How to Lower Your BMI: The Numbers Behind the Goal

BMI is your weight in kilograms divided by your height in meters squared, so for an adult whose height is fixed, the only way to lower BMI is to lower body weight. This article does the arithmetic: how many kilograms equal one BMI point at your height, how much change a given BMI target needs, and how long that takes at a pace the CDC describes. It does not prescribe a diet or exercise plan, which should come from a clinician or registered dietitian who knows your health.

How much weight is one BMI point?

One BMI point equals your height in meters squared, in kilograms. At 1.70 m that is 1.70 x 1.70 = 2.89 kg per point, so a 5-point drop takes 14.45 kg. Taller people need more weight change per point, which is why the same BMI target means a different number of kilograms at each height. Conversely, 1 kg changes BMI by 0.44 at 150 cm, 0.35 at 170 cm and 0.28 at 190 cm, so day-to-day scale fluctuations barely move it.

The CDC adult categories used here are: underweight below 18.5, healthy weight 18.5 to less than 25, overweight 25 to less than 30, and obesity 30 or greater. Reaching BMI 25.0 is the boundary, so getting just inside the healthy weight range takes a little more than the table shows. The calculator's own BMI formula gives the same result: 86.7 kg at 170 cm returns a BMI of 30.0.

What pace does the CDC describe?

The CDC says people who lose weight at a gradual, steady pace of about 1 to 2 pounds a week are more likely to keep the weight off. The two pace columns above use those figures purely as arithmetic. At 175 cm, 1 pound a week is about 1.97 kg a month, or 0.64 BMI points a month; 2 pounds a week is about 3.94 kg a month, or 1.29 points. A straight line is a simplification, since weight change in real life is uneven.

The CDC also gives a modest example: a 5% weight reduction for a person who weighs 200 pounds is 10 pounds, and this change could lower the risk of some chronic diseases such as heart disease, prediabetes and type 2 diabetes. A 5% loss from BMI 30 lands at BMI 28.5 regardless of height, so a meaningful health change can happen well before a BMI category changes.

What a BMI number cannot tell you

• BMI is weight relative to height, not body composition. The CDC describes it as a screening measure that should be considered with other factors when assessing an individual's health. A lower BMI may come from fat, muscle, water or a mix.
• The adult categories apply to ages 20 and over in CDC guidance and 18 and over in the calculator. Children and teens are assessed differently, so use a pediatrician's advice, not this table.
• A BMI of 25 or more is not a diagnosis, and a BMI in the healthy range does not rule out health concerns. Waist measurements and a clinician's assessment add information.
• If your BMI is already in the healthy range or below it, lowering it is not an automatic goal. Below 18.5 is classed underweight.

Common mistakes with BMI targets

• Treating BMI 25.0 as a finish line. The category boundary is a convention, and health depends on more than one number.
• Expecting a smooth, linear drop. The table assumes a constant pace, which real life rarely follows.
• Ignoring height in goal-setting. A 14 kg change means very different BMI shifts for a 150 cm and a 190 cm person.
• Comparing yourself with a table built for another height. Use the Calculator with your own measurements.
• Chasing a fast result. The CDC's pace guidance links gradual change with keeping weight off.
• Using BMI alone for athletes, older adults or anyone with a lot of muscle, where it can mislead.

Worked check with the calculator

Enter 170 cm and 86.7 kg: BMI is 30.0. Enter 72.2 kg instead (about 14.5 kg lighter): BMI is 25.0, since 25 x 2.89 = 72.25. For a 5% reduction, 86.7 x 0.95 = 82.4 kg gives a BMI of 28.5. These are just numbers on a formula; whether to change your weight, and how, is a question for you and your clinician.