BMI Weight Loss Calculator
Enter your height, current weight, and a target BMI to instantly see your target weight and exactly how much you need to lose or gain. The calculator also shows your current BMI category, BMI Prime, and a week-by-week timeline to your goal based on a safe, sustainable rate. Switch between metric and imperial units and set your own target BMI or choose a category.
What this calculator does
This calculator solves two problems at once. First it computes your current BMI from your height and weight. Then it runs the BMI formula in reverse: given a target BMI you choose, it calculates the exact body weight that corresponds to that BMI for your height. The difference between your current weight and that target weight is how much you need to lose or gain. Enter a weekly rate and the calculator adds a timeline showing how many weeks it will take at a safe, sustainable pace.
How the reverse BMI formula works
The standard BMI formula is BMI = weight (kg) / height (m)². Rearranging it gives: target weight (kg) = target BMI x height (m)². For example, if you are 175 cm tall and want a BMI of 22, your target weight is 22 x 1.75² = 22 x 3.0625 = 67.4 kg. If you currently weigh 85 kg, you need to lose 85 minus 67.4 = 17.6 kg. In imperial units the calculator converts internally and shows results in pounds.
Choosing a realistic target BMI
A target BMI somewhere in the middle of the healthy band, around 21-23, gives a sensible buffer. A BMI of exactly 24.9 is healthy but leaves little margin if weight fluctuates. Targets below 18.5 fall in the underweight range and carry their own health risks, including bone loss and reduced immune function. Targets above 25 are overweight by the WHO definition but may be appropriate short-term milestones for people who are starting from a higher BMI. The most important target is the one you can sustain, not the lowest possible number.
Safe rates of weight change and the timeline
Most clinical guidelines recommend losing no more than 0.5 to 1 kg (1 to 2 lb) per week, which roughly corresponds to a daily energy deficit of 500 to 1,000 kcal. Faster rates increase the proportion of weight lost from muscle rather than fat. Very low calorie diets (below 800 kcal/day) should only be undertaken with medical supervision. For weight gain, 0.25 to 0.5 kg per week is typical for someone adding lean mass through resistance training with a modest calorie surplus. The weekly rate you enter here is used only for the timeline; the calculator does not assume a specific diet.
BMI Prime: a quick check on where you stand
BMI Prime is your BMI divided by 25, the upper limit of the WHO healthy band. A value below 1.0 means you are within the healthy range. A value of 1.20 means you are 20 percent above the upper limit. The same scale applies to your target: a target BMI of 22 gives a BMI Prime of 0.88, while a target BMI of 27 gives a BMI Prime of 1.08. BMI Prime makes it easy to see at a glance how far above or below the healthy threshold you are, without having to remember the numeric cut-offs.
Limitations of BMI-based weight goals
BMI cannot distinguish muscle from fat, so athletes and people with high muscle mass will appear heavier than their body composition warrants. It also ignores fat distribution: two people with identical BMIs can have very different health profiles if one carries fat around the abdomen and the other around the hips. Older adults often lose muscle as they age, which can make BMI underestimate cardiovascular risk. Use this calculator as a starting point, and consider pairing it with waist circumference, waist-to-height ratio, or a body-fat estimate for a fuller picture.
BMI categories and weight classification
| BMI range (kg/m²) | Category | Health risk |
|---|---|---|
| Below 18.5 | Underweight | Increased |
| 18.5-24.9 | Healthy weight | Lowest |
| 25.0-29.9 | Overweight | Increased |
| 30.0-34.9 | Obesity class I | High |
| 35.0-39.9 | Obesity class II | Very high |
| 40.0 and above | Obesity class III | Extremely high |
World Health Organization adult cut-offs for ages 20 and over. Children and teenagers use age- and sex-specific percentile charts instead.
Frequently asked questions
How do I calculate how much weight to lose to reach a certain BMI?
Rearrange the BMI formula: target weight = target BMI x height (m)². Subtract your current weight from the result to get the weight you need to lose (or gain). For example, a person who is 170 cm tall and wants a BMI of 23 needs to weigh 23 x 1.70² = 23 x 2.89 = 66.5 kg. If they currently weigh 80 kg, they need to lose 13.5 kg. This calculator does that arithmetic automatically in metric or imperial.
What target BMI should I aim for?
For most adults a target somewhere in the middle of the healthy band, say 21 to 23, is practical. It leaves room for normal weight fluctuations without tipping outside the healthy range. If your current BMI is 35 or above, an intermediate target such as 30 or 27 is often more achievable and still delivers meaningful health benefits. Avoid targets below 18.5 (underweight) without medical guidance.
How long will it take to reach my target BMI?
At 0.5 kg/week, losing 10 kg takes about 20 weeks. At 0.25 kg/week the same journey takes 40 weeks. Enter your own weekly rate in the calculator and it will compute the exact number of weeks for your personal weight change. Clinical guidelines recommend 0.5-1 kg/week as a safe maximum for most people.
Is a BMI of 25 considered overweight?
Yes, by the WHO definition a BMI of 25.0 or above is classified as overweight (25.0-29.9) and 30.0 or above is obesity. However, BMI is a population-level screening tool, not a clinical diagnosis. A BMI of 25-26 in a muscular person may carry no excess health risk, while a BMI of 24 in a sedentary person with central obesity may be a concern. The number should be interpreted in context.
Why does the Asian-Pacific cut-off matter?
Research has shown that people of South and East Asian descent tend to have higher body-fat percentages and greater cardiovascular risk at a given BMI compared with people of European descent. Several health authorities therefore recommend a lower healthy-weight upper limit of 23 rather than 25 for this group. Selecting the Asian-Pacific option in the calculator uses BMI 18.5-23 as the healthy band when computing your healthy weight range and BMI Prime.
Does this calculator work for children and teenagers?
No. The fixed adult BMI cut-offs used here do not apply to people under 20. Children and teenagers are assessed using age- and sex-specific BMI percentile charts because their body proportions change as they grow. Use a dedicated child or adolescent BMI percentile tool for anyone under 20.
What is BMI Prime and how is it different from BMI?
BMI Prime is simply your BMI divided by 25. It rescales BMI so that 1.0 is exactly the top of the healthy range. A BMI Prime below 1.0 is healthy; 1.10 means 10 percent above the upper limit. The same calculation applies to any target BMI you choose, so you can instantly see how far a goal falls relative to the healthy band without memorising the raw thresholds.