You can't... (unless you put it in a full bath and measure the displacement, but lets not get ahead fo oursleves...)
If your dog does not like the scales, you could always hold your dog and stand on them, then subtract your weight.
There is no true way to weigh a cat without a scale. You can try picking him up and comparing his weight to other things.
I would say just buy a normal scale and do it that way. Or just take it to the vet and let them do it.
Use a piece of string - if the cat will stay still long enough for you to run it the length of the body. That way you can measure it properly against a ruler.
I would measure it with a ruler.
You don't measure with length. You measure the length of an object with a ruler. You would use inches or metres as your units.
a ruler
a ruler!
A ruler
A ruler :/
A ruler.
A yardstick or ruler!
A ruler would do the trick.
With a ruler, or a measuring tape.
You would use a ruler and measure in millimeters or centimeters.
ruler