Medical disclaimer
Last reviewed: 19 October 2026
Free Fasting Calc does not provide medical advice. The content, calculators and guides on this site are for general information and education only. They are not a substitute for professional medical advice, diagnosis or treatment. Never disregard professional advice, or delay seeking it, because of something you read here.
Every result is a guide
Calculator results, eating-window times, calorie and protein figures, water amounts and every other number on this site are general estimates. They are a guide only, not a measurement of your body or a plan made for you. Consult a medical doctor about fasting, weight loss and any change to how you eat.
Talk to a professional first
Intermittent fasting affects everyone differently and is not safe for everyone. Speak to your doctor, pharmacist or a registered dietitian before you start fasting, change how you fast, or act on any information on this site. This matters most if you take medication or have a health condition.
Fasting is not appropriate if you
- are pregnant (intentional weight loss during pregnancy is not recommended) or trying to conceive;
- are breastfeeding, unless your midwife or doctor agrees;
- are under 18 (guidance for children and teenagers puts healthy habits ahead of weight and dieting);
- have a current or past eating disorder, or a difficult relationship with food;
- are underweight or have a history of malnutrition;
- have type 1 diabetes, or diabetes treated with insulin or other medicines that can cause low blood sugar (such as sulfonylureas);
- take medication that must be given with food, or that affects blood sugar, blood pressure or fluid balance;
- are recovering from illness, surgery, or are frail or elderly;
- have any condition your clinician has told you requires regular eating.
This list is not exhaustive. If you are unsure, that uncertainty is itself a good reason to ask a professional.
Never stop or delay medication for a fast
Do not skip, delay or change any prescribed medication to protect a fast. If a medicine needs to be taken with food, take it with food. Speak to your prescriber or pharmacist about how to fit medication around eating, never the other way around.
Stop and seek help urgently if you experience
Chest pain, fainting, confusion, a fast or irregular heartbeat, severe or persistent vomiting, or changes to your vision. These are reasons to break a fast immediately and seek medical help, not to push through. In an emergency, call your local emergency number (for example 999 in the UK, 911 in the US and Canada, 112 in the EU).
Signs of a stroke need an ambulance straight away: Face drooping, Arm weakness, Speech problems, Time to call your local emergency number.
Fasting longer than 24 hours
Talk to a doctor before any fast longer than 24 hours. After several days of eating little or nothing, restarting food needs medical care because of the risk of refeeding syndrome, which can be dangerous.
Eating-disorder safety
Fasting content sits close to eating-disorder risk. Tracking food, weight or fasting may be causing you anxiety, or breaking a fast may reliably turn into a binge. If so, or if your eating is affecting your mood or your relationships, please step back and talk to a doctor. The footer of every page links to a directory of free helplines in your country, and to support services in the UK and the US.
No guarantees
We make no promise of any particular health or weight outcome. We do not publish "lose X by Y" projections and we never present fasting as a cure for any condition. Results in the research vary, and so will yours.