What this calculator works out
This calculator works BMI backwards. Instead of turning your weight into a BMI figure, it takes your height and a pair of BMI limits, and returns the weight range those limits correspond to in kilograms.
The BMI limits are editable, which matters: the standard 18.5 to 24.9 range is not appropriate for everyone, and lower thresholds are recommended for several population groups.
Enter your details
Before you set the limits
- The default limits of 18.5 and 24.9 are the standard adult range used across the NHS.
- Lower limits are recommended for people of South Asian, Chinese, Black African and African-Caribbean family background — commonly around 18.5 to 23.
- The range does not apply to children, during pregnancy, or to people with high muscle mass.
- Use height in centimetres. The calculator converts to metres itself.
The output is a span of weights, often 15 to 20 kg wide, and every point within it sits inside the same category. There is no single correct weight within the range, and moving from one end to the other changes nothing about which category you are in.
How this calculator works
The BMI formula rearranged to solve for weight:
Lower weight = lower BMI × (height in metres)²
Upper weight = upper BMI × (height in metres)²
Midpoint = (lower + upper) ÷ 2Because the calculation squares the height, the range widens as height increases. A range that spans 15 kg at 160 cm spans nearly 20 kg at 180 cm, which is a direct consequence of the arithmetic rather than anything about bodies.
Worked example: 175 cm
Using the default figures — 175 cm with the standard 18.5 to 24.9 BMI limits:
- Height squared: 1.75² = 3.0625
- Lower weight: 18.5 × 3.0625 = 56.7 kg
- Upper weight: 24.9 × 3.0625 = 76.3 kg
- Midpoint: 66.5 kg, across a range of 19.6 kg
Nearly twenty kilograms separate the ends of that range, and every weight between them falls in the same category. That width is the most useful thing this calculation produces, because it shows how coarse the underlying measure is. Change the upper limit to 23 for a South Asian background and the range becomes 56.7 to 70.4 kg — a materially different answer from the same height.
Common mistakes
- Treating the midpoint as a goal. It is the middle of a range, not a recommendation.
- Using standard limits regardless of background. Lower thresholds apply to several population groups.
- Applying it to children or during pregnancy. Different assessments are used in both cases.
- Entering height in metres. The field expects centimetres.
- Reading the range as a health assessment. It describes a BMI category and nothing else.
Frequently asked questions
What if I am outside the range?
It is a prompt to look further, not a verdict. BMI-derived ranges do not account for muscle mass, frame or body composition, and being slightly outside a population range tells you very little on its own. A GP can look at blood pressure, cholesterol, blood glucose and waist measurement together, which is a far more informative picture than a weight range.
Which limits should I use?
18.5 to 24.9 is the standard adult range. NHS guidance recommends lower thresholds for people of South Asian, Chinese, Black African and African-Caribbean family background, often given as 18.5 to 23. If you are unsure which applies, a GP or practice nurse can advise, and the fields are editable so you can use whichever figures you have been given.
Does this work for older adults?
Less well. Some evidence suggests a slightly higher BMI is not disadvantageous in later life, and unintentional weight loss in older adults is often a more pressing concern than being above a threshold. Frailty, muscle loss and nutrition matter more than a category at that stage.
Why is the range so wide?
Because BMI categories are broad by design — they were set to identify population-level risk, not to define an individual's ideal weight. A twenty-kilogram range at average height is a fair reflection of how much variation is compatible with the same category.
Is my height stored?
No. Everything you enter is processed in your browser and never transmitted or retained. If thinking about weight, food or body shape is causing you distress, these tools are unlikely to help and may make things harder. Beat, the UK eating disorder charity, runs a free helpline and online support, and a GP can refer you to specialist services.
Related tools
References
- NHS — healthy weight ranges and the recommended thresholds by ethnicity
- National Institute for Health and Care Excellence — guidance on identifying and managing overweight and obesity
Sources are checked at publication and can change — how I choose and check references.