BMI and weight loss before hip or knee replacement
Weight loss before hip or knee replacement can ease joint pain and help you get moving sooner after surgery. If you are waiting for an operation in Swansea or Neath Port Talbot, this page explains how to lose weight safely. It also gives you a free calorie planner and a meal checker to track what you eat.
At a glance
- Losing weight can reduce joint pain and improve how well you move, even before surgery.1
- Any weight loss helps. Losing 10% of your body weight is likely to help more than 5%.1
- Aim to lose weight slowly, at 0.5 kg to 1 kg (1 lb to 2 lb) a week.2
- At Swansea Bay University Health Board, planned hip and knee replacement is not carried out if your BMI is above 40.
- The calorie planner and meal checker below are free. No access code is needed until 3 January 2027.
Why weight matters before a joint replacement
Hips and knees carry your body weight with every step. Extra weight puts more load on a worn joint. That is why losing weight often eases pain, even while you wait.
NICE guidance for osteoarthritis says weight loss improves quality of life and physical function. It also reduces pain.1 Any amount of weight loss is likely to help.
Weight also matters for the operation itself. A large review of joint replacement studies found a higher risk of joint infection in people with a higher body mass index (BMI).4 The same review linked smoking and diabetes with a higher risk. All three can be improved before surgery.
National guidance on preparing for surgery describes waiting lists as "preparation lists".3 The weeks before your operation are a chance to get stronger and healthier. Small, steady changes add up.
BMI limit for hip and knee replacement in Swansea Bay
At Swansea Bay University Health Board, planned hip and knee replacement is not carried out if your body mass index (BMI) is above 40. If yours is above 40, the aim is to bring it down before surgery. The planner below shows how long that could take. Your surgical team will talk with you about your own risks and the support available.
Safe weight loss before hip or knee replacement
The goal is steady weight loss while keeping your strength. Crash diets are not a good idea before an operation. They can cost you muscle, which you need for walking and rehabilitation afterwards.
- Go steadily. The NHS suggests 0.5 kg to 1 kg (1 lb to 2 lb) a week.2
- Keep your protein up. Include lean meat, fish, eggs, dairy, beans or lentils at each meal. Protein helps protect muscle while you lose weight.
- Fill half the plate with vegetables. They add fibre and bulk for few calories.
- Watch the hidden extras. Cooking oil, butter, sauces and sugary drinks add up quickly.
- Stay as active as your joint allows. Swimming, an exercise bike and short, regular walks all count.
As a general guide, the NHS weight loss plan suggests about 1,900 kcal a day for men and 1,400 kcal a day for women.2 Your own figure depends on your height, weight, age and activity. The planner below works it out for you.
Check first if you take medicines for diabetes. Eating less can lower your blood sugar. Speak to your GP or diabetes nurse before you cut your calories, especially if you take insulin or tablets such as gliclazide.
Your BMI is already in the healthy range? You may not need to lose weight. Check with your GP or surgical team first.
Your calorie planner and meal checker
Use the planner to work out a daily calorie allowance and see how your weight could change over time. Then check your meals against it. Photograph a cooked meal, type a simple snack, or log a meal you eat often with one tap.
The meal checker is free. No access code is needed until 3 January 2027. After that, patients under Atanu Bhattacharjee's care receive a code at clinic or at their pre-operative assessment. The calorie planner never needs a code.
How it works
- Set your allowance. Enter your height, weight, age, sex and activity level in kg and cm, or in stone and feet. Choose a calorie deficit and a number of weeks. The planner shows your daily allowance and a week-by-week projection.
- Take a photo before you eat. Put the plate on the table, tap Take a photo and your camera opens. Then tap Analyse my meal. If you are asked for an access code, scan the QR code on your card or type the code in.
- Type simple foods. Use Type it for items such as "2 boiled eggs". Save meals you eat often and log them again with one tap.
- Track your day. Each meal shows calories, protein, carbohydrate and fat, and how much of your allowance is left today.
How the calorie allowance is worked out
The planner uses the Mifflin–St Jeor equation to estimate the energy your body uses at rest.5 It multiplies this by your activity level. Your chosen deficit is then taken away. The projection allows for weight loss slowing as you get lighter, so it does not promise a straight line.
| Safety limit | What the planner does |
|---|---|
| Lowest daily allowance | Never below 1,200 kcal for women or 1,500 kcal for men without medical supervision |
| Rate of loss | Warns if loss would be faster than 1 kg (2 lb) a week |
| Healthy weight | Sets no weight-loss target if your BMI is under 18.5, and asks you to check first if it is under 25 |
| Age | For adults aged 18 and over only |
Getting the most accurate meal estimate
- Photograph the whole plate or bowl from above, in good light.
- Leave your knife, fork or spoon on the plate. It helps judge portion size.
- Add a second photo from the side so the height of the food shows.
- Photograph any packet or nutrition label. The checker reads it.
- Type simple foods instead of photographing them. It is quick and accurate.
- Answer the short questions the checker asks, such as how much butter was used.
How accurate are photo estimates?
They are a guide, not a measurement. Even good estimates can be out by a fifth or more. Mixed dishes, sauces and cooking oil are the hardest to judge. That is why each result shows a likely range as well as a single figure. A side view, a visible label and your answers to the questions all narrow the range.
