I think I have found the solution if I'm right its 83.39 cubic inches to fill this 3lb loaf tin 21.5x13x12 tin. 32 cubic inches per pound so would take 3lb of gel to fill the tin which is 83.39 cubic inches If anyone can confirm please reply thx.

# How many cubic inches would it take to fill this size loaf tin 21.5x13x12 1lb of gel is 32cubic inches would I need 1/3?

A pan of those dimensions can hold (21.5*13*12) = 3,354.0 cubic inches of anything.

If a pound of gel occupies 32 cubic inches, then 3354 / 32 = 145.83 lbs of gel would be necessary.

.....If I understood your question correctly.