The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
3 Weeks in a Calorie Deficit But No Weight Loss — What's Happening?
If you've been maintaining a calorie deficit for 3 weeks with no weight loss, it's frustrating but not unusual. Weight loss isn't always linear, and several factors can mask fat loss temporarily. This guide explains why your scales might not be moving yet, when to worry, and what steps to take next. Understanding these patterns helps you stay on track with sustainable weight management in the UK.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Common Reasons for No Weight Loss in a Calorie Deficit
Water retention is the most common culprit. When you reduce calories, your body sometimes holds onto water as it adapts to change. This can completely offset fat loss on the scales for 2–4 weeks. Additionally, new or increased exercise causes muscle damage and inflammation, which triggers water retention to aid recovery — even if you're losing fat underneath.
- Measurement timing matters: Weigh yourself at the same time each day (morning, after toilet, before eating) and track weekly averages, not daily fluctuations.
- Sodium and carbohydrate intake affect water balance — higher intake can retain 1–2 kg of water temporarily.
- Hormonal cycle: Women may experience 1–2 kg water retention mid-cycle or before menstruation.
- Metabolism adaptation: After 3 weeks, your metabolic rate may slightly decrease as your body adjusts to fewer calories — though this effect is modest in the first month.
True fat loss may be happening even if scales don't show it yet.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
What to Do Next: Evidence-Based Steps
First, check your calorie counting using a food tracking app — portion sizes are often underestimated, especially oils, nuts, and dressings. The NHS recommends a deficit of 500 kcal daily for approximately 0.5 kg weight loss per week. If you're genuinely in deficit, give it another 2–3 weeks before making changes. Use our free BMI calculator and weight converter to confirm your current position and set realistic targets. Consider taking body measurements, progress photos, or how clothes fit — these often shift before scales do. If you've been in deficit for 6+ weeks with no weight loss, consult your GP to rule out thyroid or metabolic issues.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
How long should I wait before changing my diet if I'm not losing weight?
The NHS recommends giving any calorie deficit at least 4–6 weeks before significant adjustments. Water retention commonly masks fat loss for 2–4 weeks. If you're genuinely in a 500 kcal deficit and see no change after 6 weeks, consult your GP to rule out medical causes like thyroid dysfunction.
Can water retention hide 1 kg of fat loss?
Yes, absolutely. It's common to lose 1–2 kg of fat while retaining 1–2 kg of water, so scales show no change. This is especially likely if you've started exercising, increased sodium intake, or are in the luteal phase of your cycle. Tracking weekly averages rather than daily weights reveals the true pattern.
Should I try prescription weight loss medication if diet alone isn't working?
Prescription weight loss medications like GLP-1 injections are available in the UK for adults with a BMI of 30 or above, or 27+ with weight-related health conditions. These require assessment by a prescriber and are used alongside calorie reduction and exercise. Your GP can discuss whether you're eligible and refer you to specialist weight management services.
Is my metabolism broken if I'm not losing weight in a calorie deficit?
Metabolic adaptation (slight decrease in calorie burn) is real but modest — typically 5–10% after weeks of dieting. It's rarely the main cause of stalled weight loss in the first month. More likely causes are underestimated calorie intake, water retention, or measurement timing. True metabolic damage is extremely rare without severe restriction.
When should I contact my GP about weight loss problems?
Contact your GP if: you've been in a genuine calorie deficit (confirmed by tracking) for 6+ weeks with no weight loss; you're experiencing fatigue, hair loss, or irregular periods; you have a history of thyroid disease; or you're taking medications that might affect weight. Your GP can check for underlying conditions and refer you to NHS weight management services or specialist support.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.