OK let try and take away some of the guess work out of Barb sizes.
Firstly the UK standard of BSP (British Standard Pipe) and under the heading of BSP fall two commonly used standards, BSPT the T standing for Tapered Thread and BSPP the P standing for Parallel Thread.
BSPT barbs should always be married up with a BSPT hole thus acting like a plug and all threads working against each other forming an even bond, and to make totally water/air tight wrap the barb 3 or 4 times with PTFE tape:-
Otherwise if you put a tapered barb into a parallel threaded hole you are literally relying one thread doing all the work:-
And also BSP (UK standard) are not interchangeable with NPT (USA standard) as the actual pitch of the thread is different - BSP being 55 degrees and NPT being 60 degrees so be careful trying to squeeze BSP into NPT holes (especially brass into plexi) and vice versa otherwise you could end up doing thisâ€¦â€¦â€¦â€¦â€¦see below pic of 1/4" BSPT thread barb being forced into a 1/4" NPT hole.
Note: This isn't to say that mixing Parallel, Taper, BSP and NPT thread can't and shouldn't be done as on many occasions it can be done successfully but nevertheless doesn't mean because it works it is correct and care should always be taken!
When choosing a Barb or Fitting there are three sizes which need taking into account.
a] Hose Tail (OD) Size
b] Thread Size
c] Diameter (ID) Size
a] The Hose Tail Size commonly called OD or Outer Diameter is pretty much self explanatory for example on a 1/2" Hose Tail the measurement is taken form the widest overall diameter of the hose tail (see pic below)
b] The Thread Size is much more complicated to calculate and also to comprehend as on first inspection bears no resemblance to measurement as you would have thought.
So to keep this as a simple guide it is easier to assume that in most basic water cooling set-ups fall into two categories and then obviously the various consequence sizes of both, usually in variants of inches and not metric:-
BSP (British Standard Pipe)
NTP (American Standard)
In the pic below you can see a 1/4" NPT Thread Chrome Barb against a 1/4" BSP Thread Brass Barb and the thread pitch /\ doesn't actually marry up due to the pitch of the thread being different and as most people will not have the necessary instruments to measure thread pitch, trying to explain the complicated procedure of measuring the thread size whether BSP or NPT will be left at this stage but a general rule of thumb is to consider most British and European manufacturers use 1/4" and 3/8" BSP and most American use mainly 1/4" NPT
To find a more comprehensive view on measuring thread sizes you may want to Google it and look for a chart which will equate the number of threads into a measurement!
c] Finally, Inner Diameter or commonly called ID is also self explanatory and is the overall widest inner diameter of the barb which can be measured and seen in the pic below.
This size can vary from each individual manufacture and comes down how wide they have milled the inner bore in the production stage.
The inner Diameter measurement is very important in terms of flow rate and a larger Inner Diameter will be less restrictive than a smaller one.
Remembering that on a 1/2" set-up called such by the Inner Diameter of the hose and Barb Hose tail size can have the flow rate dramatically reduced by the use of a very narrow ID Barb diameter.
Anyway, I hope this may help take away some of the confusion that these fittings seem to inevitable create but remember it is only a small guide outlining and identifying some of the basics on these small yet very confusing fittings.