Not medical advice
Free Fasting Calculator calculates fasting and eating windows, timers, and schedule options from times you provide; reverse diet calorie schedules from intakes you choose; and body-measurement estimates (ABSI and estimated visceral fat area) from measurements you enter. All of it is provided for informational, planning, and educational purposes only. Nothing on this site is medical advice, diagnosis, or treatment, and no output of any calculator should be read as a recommendation that fasting, a calorie change, or any interpretation of a body measurement is appropriate or safe for you personally.
Talk to a professional if fasting may be complicated for you
Fasting can be medically significant. In particular, consult a qualified healthcare professional before fasting if you have diabetes or take medication that affects blood glucose, are pregnant or breastfeeding, are under 18, have a current or past eating disorder, or have any other medical condition or take any medication where meal timing matters. Stop fasting and seek appropriate care if you feel unwell.
What our numbers mean (and don't)
Our calculators produce arithmetic, not physiology. We deliberately do not display hour-by-hour claims about autophagy, ketosis, "fat-burning stages", or predicted weight loss, because current human evidence does not support assigning such transitions to precise clock times for a given individual. The same principle governs the body-measurement tools: ABSI results are population-level statistical comparisons, and visceral fat results are anthropometric estimates from a published model, not CT or MRI measurements. They carry no diagnosis, no mortality prediction, and no claim to have measured the fat around your organs. Model performance can vary by population, and the reverse diet planner does not measure metabolic adaptation or predict weight outcomes. Where our pages discuss research, they aim to describe what studies actually found, including uncertainty.
Use at your own discretion
The site is provided "as is", free of charge, without warranties of any kind. You are responsible for decisions you make based on its output. To the maximum extent permitted by law, the operator is not liable for damages arising from use of the site.
Accuracy of your own inputs
Calculations are only as accurate as the times you enter, and locally stored data can be lost if your browser storage is cleared, so export backups from the history page if your records matter to you.
Tool by tool, what is and is not claimed
General disclaimers are easy to write and easy to ignore. This one is specific, because each calculator on this site has a different relationship to evidence and you deserve to know which is which before you act on a number.
| Tool | What it gives you | What it never claims |
|---|---|---|
| Fasting calculators and timer | Exact clock times from arithmetic you could do yourself | That any schedule suits you, or an hour when a metabolic process begins |
| Schedule planner | Whether a duration collides with your sleep, work or meals | A health score, or that one protocol beats another |
| Visceral fat calculator | An estimate in cm² from a CT-validated published model | A diagnosis, a level or rating, or a substitute for imaging |
| ABSI calculator | An index and its position in a published NHANES reference band | Mortality risk, life expectancy, or an individual prediction |
| Water intake calculator | A range bracketed by two published reference methods | A personal requirement, or that hitting it does anything specific |
| Reverse diet calculator | A week-by-week ramp between two numbers you supply | An optimal pace, a true maintenance figure, or regain risk |
| Weight loss percentage calculator | Arithmetic on two weights you already measured | Any prediction about future weight change |
| Fasting before a blood test | A stop-eating time from a duration your clinic gave you | How long your test requires, or which tests need fasting |
The pattern across that right-hand column is deliberate. Every one of those refusals is a thing competing tools do offer, and each is a place where producing a confident number would mean inventing it. We would rather hand you a smaller answer that is true.
When to stop reading and ask someone
There are situations where no calculator on this site is the right next step, and it is worth naming them rather than burying them in a paragraph of legal text. Speak to a qualified healthcare professional before fasting if you have diabetes or take medication affecting blood glucose, are pregnant or breastfeeding, have a current or past eating disorder, are under 18, take medication that must be taken with food, or are planning a fast beyond roughly 24 hours.
That list is not defensive boilerplate. Each item is a specific mechanism by which a fasting schedule interacts with something a web form cannot see, and in every case the person who can see it is a clinician who knows your history.