Most of them.
I was working on a thing where I was giving each spell a section at the bottom called "
As a Ritual." It would come after the "
At Higher Levels." for spells that have that (which should also be most of them).
The reason I made a section for each spell is that the ritual requirements would be slightly different for each spell. Spells that are already rituals would say:
As a Ritual. When you cast this spell as a ritual, you don't expend a spell slot. The casting time increases by 10 minutes, and you cast the spell at its lowest level.
A common entry for higher-level and higher-scaling spells (practically every spell of 6th level or higher) was like:
As a Ritual. When you cast this spell as a ritual, you don't expend a spell slot. The casting time increases by 1 hour per spell level.
For some spells (such as
fireball), I would instead add a material component:
As a Ritual. When you cast this spell as a ritual, you don't expend a spell slot. The casting time increases by 10 minutes, and the spell consumes a special material component: a ruby worth 100 gp per spell level.