Your privacy
- Your height, weight, saved meals and daily log stay on your own phone. They are not sent to this website.
- Meal photos and descriptions are sent securely to an AI service to work out the estimate. They are not stored on this website.
- Access codes are not linked to your name.
Full privacy information for the meal checker
- Who is responsible: Atanu Bhattacharjee runs this website and the meal checker. Contact him through the contact page.
- What is sent: only the meal photo or description you choose to send, and any note you add. Do not include your name or other personal details in a photo or note.
- Who analyses it: Anthropic, the AI provider, on our behalf. It may process the data outside the UK under data protection safeguards. Under its commercial terms it does not use the data to train its AI.
- What this website keeps: no photos and no descriptions. To apply daily fair-use limits it keeps a count of meals checked against a scrambled form of your internet address, for that day only. If you use an access code, the code is linked to a scrambled phone identifier, not to your name.
- Why: to give you the estimate you asked for. You can stop at any time and clear your saved details with your browser settings.
- Your rights: you can ask what is held about you, or complain to the Information Commissioner's Office at ico.org.uk.
Who can use this
The calorie planner and meal checker are for adults aged 18 or over. Until 3 January 2027 anyone can use the meal checker. After that it is for patients under Atanu Bhattacharjee's care at Swansea Bay University Health Board. Planned hip and knee surgery is for patients referred within the health board, which serves Swansea and Neath Port Talbot in South Wales. Referral is through your GP or another clinician.
Get help straight away if you feel faint, dizzy or very unwell while cutting back on food. Call NHS 111 Wales on 111, or 999 in an emergency.
Frequently asked questions
What is the maximum BMI for hip or knee replacement?
At Swansea Bay University Health Board, planned hip and knee replacement is not carried out if your BMI is above 40. Other health boards and NHS areas set their own policies. If your BMI is above 40, the calorie planner on this page shows your projected BMI week by week, so you can see how long it could take to bring it down.
Do I have to lose weight before a hip or knee replacement?
If your BMI is above 40, yes. At Swansea Bay University Health Board, planned hip and knee replacement is not carried out above a BMI of 40. Below that, losing weight can still ease pain and help you get moving sooner after surgery. Your surgical team will discuss your own situation with you.
How much weight should I try to lose before surgery?
Any amount helps. NICE says losing 10% of your body weight is likely to help more than 5%.1 For someone weighing 100 kg, 5% is 5 kg and 10% is 10 kg. Aim for 0.5 kg to 1 kg a week.2
How many calories should I eat to lose weight?
It depends on your height, weight, age, sex and how active you are. The planner on this page works it out for you. As a general guide, the NHS weight loss plan suggests about 1,900 kcal a day for men and 1,400 kcal a day for women.2
Is it safe to diet before an operation?
Steady weight loss is safe for most people. Crash diets are not. They can cost you muscle and leave you less able to recover. Keep your protein up and stay active. If you take medicines for diabetes, speak to your GP or diabetes nurse first.
Should I keep losing weight after my operation?
In the first weeks after surgery your body needs energy and protein to heal. Focus on balanced meals with enough protein rather than strict dieting. Ask your surgical team when to restart a weight loss plan.
How does the photo meal checker work?
You photograph your plate. An AI service identifies each food, estimates the portion from the plate and cutlery size, and calculates calories, protein, carbohydrate and fat using UK food composition data. It shows a best estimate and a likely range.
How accurate is the meal checker?
It gives an estimate, not a measurement. Results can be out by a fifth or more, especially for mixed dishes and oils. Adding a side view, photographing labels and answering its questions all improve accuracy.
How do I get an access code?
No code is needed until 3 January 2027. After that, patients under Atanu Bhattacharjee's care receive a free code at clinic or at their pre-operative assessment. The code works on one phone. The calorie planner never needs a code.
Is my information stored?
Your details, saved meals and daily log stay on your phone. Meal photos and descriptions are sent securely to an AI service for analysis and are not stored on this website. Codes are not linked to your name. Full details are in the privacy section of this page.
I am worried about my eating. Where can I get help?
Talk to your GP. You can also contact Beat, the UK eating disorder charity, for confidential support. Please do not use the calorie planner if counting calories makes you anxious.
References
- National Institute for Health and Care Excellence. Osteoarthritis in over 16s: diagnosis and management (NG226), 2022. Recommendation 1.3.5. nice.org.uk/guidance/ng226. Accessed 5 October 2026.
- NHS. Weight loss plan: 12-week guide. assets.nhs.uk. Accessed 5 October 2026.
- Centre for Perioperative Care. Preoperative assessment and optimisation for adult surgery, June 2021. cpoc.org.uk. Accessed 5 October 2026.
- Kunutsor SK, Whitehouse MR, Blom AW, Beswick AD, INFORM Team. Patient-related risk factors for periprosthetic joint infection after total joint arthroplasty: a systematic review and meta-analysis. PLOS ONE 2016;11(3):e0150866. doi.org/10.1371/journal.pone.0150866.
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition 1990;51(2):241–247.
This page gives general information about weight and nutrition before hip or knee replacement. It does not replace advice from your own surgeon, GP, dietitian or diabetes team. The calorie planner and meal checker give estimates only.
