BMI Weight Loss Calculator
Find out exactly how much weight you need to lose (or gain) to reach any target BMI, with a realistic timeline estimate.
⚖️ What is a BMI Weight Loss Calculator?
A BMI Weight Loss Calculator translates a BMI target into a concrete weight goal by working backwards from the BMI formula. Instead of asking "what is my BMI?", it asks "what weight do I need to be at a given BMI?" The answer is simply Target BMI multiplied by height in metres squared. For a 170 cm person targeting a BMI of 22, the target weight is 22 x (1.70 x 1.70) = 22 x 2.89 = 63.6 kg. The weight to lose is the difference between the current weight and this target.
This calculator is useful in several practical scenarios. A person with a BMI of 30 who wants to move into the overweight category (below 30) can see exactly how many kilograms separate them from that threshold. Someone targeting the midpoint of the healthy range (BMI 22) can see their specific goal weight and a realistic timeline based on safe loss rates. And someone in the normal range who wants to understand where the boundaries are can use the Healthy Weight Range tab without entering their current weight at all.
The timeline estimate uses the widely accepted safe loss rate of 0.5 to 1 kg per week (approximately 1 to 2 lbs per week). The conservative estimate of 0.5 kg per week corresponds to roughly a 500 kcal daily deficit, achievable by a modest reduction in food intake combined with regular moderate exercise. The faster end of 1 kg per week requires a larger deficit of around 1,000 kcal per day, which is the upper limit recommended by the CDC and NHS for most adults. Rates significantly faster than this typically cause disproportionate muscle loss and metabolic adaptation.
It is worth noting that BMI is a population screening tool rather than a precise individual metric. A person with high muscle mass may have a BMI in the overweight range despite low body fat and excellent metabolic health. Conversely, an older adult with normal BMI may carry excess visceral fat not captured by the scale. For most people, however, BMI provides a useful and practical benchmark. Pairing a BMI target with waist circumference measurements (below 94 cm for men, below 80 cm for women) gives a more complete picture of abdominal fat reduction progress.
📐 Formula
To find the target weight for a given BMI, rearrange the formula:
Timeline estimate (per CDC and NHS safe loss guidelines):
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 - Moving from Obese to Overweight
Person: 175 cm, 100 kg, targeting BMI 29 (below obese threshold)
Example 2 - Reaching Normal BMI from Overweight
Person: 165 cm, 82 kg, targeting BMI 24.9 (top of normal range)
Example 3 - Finding Healthy Weight Range (Healthy Weight Range Mode)
Person: 180 cm, wants to know target weight range
❓ Frequently Asked Questions
🔗 Related Calculators
How much weight do I need to lose to reduce my BMI by 1 point?
Each BMI unit equals your height in metres squared in kilograms. For a 170 cm person, 1 BMI unit equals 1.70 squared = 2.89 kg. For a 180 cm person, 1 unit equals 3.24 kg. To reduce BMI by 3 units at 175 cm you would need to lose approximately 9.2 kg. Use this calculator to compute the exact weight difference for your height.
What is a realistic timeline to lose enough weight to reach a normal BMI?
At a safe rate of 0.5 to 1 kg per week, losing 10 kg takes 10 to 20 weeks (roughly 3 to 5 months). Losing 20 kg takes 20 to 40 weeks (5 to 10 months). Faster rates of loss increase the proportion of lean muscle lost. The CDC and NHS recommend no more than 1 to 1.5 kg per week, even for people with significant weight to lose.
What BMI should I aim for when trying to lose weight?
Most health organisations recommend targeting a BMI in the normal range of 18.5 to 24.9. A practical first goal is to reach the overweight threshold of 25 if you are currently in an obese category, then aim for 22 to 23 as a long-term healthy target. The midpoint of the normal range (around BMI 21.5) is associated with the lowest all-cause mortality in large population studies.
How is the target weight calculated from a target BMI?
The formula is: Target Weight (kg) = Target BMI x (Height in metres squared). For a person who is 170 cm tall targeting a BMI of 22: Target Weight = 22 x (1.70 x 1.70) = 22 x 2.89 = 63.6 kg. The weight to lose is simply the difference between your current weight and this target weight.
Is a BMI of 25 still healthy for someone with a lot of muscle?
BMI does not distinguish between fat and muscle mass, so muscular individuals often have inflated BMI readings. A person with significant muscle mass at BMI 25 to 27 may have excellent metabolic health. In these cases, body fat percentage (measured by DEXA, hydrostatic weighing, or Navy method) is a more informative metric than BMI alone. Waist circumference below 94 cm for men and 80 cm for women indicates low abdominal fat regardless of BMI.
How many calories do I need to cut to lose 1 BMI unit?
One BMI unit corresponds to roughly 2.5 to 3.5 kg for most adult heights. Since 1 kg of fat is approximately 7,700 kcal, losing one BMI unit requires a total calorie deficit of about 19,000 to 27,000 kcal. Spread over 4 to 6 weeks, this means a daily deficit of 600 to 800 kcal, achievable by combining modest dietary reduction with increased physical activity.
Does losing weight always lower BMI proportionally?
Yes. Since height does not change, BMI change is directly proportional to weight change. Losing 5 kg always reduces BMI by exactly 5 divided by (height in metres squared), regardless of your starting weight. This is a fixed mathematical relationship. A 175 cm person who loses 5 kg reduces BMI by exactly 5 / 3.0625 = 1.63 BMI units every time.
What is the healthy weight range for a 160 cm person?
For a 160 cm person, the normal BMI range of 18.5 to 24.9 corresponds to a weight range of 18.5 x (1.60 x 1.60) = 47.4 kg at the lower end and 24.9 x 2.56 = 63.7 kg at the upper end. The midpoint at BMI 22 is 56.3 kg. Use the Healthy Weight Range tab and enter 160 cm to see this automatically.
What is the difference between BMI and ideal body weight?
BMI is a population-level classification that uses a continuous scale with defined category cutoffs. Ideal body weight formulas (Devine, Hamwi, Robinson, Miller) give a single target weight based on height and sex. They generally correspond to a BMI of approximately 22 to 23. BMI is more widely used clinically today because it captures a healthy range rather than a single number and applies equally to men and women.
How accurate is the weight loss timeline estimate?
The timeline is an estimate based on a safe loss rate of 0.5 to 1 kg per week (or 1 to 2 lbs per week). Actual loss depends on calorie deficit, exercise level, metabolic rate, and other individual factors. The conservative end of the range (0.5 kg per week) reflects a moderate 500 kcal daily deficit. The faster end (1 kg per week) reflects a 1,000 kcal daily deficit, which is the typical upper limit recommended by NHS and CDC guidelines.
Should I try to reach a BMI below 18.5 to lose more weight?
No. A BMI below 18.5 is classified as underweight and is associated with increased risk of osteoporosis, immune suppression, anaemia, and hormonal disruption. The healthy target range is 18.5 to 24.9. People who reach a BMI below 18.5 through dieting are typically losing both fat and significant muscle mass, which is harmful to long-term metabolic health.
How does BMI weight loss apply differently for men and women?
The BMI formula is identical for men and women, but the health implications at the same BMI differ. Men typically have more lean mass, so a BMI of 25 in a man generally reflects less body fat than in a woman of the same age. However, the standard healthy range of 18.5 to 24.9 and the target weight calculation are the same. For context-specific assessment, pair this BMI tool with a body fat calculator.