Fasted FASTED

Extended Fast Planner

An hour-by-hour plan for a 24, 48 or 72-hour water fast — phases, electrolyte checkpoints, sleep and training, and the part most people skip: how to eat again. It asks who you are before it plans anything, and it runs as a live water fasting timer once you start the clock.

Before you start: Extended fasting is not for everyone, and this planner will not draw a plan for you if you say it is not for you. Do not fast beyond a day without talking to a clinician if you have diabetes or take any medication for blood sugar, take medication that must be swallowed with food, are pregnant or breastfeeding, are under 18, or have any history of an eating disorder. Stop a fast and eat if you feel faint, confused, or your heart is racing or irregular. No plan on this page goes past 72 hours: past that point a fast belongs under medical supervision, not on a website.
Step 1 of 2 · the safety check

Does any of this apply to you?

Tick anything that is true. Every item here is a reason to talk to a doctor before you skip meals for a day or more, and if you tick one this planner will not draw you a schedule. That is the whole point of asking first.

Your answers are never saved — not to a server, not to this browser. Only a plan you generate is kept, and only on this device.

Tick all that apply

Answer the screen to continue.

Step 2 of 2 · the plan

How long, and when do you start?

Length of fast

No plan here goes past 72 hours. Past that a fast belongs under daily medical supervision, not on a website — the reassuring safety data on multi-day fasting comes from clinics that monitored people every day, and it says nothing about doing the same thing alone.

Your fast

The shape of it

The bands are this site's own fasting zones, laid over the hours you chose. Hatched stretches are the nights.

These are population estimates, not measurements. How fast you reach each zone depends on your last meal, your activity, your body composition and how often you fast.

Hour by hour

Breaking it: the part that actually goes wrong

    Eating heavily after a long stretch of little or no food is the setting in which refeeding problems occur: insulin returns, phosphate, potassium and magnesium move out of the blood and into cells, and the shortfall can affect the heart and the nerves. NICE names little or no intake for more than five days as a risk factor on its own, so a 72-hour fast is under that line rather than over it — but the mechanism is the same one, and it is the reason no plan here ends with “eat whatever you want”.

    Read the full piece on refeeding syndrome before your first meal, and use the break-a-fast tool to put actual food on the plan.

    Stop the fast and eat

    A fast is not something to see through at any cost. Ending one early is a decision, not a failure.

    • Feeling faint, or the room going grey when you stand up
    • Confusion, or trouble following a conversation
    • A racing or irregular heartbeat, or chest pain
    • Repeated vomiting, or being unable to keep water down
    • A headache or cramps that electrolytes and water do not touch
    • Shaking, sweating, or the cold-and-clammy feeling of low blood sugar

    Eat something first — something small. If it does not settle, or you have chest pain or cannot stand, get emergency help.

    Saved on this device only: the length you chose and when you started. Your answers to the safety screen were never written down anywhere. Clear your browser data and the plan is gone.

    Why this planner asks first

    Almost every extended-fast plan on the internet is a table of hours. You arrive, you read it, and nothing on the page has the faintest idea who is reading. That is fine for a healthy adult skipping a day of meals. It is not fine for someone on insulin, or a sulfonylurea, or a drug that has to go down with food, because for them the failure mode is not disappointing results — it is hypoglycemia, fainting, or a heart rhythm that needs a hospital. So this page asks the six questions first, and if one of them lands it gives you a clinician rather than a chart.

    The same reasoning sets the ceiling. Plenty of people fast for a week or longer, and there is published observational data suggesting they mostly come through it well: Wilhelmi de Toledo and colleagues followed 1,422 people through fasts of four to twenty-one days and reported no serious adverse events. Read the method, though, and the finding is narrower than the headline — every one of those fasts ran inside a clinic, with daily medical checks and staff watching for exactly the problems that make long fasts risky. It is evidence that supervised fasting is well tolerated. It is not evidence that unsupervised fasting is, and nobody has produced that second study. So the cap here is 72 hours, and the reason is stated rather than dressed up as physiology.

    What the plan does draw is honest arithmetic. The metabolic zones are the ones this site already uses on its fasting zones chart — glycogen thinning out through the first half-day, fat and ketones taking over, autophagy thought to climb after that — laid over the hours you picked and your own clock. Those boundaries are population estimates. Anton and colleagues describe the switch to fat and ketone fuel arriving somewhere between twelve and thirty-six hours depending on the person and their last meal, which is a wide enough window that any page giving you a precise hour is guessing.

    The electrolyte checkpoints are there because they are the difference between a long fast that feels manageable and one that feels awful. When insulin falls, the kidneys hold on to less sodium, and water follows it out. The headache on the second afternoon, the cramp at night, the light-headedness on standing: those are usually minerals, not hunger, and salt fixes them faster than willpower does. The checkpoints say when; the electrolyte calculator says how much.

    And every plan ends in a refeeding section, which is the part people skip and the part that hurts. After two or three days without food the gut has gone quiet and a big meal is the fastest route to feeling wretched. The serious version of the same problem is refeeding syndrome: when eating restarts, insulin drives phosphate, potassium and magnesium out of the blood, and the shortfall can reach the heart. NICE treats more than five days of little or no intake as a risk factor by itself, so a 72-hour fast sits under that threshold rather than over it. The mechanism does not switch on at a line, though, which is why the last screen of every plan here is about food rather than about finishing.

