You enter 5 ft 9 in (1.75 m) and 170 lb (77.1 kg) into a BMI calculator. It says 25.2 and labels you "overweight". The day before, the scale read 168.2 lb (76.3 kg), which is 24.9 and counts as healthy. The limit stayed where it was. Your weight moved by 1.8 lb (0.8 kg).
For adults, a healthy BMI runs from 18.5 to under 25. You calculate BMI as weight in kilograms divided by height in metres squared. At 1.75 m, the healthy range therefore lies between 56.7 and 76.6 kg (125 to 169 lb).
The BMI classes for adults
The CDC sorts adult BMI into four classes. The WHO puts overweight at a BMI of 25 or more and obesity at 30 or more.
| Class | BMI |
|---|---|
| Underweight | below 18.5 |
| Healthy weight | 18.5 to below 25 |
| Overweight | 25 to below 30 |
| Obesity | 30 or more |
The limits are fixed lines on a continuous scale. A person at BMI 24.9 hardly differs from one at 25.1, yet the label changes. Treat the classes as a rough grid. They apply to adults and say nothing about children or teenagers, who are judged by other standards.
What the limits mean in kilos
You can turn the formula around: the lower and upper limit come from 18.5 and 25 times height squared. For four heights, that looks like this:
| Height | Healthy range (BMI 18.5 to below 25) | Obesity from (BMI 30) |
|---|---|---|
| 1.60 m (5 ft 3 in) | 47.4 to below 64.0 kg (104 to 141 lb) | 76.8 kg (169 lb) |
| 1.70 m (5 ft 7 in) | 53.5 to below 72.3 kg (118 to 159 lb) | 86.7 kg (191 lb) |
| 1.80 m (5 ft 11 in) | 59.9 to below 81.0 kg (132 to 179 lb) | 97.2 kg (214 lb) |
| 1.90 m (6 ft 3 in) | 66.8 to below 90.3 kg (147 to 199 lb) | 108.3 kg (239 lb) |
Ten centimetres of extra height push the upper limit up by about 8 to 9 kg (18 to 20 lb). The band is wide: at 1.75 m it spans nearly 20 kg (44 lb). Your goal weight has plenty of room inside it, and it pays to use that room on purpose.
An example: at 1.75 m and 82 kg (181 lb), BMI is 26.8. The upper limit of 76.6 kg is 5.4 kg (12 lb) away. Take that number as a guide for your goal. Whether it suits your body is something to clear with a doctor if you are unsure.
What BMI does not see
BMI knows two numbers, height and weight. Everything else is missing.
Muscle mass. Muscle is denser than fat. Someone who lifts weights regularly can sit in the overweight range with little body fat. Someone with little muscle can sit in the healthy range and carry a high share of fat.
Age. Over the years, the ratio of muscle to fat shifts at the same weight. The same number describes a different body at 25 and at 65.
Fat distribution. Two people with the same BMI carry fat in different places. Waist circumference adds to the single number. Trendcurve reads body fat and waist from Apple Health when you switch these measurements on, and body measurements are a Pro feature.
In the end, a doctor decides whether a weight is healthy for you. That applies above all below 18.5, with existing conditions, and if you have or had an eating disorder: talk to a doctor before you set a goal weight. Trendcurve is not a medical device.
Reading the limit in your trend
Back to the number at the start. With a height of 1.75 m and an upper limit of 76.6 kg, one week could look like this: 77.1, 76.3, 77.4, 76.0, 76.8, 77.2 and 76.5 kg.
Three days sit below the limit and four above it. Each single value would send you into the green zone once and the orange zone once. The average of the seven days is 76.8 kg, 0.2 kg above the limit. That number answers where you stand. Why your weight fluctuates daily explains why the daily values scatter like this.
Trendcurve smooths that scatter into a trend weight. Trend weight instead of the daily value explains how. You then compare the trend with your upper limit and leave the daily values aside. In the app, the BMI scale sits in the Analysis tab in the "Body" section and is part of the free version. It needs your height, which you enter in the settings.
When you create a goal, Trendcurve also guards you against a goal weight in the underweight range.
Treat the limit as crossed only when the trend holds below it for several days. An example for the moment of entry: your trend stands at 76.8 kg and falls by 0.4 kg per week. It reaches 76.6 kg in the middle of the first week. If the trend stands at 79.0 kg, that is 2.4 kg (5.3 lb), or about 6 weeks. A single morning at 76.3 kg does not change this calculation. The forecast with a target date is a Pro feature.
Placing your goal inside the band
A goal exactly on the upper limit feels like success when the scale shows 76.5 kg. Two days later it tips back to 77.0 kg and the success is gone. Plan a buffer instead.
At 1.75 m, a goal weight of 75 kg (165 lb) gives a BMI of 24.5. You sit 1.6 kg (3.5 lb) below the limit, and normal daily swings do not lift you over it. From a trend of 76.8 kg that is 1.8 kg, or 4 to 5 weeks at 0.4 kg per week. Which rate fits is covered in How fast should you lose weight?.
The lower limit of 56.7 kg at 1.75 m is a warning line and no goal. Anyone landing near it needs no further loss.
After the goal comes maintenance. In maintain mode, Trendcurve draws a corridor around your goal weight, by default plus and minus 1.5 percent. At 75 kg it runs from 73.9 to 76.1 kg and stays entirely inside the healthy range. The band in the chart and the status are free, while corridor history and early warning belong to Pro.
Work out your upper limit today: 25 times your height in metres squared. Set your goal 1 to 2 kg (2 to 4 lb) below it and from now on compare only your trend weight with that line.