When to Ti and when not to Ti

Can you guess which bolt is made from titanium in this photo? The bolt on the right is Ti. When is it right to use a Ti Bolt? I have developed a formula over the years. W(weight in grams) x L (lenght in mm) = X. When I was a kid I had no Idea what titanium was, I just wanted to go fast. My first bits of titanium were for my cherished Klein. A 118 bar from Bontrager, and some Titanium crank bolts.

Actually, there is no formula... and in most cases an aluminum bolt is lighter and less expensive. I will use titanium for bolts over 20mm and when soft aluminum threads are not strong enough. I needed a solution to allow me to use my Reynolds Ouzo Pro seat post. I went from a M5x30 Stainless bolt to a M5X35 Ti bolt to solve my thick carbon-fiber seat rail issue. The titanium saved me 5 grams at a cost of $15. You can make your own formula to figure out if titanium is worth the cost.