    The written guides sit alongside this: extended fasting for the overview, then 24 hours, 48 hours and 72 hours for the length you picked, and refeeding syndrome before you break it. For running the clock itself, the best intermittent fasting app in 2026 comparison covers the field, ours included, along with what it does not do.

    How this is worked out

    The timeline is the site's own fasting-zone model — glycogen depletion, the shift to fat and ketone fuel, and the point where autophagy is thought to rise — laid over the hours you choose. These are population estimates, not measurements: how fast you get there depends on your last meal, your activity, your body composition and how often you fast. Electrolyte checkpoints are placed because insulin falling during a fast makes the kidneys excrete more sodium and water, which is why headaches and cramps on a long fast are so often mineral loss rather than hunger. The 72-hour ceiling is a deliberate cap rather than a physiological limit. The observational safety data on longer fasts comes from programmes with daily medical supervision, so it cannot be read as evidence that an unsupervised fast of that length is safe, and this planner will not draw one. Every plan ends in a refeeding section for the same reason: refeeding problems are a recognised hazard of eating again after a long period of little or no food, and NICE identifies little or no intake for more than five days as a risk factor on its own.

    Sources

    1. 1. Nutrition support for adults: oral nutrition support, enteral tube feeding and parenteral nutrition (CG32) — National Institute for Health and Care Excellence (NICE)
    2. 2. Refeeding Syndrome (StatPearls) — NCBI Bookshelf, National Library of Medicine
    3. 3. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects — Wilhelmi de Toledo et al., PLOS ONE, 2019 (PubMed Central)
    4. 4. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting — Anton et al., Obesity, 2018 (PubMed Central)

    Nothing on this site is medically reviewed — it cites its sources so you can check them. Read our editorial policy.

    Frequently Asked Questions

    Is a 72-hour fast safe?
    For some healthy adults, and not for others, and the honest answer starts with who you are rather than with the number. A 72-hour fast is not safe without medical supervision if you have diabetes or take anything that lowers blood sugar, take medication that has to go down with food, are pregnant or breastfeeding, are under 18, or have any history of an eating disorder. This planner asks all of that before it will draw anything, and if any of it applies it stops and tells you to talk to a clinician instead. Even when nothing applies, longer is not automatically better — a 72-hour fast is not three times as useful as a 24-hour one.
    Why does the planner stop at 72 hours?
    Because past that point a fast belongs under medical supervision, not on a website. The reassuring safety data on multi-day fasting comes from programmes with daily clinical monitoring, and it cannot be read as evidence that doing the same thing alone at home is safe. Rather than draw a plan we cannot stand behind, we cap it and say why.
    Do I need electrolytes on an extended fast?
    Almost certainly, and this is the part people skip. When insulin falls during a fast the kidneys excrete more sodium and water, which is why headaches, cramps, dizziness and flatness on a long fast are so often mineral loss rather than hunger. The plan places checkpoints through each day and links the electrolyte calculator for the amounts.
    How should I break a 48 or 72-hour fast?
    Slowly, and the plan treats this as part of the fast rather than the end of it. After a long fast the gut has quietened down and a large meal is the fastest route to feeling awful. More seriously, eating heavily after a long stretch of little or no food is the setting in which refeeding problems occur — NICE names little or no intake for more than five days as a risk factor on its own. Every plan here ends with a refeeding section, and none of them ends with "eat whatever you want".
    Is there a water fasting timer on this page?
    Yes — the planner is one. A water fast is a fast where only water, and usually electrolytes, go in, which is exactly what every plan here assumes: the schedule it draws for 24, 48 or 72 hours is a water fast timed hour by hour, with the electrolyte checkpoints marked. Start the clock and the page counts down against that plan and tells you which phase you are in. For an everyday 16 or 18-hour window rather than a multi-day one, the fasting timer is the simpler tool.
    Can I train during an extended fast?
    Light movement is usually fine. Heavy or high-intensity training deep into a multi-day fast is not a good trade: performance drops, and the risk of feeling faint rises exactly when you are least able to judge it. The plan places any training early, before glycogen is gone, and marks the hours where you should be doing nothing more strenuous than a walk.

    Go deeper

    science · 11 min read

    Refeeding Syndrome: What It Is and Who Is Actually at Risk

    Refeeding syndrome explained: the electrolyte mechanism, the NICE criteria that decide who is at high risk, and an honest answer on whether a 48-hour fast puts you there.

    fasting methods · 8 min read

    36-Hour Fast: Benefits, Safety, and How to Do It

    A complete guide to 36-hour fasting: what happens in your body, the proven benefits, safety precautions, and step-by-step instructions for your first extended fast.

    nutrition · 7 min read

    Electrolytes During Fasting: Do You Need Them?

    Do you need electrolytes during fasting? Learn which minerals matter, how much you need, and how to prevent headaches, cramps, and fatigue while fasting.

    More from The Lab

    Fasting Electrolyte Calculator

    Calculate your exact electrolyte needs while fasting. Get daily sodium, potassium, and magnesium targets based on your weight, fast duration, and activity level.

    How to Break a Fast

    Get a personalised plan for breaking your fast safely. Enter your fast duration, goal, and dietary needs to see the best foods, timing, and what to avoid.

    Fasting Zone Tracker

    Interactive fasting zones chart showing what happens to your body at every hour. See when fat burning, ketosis, and autophagy begin.

    Track a long fast hour by hour in the Fasted app

    The Fasted app runs the timer, shows the metabolic phase you are in, and reminds you about electrolytes — the three things that make a long fast go well or badly.

    Download on the App Store Get it on Google Play

    Free to use — no account required