A good way to check if you're a healthy weight is to use the Body Mass Index (BMI). Use our healthy weight calculator to learn your BMI, and find helpful information and advice.
The BMI ranges for adults
People come in different shapes and sizes. For adults, a range of BMI scores indicate that you're a healthy weight.
A BMI above the ideal range indicates that you're heavier than is healthy for your height.
The ranges below only apply to adults. BMI results are interpreted differently for children.

• BMI below 18.5: a score this low means that you may be underweight. There are a number of possible reasons for this. Your GP can help you find out more, and achieve a healthy weight.You can learn more by reading Nutrition for underweight adults.
• BMI 18.5-24.9: this is a healthy range. It shows that you're a healthy weight for your height. However, it's still important to eat a healthy, balanced diet and include physical activity in your daily life if you want to maintain a healthy weight.
• BMI 25.0 or more: your BMI is above the ideal range. This means that you're heavier than is healthy for someone of your height. Excess weight can put you at increased risk of heart disease, stroke and type 2 diabetes. It’s time to take action. See the section below, and learn more by reading Lose weight. 

The next step
If your BMI shows that you're overweight it's time to take action. There’s lots of information, advice and support on NHS Choices that can help you.

• Lose weight has information and advice on achieving a healthy weight.
• Good food contains information and advice on healthy eating.
• Fitness gives you information and advice on physical activity.

You can also talk to your GP or practice nurse. They can offer advice on lifestyle changes, and may refer you to a weight loss group or discuss other treatments.
They may measure your waist circumference. This can provide further information on your risk of certain health conditions, such as type 2 diabetes and heart disease. You can learn more by reading Why body shape matters.
For children who are overweight, a GP or practice nurse can give advice and support on helping your child achieve a healthy weight as they grow.
Lose weight
For adults, losing even a little excess weight has health benefits. You’ll lower your risk of serious health problems such as heart disease, stroke, high blood pressure and type 2 diabetes.
Weight loss can also improve back and joint pain. Most people feel better when they lose excess weight.
For adults and children, the key is to make small, long-lasting changes to your lifestyle. That means a healthy, balanced diet and regular physical activity.
To start with, you can cut down on excess calories by swapping high-calorie meals and snacks for healthier alternatives. Read Healthy food swaps to learn more.
Physical activity is an important part of losing weight. Adults should get at least 30 minutes of physical activity, five days a week. For children, it’s 60 minutes, five days a week.
Sport is a great way to get active, but physical activity doesn’t have to mean spending hours on a treadmill. Instead, think about practical ways that you and your children can fit physical activity into your daily routine, such as walking to school or the shops.
For more ideas on how to get you and your family active, visit Change4Life.

source[http://www.nhs.uk/Livewell/loseweight/Pages/BodyMassIndex.aspx]