When you’re weighing for food you have to make sure you know whether the nutrition information listed is for cooked or raw.
If the nutrition info is on the package, it’s info for whatever state the food is in while it’s packaged. So if it’s a raw chicken breast package and it says says 100g = 130 calories, then you would weigh it while it’s still raw. It will still be 130 calories even after cooked even though it now weighs less than 100g
You can also use a website, just make sure you know if the website is listing for raw or cooked. So if you have leftover cooked chicken breasg and you want to weigh it, pull up a website that shows you nutrition info for COOKED chicken breast and not raw chicken breasy