the English like their stocks, with length of pulls longer than Americans or Italians do. A scoped rifle with a tighter, more curved pistol grip with a 14" pull suits me too. Changes in stock configuration change the optimum length of pull for you.

Your physique is a factor too. I believe the late Jeff Cooper was about my height, 6' 2". He wanted a 12 1/2" length of pull, and I couldn't shoot a kicker that short. The difference is that I have a long neck and he had a very short neck.
http://www.frfrogspad.com/cooper.htm

Two hints.
When you try a rifle for length by the crook of the elbow method, don't stretch. Get a comfortable grip on the pistol grip.

If your nose is less than an inch from your thumb when you're holding the gun in shooting position, the stock is too short. Back when I was a skinny teenager, I had to use magnums in Dad's old shotgun. You know how it is with teenagers. :D I cut my lip more than once.

Bye
Jack

I'll repeat myself. Find a gun that fits you, then lay it in your elbow and see where the trigger comes to on your hand? That is your personal measuring point for LOP, as opposed to the standard one, which is a good starting point. Pisgah's point about being consistent in what clothing you wear when you make the measurement is a good one. I don't know how you are getting an 18" measurement or on what? 15.5" is my own measure by the standard method, though I prefer about 14.5".

Jack's point on Cooper is correct. He valued light weight and "handy" ergonomics. The way he taught us to snap shoot with a rifle, a long stock can hang up in your clothing if you don't push it out far enough forward first. I think that's part of where his preference came from. The other is that he was accustomed to military rifle stocks, which are made not to be too much of a reach for even the shortest soldier they expect to recruit. 13" is pretty much the universal military standard, showing up on everything from U.S. military rifles to military stocked Mausers, give or take an eighth of an inch or so.

13" LOP comes to the base of my trigger finger with the stock in the crook of my elbow. I have scoped my brow using a stock that short with a standard scope configuration, so I won't put a standard scope on a military length stock. A scout scope configuration or military iron sights present no problem for me at all in that stock length, and I enjoy shooting them just fine.