Often your chainsaw bar has all the data you need written on it, like the Stihl bar pictured below. But your guidebar may be old and worn and the data could be unreadable, or maybe it's just not that user friendly !! Don't worry we can figure it out.
 
                                
The PITCH is the distance measured between the centre of three link rivets, divided by 2.
                                 
 
The GAUGE is the width of the groove in the chainsaw bar.
                                                         
 
 
The LINK COUNT is determined by counting the driven teeth (sometimes called Tangs) of the chain, these are the bits that run in the groove, meaning that the Gauge is also the width of the driver teeth.
                                      
 
 

OK , NOW I KNOW WHAT PITCH/GAUGE/LINK COUNT ARE, BUT WHY DO THESE MATTER?

 
A chain with the wrong gauge for the bar may not physically fit. A thicker gauge of chain than the bar was made for simply will not fit in slot (unless bar is quite worn), though a thinner (smaller) gauge chain will fit on a larger gauge bar, this is not safe practice as the chain can twist in groove, both bar and chain could be damaged which could have serious injury consequences for the user should the chain become so worn that it snaps in use.
 
The pitch of your chainsaw's sprocket (the drive gear attached to engine) must match both the sprocket in the end of bar and the chain pitch (if a sprocket type bar is in use, there are types with no sprocket in bar end e.g. Oregon Laser Tip or Stihl Duromatic bars). If the pitches do not match the teeth of the drivers will not mesh with those on sprocket(s), the chain will jam instantly and/or be thrown off bar, very dangerous and likely to cause expensive damage to your bar and/or chainsaw. NOTE Low Profile 3/8" Pitch is not the same as a Standard 3/8" Pitch , they are not interchangeable, see Low Profile section below.
 
Your chainsaw sprocket will not care what gauge of chain you run on it, only the pitch matters if you are replacing both bar and chain. Your bar should outlast several chains (just turn it over anytime you remove chain to even out wear on the pressure side - bottom edge - of bar) and a sprocket may well last for a couple or even three bars, though I always replace sprocket at same time as bar on my own saws.
 

PITCH

 
There are four pitches of chain in common use today 1/4", .325", 3/8" and .404".
Of these by far the most common are .325" and 3/8". Inexperienced users are unlikely to have a .404" pitch saw, only the largest professional saws have chains of this pitch. 1/4" pitch is commonly found on small saws, both electric and petrol, electric Lopper machines and the likes, and is used most often on specialist chainsaw carving bars. The majority of users need .325" or 3/8" chain. A good ruler will enable you to determine the pitch of your chain by measuring as described earlier.
 

GAUGE

 
Four gauges are usually found on modern handheld saws, these are 0.043" (1.1mm), 0.050" (1.3mm), 0.058" (1.5mm) and 0.063" (1.6mm). The 0.043" (1.1mm) gauge is only ever found with 3/8" pitch chain and is fitted mostly to electric saws and some small Stihl petrols eg Stihl 017/018 , Stihl call this chain Picco Micro.
 
All the other gauges are found in all pitches. Finding gauge manually is very difficult, a micrometer helps obviously.
 
 
Sometimes called LP chain (or in Stihl speak Picco) is only ever found in .325" and 3/8" both with 0.050" gauge (the .325 version is less common, Husqvarna call it Pixel) in reality you mostly see this stuff in 3/8" 0.050" (though the 0.043" Picco Micro is indeed Low Profile). It is vital that you fit LP chain if your bar requires this. The nose radius on a LP bar is smaller than a standard bar, normal chain will not fit round the sprocket properly as cutters are too high/deep. You can often  fit LP chain to a standard bar with no ill effects (but standard chain cuts faster) but many saws have a smaller (6 tooth) drive sprocket on engine when running Low Profile chain, a Standard profile chain will not mesh on this sprocket, and a Low Profile chain may not mesh properly on a standard profile saw sprocket (not a good move to fit LP chain on a standard bar anyway). BEWARE Oregon chain Type numbers ending LP eg Type20LP are NOT Low Profile chains, the LP is a chain designation by Oregon meaning it's a Full Chisel chain, Low Profile Oregon types are 90, 91 and 95. Husqvarna call Low Profile Pixel, but its the only .325 .050" chain they market and can be safely used on normal or standard profile bars (their chain is actually manufactured by Blount Cutting who make Oregon - and SARP and Windsor and Laser and Dolmar/Makita and others). Low Profile chain is sold for bars up to 18" only (cutting length, see below).
 

