I have a small cleaning business and I am wondering how to calculate how much supplies ( toilet paper, garbage bags, soap, hand towels) my contracts would use?

This all depends on the wording of your
Cleaning contracts.
With your cleaning what are the specific supplies that you actually provide.
All the items that you mention are specific items.
If you supply them , they should be billed
Properly on the actual supply.
You have to buy them.
You sell them at selling prices.