If a batten is 1.2mm thick, how many layers of carbon should I need to recreate that thickness?

Or how many gm2 per mm of thickness?

I am using 90 gm2 weave, last time I just guessed, though maybe this time I should be a little more scientific.

It would depend on how much pressure is applied to the lamination during its curing process.
Make some small samples and experiment, would be my only suggestion.

Trial and error IS the only solution, as said above.
Thinner resin would result in thinner laminate, as would more pressure, less saturation.
Old daze, typical polyester wave fin, 34 layer's of 6 oz. BUT, under squeegee pressure, maybe 36, or thick resin saturation maybe 30 layer's would get the same thickness results.

Agree with above

Very low resin in a batten and if it is pultruded you won't be duplicating it exactly - so if repairing it then I wouldnt fuss over it much.
The manufacturer does usually supply a thickness in their specs
Anyway that weight from memory a bit over 0.05? so about 40 layers if pressure applied?

Seems 8-9 layers would do the trick, according to what this tool has just calculated for me.
www.r-g.de/laminatrechner.html
It's in German, but I guess it's rather self-explanatory and one can fool around on it to find out how heavy or thick a laminate would be.
As the company it belongs to doesn't have a 90g weave, I gave in their 93g weave: 1,2mm thickness means 8 layers, laminated by hand with epoxy resin. adding pressure might of course compress it more, resulting in less thickness and less weight.
Cheers, Sven

Oh, I forgot, you can as well calculate thickness for vacuum applied pressure (you would have to add one or two layers in your case) or vacuum infusion, which reduces resin content and thickness per layer even more.

Cheers, thanks for the replies, what i was hoping for was a rule of thumb, for grams of carbon per thickness or grams of carbon per volume. Prepreg under pressure would give and ideal ratio, with hand layup much less. I know there are formulas for this stuff. But as a guide how much carbon in cubic mm?

I think that if you were creating a batten, you should be staying away from any sort of weave and use uni. The batten shouldn't require too much torsional flex, as weave covers. If it were down to a calculation, ZeeGerman's results are fairly spot on.
I would use 5x200gsm for each mm (compressed)
Also, if you are repairing a batten, a sleeve of decent length (far away from a torsional point) would be a good fix.

Hows my maths lol

Just looked up my 100gsm unidirectional, its 0.1mm thick
So 12-14 layers under pressure?