How Much Weight Should I Lose

What this tool is actually doing

Notice what this calculator did not ask you. No age, no sex, no activity level, no diet, nothing about calories at all. Just your height and your weight. That tells you exactly what is going on under the bonnet: this is a BMI tool wearing a different hat.

It works out your Body Mass Index, looks up what the normal range would weigh for someone of your height, and subtracts. The answer comes back as two numbers, and the gap between them is wide. That range is where this page is going to spend most of its time, because it is the part people misread, and misreading it can turn a useful bit of arithmetic into a discouraging one.

The formula this calculator uses

BMI is your weight in kilograms divided by your height in metres, squared. Turn that around and you can work out what any BMI would weigh at your height:

Weight for a given BMI = BMI × height (m) × height (m)

The tool runs that twice, once at each edge of the normal range, to find the heaviest and lightest you could be while still counting as normal weight. Then it subtracts both from your current weight, which gives you two figures:

• The smaller number is the weight you would need to lose to drop just inside the normal range, at the upper edge.
• The larger number is what you would lose to arrive at the very bottom edge of normal.

If either figure comes out under 3 kilograms, the tool tells you that you do not need to lose weight, which is a sensible buffer rather than a precise verdict. It needs a height of at least 4 feet, and it reports in kilograms whichever unit you entered. The lower edge it uses is a BMI of 18.5, which is the line the WHO and CDC use, and the upper edge is 24.9.

A worked example

Take someone 170 cm tall weighing 90 kg. Their height is 1.70 m, so 1.70 squared is 2.89, and their BMI works out at about 31.1.

Now the two edges of the normal range at that height:

Upper edge: 24.9 × 2.89 ≈ 72 kg
Lower edge: 18.7 × 2.89 ≈ 54 kg

Subtract each from 90 kg and the tool reports that they should lose between 18 and 36 kg to be in the normal weight range.

Eighteen to thirty-six kilograms. Read as a single instruction, that is a daunting sentence to be handed by a web page, and it is very easy to look at the big number and quietly close the tab. So let us take it apart, because it means much less than it appears to.

Read the smaller number

Those two figures are not a target range. They are the two ends of a wide band, and only one of them is a goal.

The smaller number, 18 kg in the example, is the meaningful one. It is what it would take to step inside the healthy range. The larger number, 36 kg, is what it would take to travel all the way across that range and stop at the far edge, at a BMI of 18.7, which is a hair above the line where underweight begins. Nothing about the research says that edge is a better place to be than the other one. A BMI of 24 is not a worse outcome than a BMI of 19. They are both simply inside the range.

So the honest reading of "lose between 18 and 36 kg" is: 18 gets you there, and 36 is just where the arithmetic runs out. Anyone treating the bigger number as the real target has been misled by a subtraction. We would rather tell you that plainly than let a piece of arithmetic set you an unnecessary goal, and we think the tool would read better if it led with the smaller figure alone.

The number the research actually cares about

Now the more useful point, and the reason this page exists rather than just the calculator.

Even that smaller number is not really where the health benefit lives. The benefits of losing weight do not sit waiting for you at the BMI 24.9 line, ready to be collected only once you cross it. They arrive far earlier than that, and there is good evidence for exactly how much earlier.

A large study called Look AHEAD followed more than 5,000 adults with type 2 diabetes through a weight-loss programme and looked at what different amounts of loss actually achieved. The people who lost between 5 and 10 percent of their starting body weight, an average of around 7 kg, were substantially more likely to see clinically meaningful improvements across the board: better blood sugar control, lower blood pressure both systolic and diastolic, higher HDL cholesterol, and lower triglycerides. Losing more did deliver more, and the researchers were clear about that. But the striking finding was how much arrived with how little.

Put that next to the example. The tool tells our 90 kg person to lose 18 kg. Five to ten percent of their weight is 4.5 to 9 kg. That is a quarter to a half of what the calculator asked for, and it is the amount at which the measurable health improvements start showing up.

This is not a reason to ignore the tool. It is a reason to read it as a map rather than a demand. The BMI figure tells you roughly where the healthy range sits for your height, which is useful context. The 5 to 10 percent figure tells you where the first real milestone is, which is useful motivation, and it is a milestone that is actually within reach. Aim there first. If you get there and want to keep going, the map will still be waiting.

Where this tool is blunt

Two inputs buys you a fast answer and not much nuance. Be aware of what it cannot see:

• It cannot tell muscle from fat. This is BMI's oldest flaw. A well-muscled athlete can be told to lose weight they have no business losing, because the number only sees total mass. Our Body Fat Calculator will tell you more.
• It does not know your age, sex or ethnicity. The normal range it uses is one set of lines for every adult body. If you are of South or East Asian descent in particular, the risks associated with weight tend to appear at lower BMI values, which our BMI Calculator covers.
• It is for adults. Children and teenagers are read against growth charts for their age and sex, not fixed numbers, so use the Child BMI Calculator instead. It is not a measure for use in pregnancy either.
• It says nothing about how. It is a destination, not a route. Our Calorie Deficit Calculator handles the other half.
• It is not a diagnosis. Weight is one input into your health among many, alongside your waist, your blood work, your fitness and your history. If your number worries you, that is a conversation for a doctor, who can see all of those at once.

Questions people ask

Why does it give me two numbers?

Because the normal weight range is a range. The smaller figure gets you inside it; the larger one takes you to the far edge of it. Only the smaller one is a sensible goal.

The number looks impossible. Where do I start?

With 5 percent of your current weight. That is the milestone the research points to, it is a fraction of what the tool quotes, and it is where meaningful health improvements begin. Get there, then decide whether you want the next one.

It says I do not need to lose any weight, but I want to. What then?

That message means you are within about 3 kg of the healthy range for your height. Wanting to change how you look is a separate matter from health, and it is worth being honest with yourself about which one you are chasing. If the honest answer is a health one, this tool is already telling you the answer.

How fast should I lose it?

Slower than you would like. Around half a kilogram a week is the usual sensible pace, and our Calorie Deficit Calculator explains why faster tends to cost you muscle and rarely sticks.

References

Where the figures come from. The BMI formula and the normal weight range this tool works from are the standard adult definitions published by the CDC, which follow the World Health Organization. The finding that losing 5 to 10 percent of body weight produces clinically meaningful improvements in blood sugar, blood pressure and cholesterol comes from Wing and colleagues' analysis of the Look AHEAD study.

1. Wing RR, Lang W, Wadden TA, et al. Benefits of modest weight loss in improving cardiovascular risk factors in overweight and obese individuals with type 2 diabetes. Diabetes Care. 2011;34(7):1481-1486. https://pmc.ncbi.nlm.nih.gov/articles/PMC3120182/
2. Centers for Disease Control and Prevention. About Body Mass Index (BMI). https://www.cdc.gov/bmi/about/