A WORD ON BAR LENGTH

 
Chainsaw bar length is correctly quoted and referrred to as CUTTING length of the bar, not the overall length of the item. The overall length of a 20" bar will typically be about 22" for example, the 20" bit is what is refferred to in chain size terms and would be a 20" chain (if it was .325" pitch it would be 78 links long, if it was 3/8" - standard profile- it would typically be 72 links). The same official 20" chain in 3/8" pitch would, when measured as a loop, be found to be 26.5" (67cm) long. It should be clear I hope, that only the cutting length as quoted by manufacturer is in fact 20", therefor the ONLY RELIABLE measure of chain length is the number of driver links ! ( A 62 link Stihl chain in .325" Pitch 0.063" Gauge is sold , by Stihl, as both a 15" and a 16" ...it depends on the bar fitment it is for......)
 
 
 
Right, now we have to get down and dirty, looking at the old chain very closely. In general we are looking for numbers/letters stamped on the driver teeth (the bit that goes in slot remember). Info is usually stamped on one side only of these. It can be a single number (1 or 2 digits) or a number and letter combination. It also helps to notice if it has a maker stamped on side of the links, Husq for example is Husqvarna chain, Oregon is Oregon chain etc etc.
 
First we will consider STIHL chains, since they are very easy to size correctly. On the cutter there will be a number, 1/4, 325, 3/8 or 404, that is the pitch. Simple. If the number on the cutter is a single digit then use the list below to i.d. the pitch. On the tang or driver there will be another single digit number that is the gauge the list below will identify this for you.
 
Cutter number 1 = 1/4"
Cutter number 2 = .325"
Cutter number 3 = 3/8"
Cutter number 4 = .404"
Cutter number 6 = 3/8" Picco
 
Driver number 1 = .043" (1.1mm)
Driver number 3 = .050" (1.3mmm)
Driver number 5 = .058" (1.5mm)
Driver number 6 = .063"(1.6mm)
 
Other brands are identified purely by the number on the Tangs.
 
PITCH                                               3/8"               3/8"       3/8"       3/8"       3/8"      .325"     .325"     .325"       
GAUGE                                           .043"LP     .050"LP      .050"     .058"     .063"     .050"     .058"     .063"       
OREGON                                           90                   91        72         73          75       20/95       21         22         
CARLTON/TIGER                               N4                 N1         A1        A2         A3         K1        K2        K3            
WINDSOR/LASER/SANDVIK            43R                50R        50A       58A       63A      50J        58J        63J          
HUSQVARNA                                                       H36        H36       H42       H45      H30    H25/H21   S28                      
SARP                                                 E1                E3         D3         D5         D6     G3/G30     G5        G6                       
McCULLOCH                                                       mp370  pm370       378        373      320       328        323          
SABRE                                                                  357       980D        988      983       520        528   928D                               
DOLMAR/MAKITA                                                  092        101         093      110       082        083      109                            
  
NB: SARP G30 and OREGON 95 are the equivalent of Husq H30 Pixel (LoPro) and H36 is LoPro compatible
 
 If your chain is 1/4" Pitch then the likely makers marking on tangs will be:
Oregon 25
Carlton E1
SARP 25
Stihl  1/4 on cutter & 3 on tang
Laser/Windsor 50K
Sabre 16
Husqvarna H00

Hopefully some of you will find this useful.

If you think your chain type is 06, 16 or 56..................turn it up the other way ! Or stand on your head and look again, because it's actually 90, 91 or 95 ! If it has some other number not listed just ask a question thru a shop item, there are others around (eg 78 = old type same as modern 75 ; 33 is same as Oregon 20 and sold in North America, same goes for 34 which is equal to Oregon 21).