Weight Loss Calculator Ireland

Weight Loss Calculator Ireland: what it calculates

This calculator turns a starting weight, height and target weight into a small set of transparent planning numbers:

• kilograms between your current and target weight;
• the target as a percentage of your starting body weight;
• current BMI and BMI at the target weight;
• reference weights after 5%, 10%, 15% and 20% loss;
• an optional timeline based on a weekly pace that you choose; and
• an optional waist-to-height ratio.

It is designed for adults as an educational planning tool. It does not diagnose obesity, prescribe a calorie intake, tell you whether you should lose weight, or decide whether you are eligible for Ozempic, Wegovy, Mounjaro or another medicine.

The HSE specifically notes that BMI alone does not measure health and cannot distinguish muscle from fat. The number should be treated as one screening measure alongside other clinical information.

How the percentage weight-loss calculation works

Percentage weight loss is calculated from your starting weight:

Percentage loss = (starting weight − target weight) ÷ starting weight × 100

For example, moving from 100 kg to 90 kg is a 10 kg reduction, equal to 10% of starting body weight.

The calculator also shows the weights that correspond to 5%, 10%, 15% and 20% reductions from the starting point. These are mathematical reference points, not predictions that a particular diet, medicine or programme will produce those results.

Why show 5%, 10%, 15% and 20% milestones?

Percentage change is often more useful than kilograms alone because the same 10 kg change represents very different proportions of body weight for different people.

Clinical guidelines and medicine trials frequently report outcomes using percentage change. For example, treatment reviews may use a 5% threshold in particular contexts, while modern obesity-medicine trials commonly report the proportion of participants reaching 5%, 10%, 15% or 20% reductions.

That does not mean 20% is a universal target, or that 5% is the right target for every person. Your health, body composition, medical history, medicines and personal priorities matter.

BMI: useful, but limited

BMI is calculated as:

BMI = weight in kilograms ÷ height in metres²

The HSE uses adult BMI categories as a population-level screening framework:

• below 18.5: underweight;
• 18.5 to 24.9: healthy weight;
• 25.0 to 29.9: overweight;
• 30 or above: obesity.

These categories do not diagnose an individual health problem. The HSE notes that BMI cannot distinguish muscle from fat. A muscular athlete can have a high BMI despite relatively low body fat.

Ethnic background can also affect the relationship between BMI and health risk. The HSE advises that some people, including people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, may develop weight-related health problems at lower BMI values.

For a deeper explanation, use the Compound BMI Calculator Ireland.

Why the calculator will not project a target below BMI 18.5

A calculator should not make an unsafe-looking number appear authoritative simply because the arithmetic works.

For that reason, this tool stops the calculation if the entered target would produce a BMI below 18.5. It also does not support planning further weight loss when the starting BMI is already below 18.5.

That is a guardrail for a general public tool, not a diagnosis. If weight change is medically indicated in a situation the calculator does not cover, use an appropriately qualified healthcare professional rather than forcing the scenario through a generic calculator.

Waist-to-height ratio

Advanced mode can calculate waist-to-height ratio:

Waist-to-height ratio = waist circumference ÷ height

The HSE says adults with a BMI below 35 can use this as another way to assess whether they may have excess body fat. A ratio of 0.5 or higher may indicate increased health risk.

The HSE also notes that waist-to-height ratio is less accurate when BMI is above 35.

To measure your waist, the HSE advises measuring midway between the lowest rib and the hip bone while breathing naturally. Use the same units for waist and height.

The timeline is arithmetic, not a forecast

Advanced mode asks for a scenario pace in kilograms per week.

The calculation is simply:

Weeks to target = kilograms to target ÷ kilograms per week

It does not model plateaus, water-weight changes, appetite, metabolism, medication response, illness, adherence or changes in physical activity.

The HSE’s obesity model of care has referenced consistent losses around 0.5 to 1 kg per week in weight-management support, while current HSE guidance also warns against fad diets and very rapid loss. Appropriate rates can differ substantially between people and may be slower.

That is why this calculator labels the field as a scenario, not a recommendation.

Weight loss is not only about the scale

The HSE describes obesity as a chronic condition with many causes. Treatment can include sleep, mental-health support, nutrition, physical activity, management of other conditions, medicines and, for some people, surgery.

