The filter thread size is usually displayed on the front or side of your lens next to this symbol Ø and will look something like Ø77 or 77mm.
Your lens filter size is the diameter of your lens measured in millimeters. But you don’t need to measure it, it’s usually printed on the front or side of your lens next to this Ø symbol. Alternatively, you can find your filter size on the back of your lens cap. Please note that the focal length (e.g 24mm) is not your filter size. You can also search for your filter size by entering the specific lens model and filter size. E.g. “Canon EF 24mm f1.4L USM II filter size” and you’ll see it’s 77mm.