Weigh an adult cat once a month on a 5 g scale, log 4 numbers, and read the result as 3 trend tiers instead of a single dot. A 4 kg cat that drops 200 g in 30 days has lost 5 percent of its body weight, the line where most vets want to see the cat.
One reading is noise, 12 readings are a curve
Weight at 7 a.m. and at 7 p.m. are different cats on paper. A meal adds 30 to 60 g, water 100 to 200 mL, a full bladder 30 to 80 g, litter in the coat 10 to 20 g, so the daily swing reaches 40 to 120 g, or 1 to 3 percent of a 4 kg body. Against that noise a 5 g scale is not overkill: 5 g is 0.125 percent of 4,000 g, the resolution needed to see a drift before it becomes a diagnosis.
Twelve monthly points make a year of data, and 3 points is the minimum for a slope. A 2 percent drift, 80 g on a 4 kg cat, shows up within 3 months, while a 50 g gap between 2 months sits inside normal variation and earns a 14-day recheck, not a decision.
Same 5 g scale, same slot, 3 readings
A kitchen scale with 1 g steps and a 5 kg capacity is cheap but has a small platform, so weigh the empty carrier, then the cat in it, and subtract. A baby scale with a 5 g step and a 20 kg platform is steadier for a cat that stands still.
Routine beats equipment: before the first meal, after the litter box, same scale, same room, 3 readings with the middle one kept. Expect plus or minus 10 g of drift between 2 units, so 500 mL of water should read 500 g and 1 kg of rice 1,000 g. Check monthly, because a scale that drifts 30 g turns a flat cat into a losing one.
Log 4 numbers every month
Weight alone cannot separate muscle from fat or food from fluid; 4 fields make the number interpretable later.
| Field | Unit and precision | Why it earns a column | How often |
|---|---|---|---|
| Weight | grams, 5 g step, 3 readings | the trend line itself | 1 time a month, same slot |
| Date and time | day, hour | spot a 7 a.m. versus 9 p.m. shift of 40 to 80 g | every entry |
| Food intake | grams per day, or kcal | separates eating less from absorbing less | 3 days around the weigh-in |
| Body condition score | 1 to 9 scale, 0.5 steps | catches fat gain and muscle loss at stable weight | 1 time a month, hands on ribs |
Three trend tiers, three responses
Tier the change against the previous month, then against 90 days, as a percentage of current weight so small and large cats compare.
| Tier | Change in 30 days | Change in 90 days | Action | Vet window |
|---|---|---|---|---|
| Flat | within 2 percent, 80 g on 4 kg | within 3 percent | keep the routine, reweigh in 30 days | annual exam |
| Slow loss | 2 to 5 percent, 80 to 200 g | 5 to 10 percent | reweigh in 14 days, check teeth and food uptake | book within 3 weeks |
| Fast loss | over 5 percent, over 200 g | 10 percent or more | weigh weekly, photograph the rib area | seen within 72 hours |
| Rising | over 5 percent, over 200 g | 10 percent or more | measure the food, cut 10 percent, reweigh in 21 days | book within 4 weeks |
Flat is the target for an adult cat at a body condition score of 5 out of 9. Slow loss is the tier that gets ignored, because a cat that loses 100 g a month still looks normal in month 2 and thin in month 6. Fast loss is the tier that gets a workup: dental disease, chronic kidney disease, hyperthyroidism, diabetes, inflammatory bowel disease, and lymphoma all start as a 200 g drop with a normal appetite.
Why 5 percent is the clinic line
Unintentional loss of 5 percent in 30 days, or 10 percent over 6 months, is the threshold most clinics treat as worth a visit; a 10 percent loss in 90 days usually buys blood work, a urine sample, blood pressure, and a dental check. On the other side, 1 kg over ideal on a 4.5 kg cat is 22 percent overweight, and that is where diabetes and joint disease risk climbs. A score of 5 out of 9 means ribs under a thin fat layer and a waist seen from above; 7 out of 9 means no waist and a fat pad you can grab.
Turn the trend into grams per day
200 g in 30 days is 6.7 g a day, and 80 g in 30 days is 2.7 g a day. Both numbers are far below the 40 to 120 g daily swing, which is the whole argument for monthly averaging: the signal exists, but only after the noise is smoothed.
Food math works from the other end. Dry food runs 3.5 to 4 kcal per gram, so 10 g less kibble a day is 35 to 40 kcal, about 15 percent of the 200 to 250 kcal a 4 kg cat needs. A 20 percent cut, 40 to 50 kcal a day, is a realistic plan for an overweight cat; 250 to 500 g a month, or 1 to 2 percent of body weight a week, is the pace most vets aim for.
What the scale cannot see
Fluid, a litter-filled colon, a pregnancy from week 4, and a tumor all add grams that are not muscle: a cat can lose 300 g of muscle and gain 300 g of fat and weigh the same. That is why the 4 fields matter. Kittens are the opposite case, weighed daily on the same 5 g scale, gaining 10 to 15 g a day, with 2 flat days in a kitten under 8 weeks treated as urgent.
When one month already matters
Call now if the cat has lost 5 percent in 30 days while eating normally, has lost 10 percent in 90 days, or has dropped weight with a rising thirst. Normal water intake is roughly 40 to 60 mL per kg a day; over 100 mL per kg a day is polydipsic and needs blood work and a urine sample. Add vomiting more than 2 times a week, 3 days of poor eating, or a new pot-belly shape, and the visit moves to this week. Never use human weight-loss products; several are toxic at a single tablet.
This week: 5 steps
- Buy or borrow a scale with a 5 g step, then verify it with 500 g of water and 1 kg of rice.
- Pick one slot, before breakfast and after the litter box, and weigh 3 times, keeping the middle value.
- Start the log with 4 columns: grams, date, food grams, body condition score.
- Backfill the last 6 months, because 6 points already show a slope.
- Set the next weigh-in 30 days out, and a 14-day recheck if the first 2 entries differ by more than 80 g.
For the other side of the same problem, see obesity and weight loss and the vaccine titer guide; for human medicines that should never reach a cat, use the toxicity checker.
Frequently asked
My cat weighs the same every month but looks thinner. What is missing?
Weight hides body composition. A cat can lose 300 g of muscle and gain 300 g of fat and read identical on a 5 g scale, so add a 1 to 9 body condition score and a rib check to the same monthly session. Losing rib cover while weight holds is the pattern that deserves blood work, because muscle loss shows up there 2 to 3 months before the scale moves.
How accurate does a home scale need to be for a 4 kg cat?
A 5 g step is enough, and 1 g is better, because 5 g is 0.125 percent of 4,000 g. Verify the unit monthly with 500 mL of water and a 1 kg bag of rice, weigh 3 times and keep the middle reading, and expect plus or minus 10 g of drift between 2 scales. Anything under 50 g of change belongs in the noise, not in a decision.
Should I weigh a cat weekly instead of monthly?
Weekly is right while a cat is on a weight-loss plan, recovering from illness, or over 12 years old, and monthly is enough for a healthy adult at a stable body condition score. Weekly numbers need a 3-reading median and a 7-day average, since a single week can move 80 g on food and water alone. Keep the same slot and scale, or the trend becomes an artifact of timing.