A person’s health can improve without hitting a particular target weight. Blood pressure, blood glucose, fitness, strength, sleep, mobility and quality of life can all matter.

NICE’s current overweight and obesity guidance similarly emphasises an individualised, nutritionally balanced approach rather than one universal diet.

Can this calculator tell me if I qualify for Wegovy or Mounjaro?

No.

The HSE currently states that Wegovy may be prescribed to adults with a BMI of 30 or more, or 27 or more with at least one weight-related health problem. It gives the same broad adult BMI thresholds for tirzepatide (Mounjaro).

But a BMI threshold is only part of a clinical assessment. A prescriber may also consider medical history, other medicines, pregnancy plans, side effects, weight-related conditions and whether a medicine is appropriate for the individual.

The calculator therefore shows your BMI but deliberately does not display “eligible” or “not eligible”.

For the practical distinction between the products, read How to get Ozempic in Ireland and The rise of GLP-1 medicines in Ireland.

Ozempic is not the same as Wegovy

Both Ozempic and Wegovy contain semaglutide, but their authorised uses are not interchangeable.

The HSE describes Ozempic as a treatment for type 2 diabetes and Wegovy as a treatment for obesity. If your goal is weight management, a calculator should not convert a BMI number into a claim that Ozempic is appropriate for you.

Use the Irish GLP-1 price guide for current cost context and the nutrition needs calculator for a separate educational look at energy and protein estimates.

Worked example

Suppose an adult enters:

• current weight: 100 kg;
• height: 180 cm;
• target weight: 90 kg.

The tool calculates:

• weight to target: 10 kg;
• percentage reduction: 10%;
• current BMI: about 30.9;
• target BMI: about 27.8;
• 5% milestone: 95 kg;
• 10% milestone: 90 kg;
• 15% milestone: 85 kg;
• 20% milestone: 80 kg.

If Advanced mode uses a purely illustrative pace of 0.5 kg per week, the arithmetic timeline is 20 weeks. That is not a promise that an individual will lose exactly 0.5 kg every week.

Method and limitations

The calculator performs deterministic arithmetic in your browser. Compound does not receive the values you type into the calculator.

It does not account for:

• age-related body-composition differences;
• pregnancy or breastfeeding;
• eating disorders or disordered eating;
• muscle mass;
• fluid changes;
• medicines that affect weight;
• endocrine or metabolic conditions;
• disability or mobility limitations;
• ethnicity-specific BMI risk thresholds;
• previous weight history;
• individual response to diet, exercise or obesity medicines.

If you are under 18, pregnant, have an eating disorder or think you may have one, or have a condition affecting height, the HSE says standard adult BMI is not an appropriate tool.

Frequently asked questions

How do I calculate 10% weight loss?

Multiply your starting weight by 0.90. A person starting at 100 kg would be 90 kg after a 10% reduction.

What is 5% weight loss in kilograms?

Multiply your starting weight by 0.05 to find the kilograms represented by 5%. At 90 kg, 5% is 4.5 kg.

What is a healthy target weight?

There is no single calculator answer that is appropriate for everyone. BMI can provide a screening range, but health conditions, body composition, ethnicity, medicines and personal circumstances can change what is clinically relevant.

How long will it take me to lose weight?

The Advanced timeline divides the kilograms between your current and target weight by the weekly pace you enter. It is a mathematical scenario, not a prediction.

Does a BMI over 30 mean I need weight-loss medicine?

No. BMI can be one part of an assessment, but it does not determine treatment on its own. The HSE advises discussing obesity medicines with a healthcare professional.

Can I use this to see if I qualify for Ozempic?

No. HSE guidance describes Ozempic as a treatment for type 2 diabetes. The calculator does not assess diabetes or decide whether a prescription medicine is suitable.

Is waist-to-height ratio better than BMI?

They provide different information. The HSE says waist-to-height ratio can be used in adults with BMI below 35 as another check for excess body fat, while BMI remains a widely used screening measure.

Does Compound store my weight?

The calculator runs locally in your browser. The figures entered into the form are not submitted to Compound.

Useful sources

HSE — How obesity is diagnosed

HSE — Obesity treatment

HSE — Medicines to treat obesity

HSE — Model of Care for the Management of Overweight and Obesity

NICE NG246 — Overweight and obesity management