if you need 120ml of 1.2% then the amount is 120 x(1.2/100) or 1.44g agarose

Paul is correct but his answer is perhaps confusing to what I think your question is. 1X TAE is 1X TAE, so you use whatever volume you need to make for your gel and then add the appropriate amount of powdered agarose. The agarose does not make a significant change to the volume hence you don't need to factor it in.

So for your gel you would use 40ml of TAE and 0.48g agarose (instead of 0.4 for the 1%).

Michael J. Benedik Thank you for the clarification and explanation! I did feel confuse at the first place.

So just to confirm, in the context of I need to make a 40ml agarose gel for 1.2% agarose gel

I need to weight 0.48 agarose powder and mixed with 40ml TAE buffer ?

Correct.

Michael J. Benedik Alright, thank you so much for the confirmation! That’s definitely help me a lot!