how much do i increase my recipe by to go from an 8" round to a 12" round, is it multiply by 2, 2.5 or 3????

I usually go by volume, and double-test. I'd fill the 8" round with water to the rim, then pour into the 12" round. See how many times you need to do this. Then to double-check, I fill the 8" round 2/3-3/4 full (as you would do with cake batter to allow for the rise). See how many times you need to do this to fill the 12" round 2/3 to 3/4 full. These tests should pretty much match. This tells you how many times to multiply the original recipe. Keep notes. As PieceofLayerCake says, if in doubt, it's better to have too much than too little batter (the extra can go into cupcake tins or a smaller pan). Keep a notebook or annotate the recipe so you know what you did and can repeat it. The cake-baking book I frequently use has plenty of notes for how many x I multiplied a recipe for doll cakes or enlarged layer cakes for the kids' birthday parties, how long they baked in the different ovens at different addresses we have lived at, etc.

There are resources online (http://www.joyofbaking.com/PanSizes.html) that explain pan sizes in terms of volume. I'll usually just divide the pans into percentages of each other....if that makes sense, I don't explain math very well. This resource doesn't go past 10" pans, but you can do your own test by using water. It's inexact but it gives you a good idea.

I have been a pastry baker for years, but I just started my cake business about a year ago and in the end it took a bit of over scaling to get a good idea of the true yield of my recipes. What's a bit of extra cake? Just write everything down! The extra cakes I would freeze and pull out for company or last minute orders.

Thank you all for your replies and so super quick too! I'm in the UK and I am glad I found this site. Thank you for your top tips on good books too. Just waiting on my baking tins to arrive, then its shopping for more ingredients! lol

If you have both pans around, measure volume by filling with water & measuring the water. Then compare.
I can't find a 12" round volume on various baking pan charts commonly available for home cooks. (Maybe you can find one in Rose Levy Beranbaum, Cake Bible, a great resource, or a professional book.)
By volume calculation, the 12" pan (if 2" high) holds 226 cu in and the 8" pan (if 2" high) holds 101 cu in.
So multiplier is 2.25.
If you don't have pans or reference books handy, maybe buy them and practice before the Friday order.
Last, Piece of Layer Cake (a member here) is a pastry cook, and may be able to advise.
Good luck, and please tell us your results...

Area of your pan goes from about 50 square inches to 113 square inches, so 113/50 gives you a ratio of 2.25. Most ingredients ought to cooperate though eggs maybe a bit tricky...maybe not.