Are welding spools all the same diameter? I bought my welder from machine mart but its a long way to go for more welding wire. If I buy some from B&Q or Halfords or eBay. will the spool fit onto my machine?

I think there are two main sizes, and most machines have two steps to the plastic collar to suit them both...

Oh, I assume your using a hobby machine!

As a rule, all welding spools are of standard sizes.
So long as you get a spool of the same weight it will have the same centre and fit your welder.
What MIG have you got?
What size spool does it take?

Gas or gasless? Big roll or fairly small one?

B&Q is hell of an expensive

My welder is a Clarke 100E MIG welder. Low power and 6 settings is ideal for the 20 swg tube I'm using at the moment. The manual says it takes 2 sizes of reel, small and huge. The man in the shop looked horrified when I went to buy the huge one saying the motors are not really up to it.

Yes, B&Q are expensive, but if you really need something on a Sunday afternoon...

don't try and use more than the 5kg reel. 5kg should last you a decent amount of time and fits fine on my clarke 150. To use the 10 (or is it 20?) kg reel i had to make a special holder when i had my cebora hobby and it was a bit of a pain. Suspect it would be the same for the clarke - but don't know for certain.

Next up from 5kg is 15kg and they are heavy for diy machines if they will fit in the case.

I think the sizes were something like 0.5kg and 5kg. 0.5 doesn't last all that long and is only about half the price of the 5kg reel.

get the 5kg, its what i use all the time on my clarke - they work fine.

I would agree, get the 5kg reel.

The only thing i would add is, if you're not going to use it for a little while, roll the wire back onto the reel and wrap it in cling film, to stop it rusting.
If you have any, put a silca gel sachet(the little white packets you sometimes find in with electrical item that say "do not eat" on them) in when you wrap it, this will keep it moisture free.

I use the 5kg on my SIP with no problems, sometimes you need an insert for the bigger spool generally supplied with the welder. Dont dismiss halfords for welding consumables I paid £18.00 for a .6 mm spool a couple of months back

Also, keep your spare wire in the airing cupboard - nice and dry, no risk of rusting whilst you're waiting to use it.

As for where to buy it, look up on yellow pages for your local welding supplies, give them a call they should be cheaper.