I want to measure the weight of my cat in order to see if he is healthy according to his BMI. So I want to know how can I measure the weight. What is the correct way?
If you're concerned about healthy weight (not medication), the cat's weight in pounds or kilograms is less important than the amount of fat they're carrying.
According to Pets.WebMD, and the image shown below, the shape of the cat is a good gauge of your cat's healthy weight:
“Cats should have that hourglass figure when you’re looking down at them, they shouldn’t have a saggy belly hanging down, and you should be able to feel their ribs,” [Melissa Mustillo, DVM] says.
If your cat is overly fluffy or otherwise hard to visually assess, its weight can also be determined by feeling the spine and ribs, as described on Purina's "Assessing Your Cat's Body Condition" page:
Ideal Cat Weight(5)
A well-proportioned body – you should be able to see a waist behind the ribs; ribs can still be felt but with a slight fat covering. There will be a small paunch of fat on the abdomen. This level is a healthy weight for a cat.
In an underweight cat, "[r]ibs can be felt with a minimal fat covering" at the least severe level, down to "[r]ibs will visible on shorthaired cats, and they will not have any obvious fat." If the cat is overweight, that ranges from "[r]ibs can be felt but are covered with a slight excess fat covering" up to "[r]ibs and lumbar area are hidden under a heavy covering of fat, and heavy fat deposits are also present on the face and limbs."
It may take some practice and discussions with your veterinarian to get the hang of checking your cat's weight, but as it becomes routine to feel along it's ribs and spine, you'll begin to notice shifts in your cat's weight.
Note that the "saggy belly" may not always be the best metric for determining the weight of a cat; while it is present in overweight cats, it is often also present in cats who are not and have never been overweight, as addressed in the question "Why does my female cat stomach area hang so low?"
The "average" weight for a domestic cat is about 10 lbs, however, this varies depending on breed and build. Anecdotally, of my two adult cats, one would be underweight at 10 lbs, while the other starts edging into overweight at the same size.
If you have concerns about your cat's weight, before making any changes to his or her diet, you should consult with your veterinarian to be sure the cat is indeed over- or under-weight and develop a plan to get your cat to the ideal weight for his or her size.
I use a pet carrier and a digital fish scale.
With the cat, small dog, rabbit, etc in the pet carrier weigh with a digital fish scale. Hang the carrier by the handle on the fish scale hook.
Let the animal out of the carrier and weigh the pet carrier.
IMHO This gives a more accurate weight for medications.
Related Putting a cat into a carrier
When we had a failing cat that we needed to put weight on, we actually bought a digital deli meat scale on eBay and that way we could accurately track his weight gain/loss. It works really well as long as the cat doesn't move around too much. And as I like to joke with our healthy cats, as a benefit, if the cat gets to be too much trouble, this method is accurate enough to determine how much I'd get if I ground up the cat and sold him for dog food.
For cats that squirm too much (especially kittens) a new approach we've taken is to use our hand-held luggage scale. Normally you attach this to the handle of a piece of luggage and lift it up and the scale registers the weight of the luggage. In this case, you can simply use a reusable cloth shopping bag. Plop the cat in the bag and lightly tie the handles so the cat can't escape, and weigh the bundle with the luggage scale. To be most accurate, you'd want to tare the scale with just the bag then weigh the cat, but to be honest the shopping bags we use are so flimsy they don't really have any weight to speak of.
Locate a bathroom scale that is continuous reading (either mechanical, medical or externally powered type) and place it under the litter box. If your cat is happy with you snooping when doing its business then peek at the scale and wait for the cat to jump out and determine the difference.
If you cannot find a bathroom scale that is continuous reading you can probably get a mechanical fish or bag scale that you could suspend the litter box from but your cat may get a bit irritated with you for making their toilet shaky and take feline revenge by not using it so use only as a last resort.
If you cat prefers privacy when in the toilet you can use a Go-Pro or other cheap clone recording/time-lapse digital camera to record the scale display for a day and then look for the min and max value before a visit.
As an added bonus you can vicariously monitor how large the number one and two are to monitor eating habits and general condition of your cat. You can monitor multiple cats continuously if you want.
Obviously you can locate a scale that has a built in data-logger or communications port and it will save you the trouble of watching boring cat toilet videos.