Weigh the cat once a week, on the same scale, at the same time of day, and write the number down. Compare it with the weight from 30 days earlier, and treat a drop of 5 percent or more as a vet visit. Everything else is detail: 1 measurement a week, 1 comparison over 30 days, 1 line at 5 percent. It catches kidney disease, hyperthyroidism, diabetes and dental pain months before a cat looks thin.
Why weekly, not daily
A 4.5 kg cat swings 60 to 100 g across a single day: a 70 g meal, 200 ml of water, a full bladder and a litter box visit all move the scale. Daily weighing measures lunch, not the cat. Weekly weighing averages that noise out, and 4 points make a line you can read.
Weekly is also the shortest interval a home scale can resolve honestly. On a scale with 100 g error, a 50 g change per week is invisible, but a 4-week trend of 250 g is not. If you want to see change sooner than 30 days, you need a better scale, not more weighing.
The 3 scales, and what each one lies about
Baby or kitchen scale, flat tray. Resolution 1 to 5 g, real accuracy about 5 g for a cat under 8 kg. This is the best home option and the cheapest, 20 to 40 USD, and the cost tables list what a clinic scale visit adds to the bill. Put a plastic box on it, tare it, and the cat will often sit in it without a fight.
Electronic human scale, cat in arms. The error doubles because you weigh twice and subtract: 2 readings of plus or minus 100 g give a difference accurate to about 150 to 200 g. That is 4 percent of a 4.5 kg cat, which is the same size as the entire 5 percent line. Use it only if nothing else is available, and take 3 readings and average.
The clinic floor scale. Plus or minus 10 to 20 g, and the number goes straight into the record. Ask the vet to weigh the cat at every visit, even a vaccine appointment, and ask for the number in kilograms to 2 decimals rather than a comment about the cat looking well.
The 60-second protocol
Same day of the week, same hour, before the first meal, after the litter box. Weigh 3 times and take the median, not the average, because one bad reading of a moving cat can be 200 g out. Write the date, the weight to 2 decimals, and the scale you used, on the same sheet as a 1-line-a-day log like the care calendar format. If the cat is weighed in a carrier, subtract the carrier weight once and write it on the carrier.
Two habits kill the data: changing scales without noting it, and weighing after a meal. A cat that eats 70 g of wet food weighs 70 g more for about 2 hours, which is 1.6 percent of its body weight and enough to fake a trend in either direction.
The 5 percent math, in 4 steps
Step 1: today's weight, the median of 3 readings. Step 2: the entry closest to 30 days ago, within 27 to 33 days. Step 3: change in percent equals today minus the older weight, divided by the older weight, times 100. The denominator is always the older number, not today's. Step 4: compare the result with minus 5 percent.
Worked example. Thirty days ago the cat was 4.50 kg. Today it is 4.20 kg. The difference is minus 0.30 kg, divided by 4.50, which is minus 6.7 percent. That is past the 5 percent line, so it is a vet call within 7 days, and the cat should be weighed again in 7 days to see whether it is still falling.
For a 4.5 kg cat, 5 percent is 225 g, so the line sits at 4.27 kg. The arithmetic is worth doing on paper once, because most owners guess that 300 g is nothing on a 4.5 kg cat, and it is 6.7 percent.
Two scores that go on the same page
Body condition score on the 1 to 9 scale, where 5 is ideal, and muscle condition score: mild, moderate or severe loss over the spine and hips. A cat can hold its weight while losing muscle, and that reads as a flat line with a body score climbing from 5 to 7. Breed care calendars such as the Levkoy calendar usually print a target weight band, which makes a useful second reference for a purebred, with the 30-day 5 percent rule as the one that actually triggers a call.
Reading the curve, not the point
Plot weight in kg on the vertical axis with a 0.1 kg grid, and weeks on the horizontal. Draw the 5 percent band 30 days back as a shaded strip, and plot a 3-point moving average as a second line. A single point outside the band is a prompt to reweigh; 2 consecutive points outside is a trend; 3 is a reason to book.
Look at the slope, not the level. A cat that has always been 3.2 kg is not thin, and a cat that dropped from 4.8 to 4.5 kg in 6 weeks has a problem even though 4.5 kg sounds normal. The same 5 percent rule applies upward: a gain of 5 percent in 30 days on a cat that is not a growing kitten means too much food, or fluid.
When the number becomes a vet visit
Minus 5 percent in 30 days: book within 7 days. Minus 10 percent at any interval: within 48 hours, because that is the range where kidney disease, diabetes, hyperthyroidism and cancer all live. A gain of 10 percent in 30 days, vomiting more than 2 times in a week, or a cat that eats well and still loses weight: same week.
Senior cats over 11 years deserve monthly weighing, because 1 kg lost over 6 months is easy to miss on a fluffy cat and hard to miss on a chart. Kittens go the other way: weigh weekly and expect 50 to 100 g a week, with a stall of 2 weeks worth a call.
What to change when the line moves
A confirmed 5 percent drop with no other sign: reweigh in 7 days, check the teeth and the water bowl, and count calories for a week before changing anything. A confirmed drop with a raised water intake is a blood panel, not a diet change. For a gain, cut the daily ration by 10 percent and reweigh in 14 days; cutting by 30 percent at once causes most cats to beg and most owners to give in.
Take the chart to the clinic. One page of 12 weeks of numbers does more than a description of appetite, and it costs 12 minutes of your time in total.
| Weight, kg | 5% line, g | 10% line, g | Normal weekly swing, g | Reweigh interval if flagged |
| 3.0 | 150 | 300 | 30-60 | 7 days |
| 3.5 | 175 | 350 | 35-70 | 7 days |
| 4.0 | 200 | 400 | 40-80 | 7 days |
| 4.5 | 225 | 450 | 45-90 | 7 days |
| 5.5 | 275 | 550 | 55-110 | 48 hours if 10% |
| 6.5 | 325 | 650 | 65-130 | 48 hours if 10% |
Frequently asked
What does the 30-day 5 percent line actually mean?
Compare today's weight with the reading closest to 30 days ago, divide the difference by the older weight, and multiply by 100. A result of minus 5 percent or worse means a vet visit within 7 days, because that is where early kidney disease, hyperthyroidism and dental pain show up. For a 4.5 kg cat the line sits at 4.27 kg, which is only 225 g.
Which scale should I buy for weighing a cat at home?
A baby scale or kitchen scale with a flat tray and 1 to 5 g resolution, around 20 to 40 dollars, beats everything else for a cat under 8 kg. Weighing the cat in your arms on a human scale doubles the error to roughly 150 to 200 g, which is the same size as the whole 5 percent line on a 4.5 kg cat. Weigh 3 times and use the median.
My cat's weight went down 300 g. Is that serious?
On a 4.5 kg cat, 300 g is 6.7 percent, which is past the 30-day line, so it is worth a vet visit within a week and a reweigh in 7 days. On a 6.5 kg cat, 300 g is 4.6 percent, which means reweigh rather than book, unless water intake is up or the cat is eating less. The percentage matters, not the grams.