Intermittent Fasting Calculator & Timer
Your eating window for 16:8, 5:2 and more, plus a fasting timer with history.
Safety check
Please speak to your doctor before fasting
Your plan
- Eating
- Fasting
- Asleep (fasting)
Fasting timer
History
| Started | Ended | Length | Remove |
|---|
For general information only, not medical advice. Consult a qualified health professional for advice about your situation.
About the Intermittent Fasting Calculator & Timer
Choose a fasting pattern — 12:12, 14:10, 16:8, 18:6, 20:4 or one meal a day, or the weekly 5:2 and alternate-day plans — enter when you usually wake up and go to bed, and see your eating window and fasting window laid out on a 24-hour clock, with how long before bed your last meal is. For 5:2 and alternate-day fasting it marks your fasting days for the next two weeks and works out the 20–25% of your usual energy that research protocols allow on those days.
A fasting timer counts how long you have fasted against your target and keeps a history of finished fasts — on this device only, nothing is uploaded. You can add your eating window or fasting days to your own calendar as an .ics file.
A short safety check comes first: fasting is not for everyone. If you are pregnant or breastfeeding, under 18, have had an eating disorder, or take insulin or sulfonylurea tablets for diabetes, do not start fasting without advice from your doctor.
How to use it
- Answer the safety check. If any of it applies, read the advice and speak to your doctor before you start.
- Pick a pattern. For a daily pattern such as 16:8, enter your usual wake-up time and bedtime and choose where the eating window goes — earlier, in the middle of your day, later, or from a first-meal time you choose.
- For 5:2, tick your two fasting days (not next to each other); for alternate-day fasting, choose your first fasting day. Enter your usual daily calories to see the 20–25% target for fasting days — the TDEE calculator can estimate them.
- Download the plan to your calendar if you like (.ics works with Google, Apple and Outlook calendars).
- When you start a fast, press Start fast (or set the time you actually started). Press End fast when you eat: the fast is saved to your history, which you can download as CSV.
Examples
Eat 11:00–19:00, fast 19:00 → 11:00 next day · last meal 4 h before bed · 8 h of the fast while awake
Eat 12:00–20:00, fast until 12:00 the next day
Monday and Thursday: about 400–500 kcal (20–25%); usual eating on the other 5 days
Meal 14:30–15:30, fast 23 hours
The patterns
Research reviews group intermittent fasting into three kinds (Patterson & Sears, Annual Review of Nutrition 2017):
- Time-restricted eating — usual eating within a set window each day, which creates a regular, longer overnight fast. The ratio is hours fasting : hours eating, so 16:8 is a 16-hour fast and an 8-hour window.
- Modified fasting — eating about 20–25% of your energy needs on scheduled fasting days. This is the basis of the 5:2 diet: severe energy restriction on 2 non-consecutive days a week and usual eating on the other 5.
- Complete alternate-day fasting — no food or drinks with energy on fasting days, alternating with usual-eating days. The same review notes reports of extreme hunger, which may make it hard to keep up.
Where the eating window sits is your choice; the review also describes evidence for the hypothesis that eating patterns that reduce night-time eating and lengthen the overnight fast may improve health.
What the evidence shows
A systematic review of 27 trials in people with overweight or obesity (Welton et al., Canadian Family Physician 2020) found weight loss of 0.8% to 13% of starting weight with no serious adverse events; 12 trials that compared intermittent fasting with ordinary calorie restriction found similar results. Most trials were small and lasted 2 to 26 weeks, so longer-term effects are less certain. For a wider review of the research on fasting and health, see de Cabo and Mattson, New England Journal of Medicine 2019.
Who should not fast without medical advice
- Pregnant, trying to conceive or breastfeeding: energy needs are higher (ICMR-NIN adds about 350 kcal a day in pregnancy and 520–600 kcal a day while breastfeeding), and the NHS says pregnancy is not the time to go on a diet.
- Under 18: still growing; the trials in the reviews above were in adults.
- An eating disorder now or in the past: strict food rules can feed it. The NHS lists “very strict habits or routines around food” among the signs of an eating disorder.
- Diabetes treated with insulin or sulfonylureas (for example gliclazide or glimepiride): the NHS says low blood sugar is common with insulin and can happen with gliclazide and glimepiride, and that missing or delaying meals makes it more likely; US labels for sulfonylureas say hypoglycaemia is more likely when calorie intake is deficient. Fasting needs a plan from your diabetes team, who may change your doses. The ADA Standards of Care in Diabetes—2026 (section 6) cover hypoglycaemia.
- Other conditions, underweight, or medicines that must be taken with food: ask your doctor or pharmacist first.
Sources
- Patterson RE, Sears DD. Metabolic effects of intermittent fasting. Annu Rev Nutr 37:371-93
- Welton S et al. Intermittent fasting and weight loss: systematic review. Can Fam Physician 66:117-25
- de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med 381:2541-51
- NHS: Low blood sugar (hypoglycaemia), Weight gain in pregnancy, Eating disorders
- US FDA prescribing information for glimepiride tablets, section 5.1 Hypoglycemia (DailyMed)
- American Diabetes Association. Standards of Care in Diabetes—2026, section 6. Diabetes Care 49(Suppl 1)
- ICMR-NIN, Nutrient Requirements for Indians (2020, revised short report 2024), energy for pregnancy and lactation
Limitations
- A planner and timer, not a diet: it does not tell you what to eat or set a calorie goal for usual-eating days.
- Window placements (1 hour after waking, mid-day, 3 hours before bed) are planning choices, not medical rules.
- The timer and history are stored in this browser only; clearing site data, private browsing or another device starts again.
- The calendar file uses your device’s local time; change the events in your calendar if you travel across time zones.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
What is 16:8 intermittent fasting?
You eat within an 8-hour window each day and fast for the other 16 hours, for example eating between 11:00 and 19:00. Much of the fast falls while you are asleep.
What can I drink while fasting?
Drinks without energy: water, and plain black coffee or tea without milk or sugar (USDA lists brewed coffee and black tea at about 1 kcal per 100 g). Milk, juice and sugary or alcoholic drinks break a complete fast.
How many calories can I eat on a 5:2 fasting day?
Research protocols for modified fasting allow about 20–25% of your usual energy needs on fasting days — about 400–500 kcal if you usually need 2,000 kcal a day.
Is intermittent fasting better than an ordinary diet for losing weight?
In a 2020 systematic review, the 12 trials that compared intermittent fasting with ordinary calorie restriction found similar results. Choose the pattern you can keep to; the trials were mostly small and short.
Who should not do intermittent fasting?
Do not start without medical advice if you are pregnant, trying to conceive or breastfeeding, under 18, have had an eating disorder, or take insulin or sulfonylurea tablets for diabetes — fasting with these medicines can cause dangerously low blood sugar.
Does the timer keep running if I close the page?
Yes. The start time is saved in your browser, so when you come back the timer shows the time since you started. Nothing is sent to a server.