The honest fasting toolkit
Know exactly
when you
eat again.
This intermittent fasting calculator shows when your eating window opens and closes. Enter your last meal and pick a schedule from 12:12 to one meal a day: you get a live countdown, where you are in the fast, and a calendar reminder. No sign-up and no ads.
Choose your protocol
You eat again at --:--
Enter your last meal to see your window.
Why this number? (show the maths)
Your fast starts when you finish your last meal and runs for the length of your protocol. The eating window is whatever is left of the 24 hours.
fast ends = last meal + fast hours
window closes = fast ends + (24 − fast hours)
So on 16:8, a meal finished at 8:00 PM means the fast ends at 12:00 PM the next day and the 8-hour window closes at 8:00 PM. Times adjust automatically across midnight.
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Add your body stats for calorie & protein targets +
Used only for the equation constants.
Daily calories
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Protein a day
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BMR
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Maintenance (TDEE)
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We won't show a target that low.
Your goal maths lands below a level we consider safe to recommend, so we're showing the floor instead. Very low intakes are hard to sustain and can do more harm than good. If losing weight matters to you, a smaller deficit you can actually keep beats a large one you can't. If food or eating feels distressing, please talk to someone — there are support links in the footer.
Why these numbers?
Calories use the Mifflin-St Jeor equation for BMR, which a 2005 review of four common equations found the most reliable, though any equation can be off for one person. It is multiplied by a common activity factor (not an official standard) for maintenance (TDEE). Fat loss shows maintenance minus 20%, which is our rule; guidelines more often suggest a deficit of 500 to 750 calories a day. Building muscle adds 10%, also our rule.
Protein: losing fat or building muscle assumes you exercise, so it shows 1.4–2.0 g per kg of body weight, the range sports-nutrition research gives for most people who exercise. "Maintain" shows 0.8 g per kg, the recommended minimum for adults. From 65, it shows at least 1.0–1.2 g per kg (1.2 or more if you are active), the range set out for older adults. The big number is the middle of the range.
These are population estimates, not a prescription. Your real needs vary. See our editorial policy for how we source this.
Where you are, hour by hour
Fasting is a state that changes as the hours pass. These stages are approximate and vary a lot between people — we label them the way the evidence actually supports, hedges included.
- 0–4 hFed
Digesting your last meal. Blood sugar and insulin are up, and glucose is the main fuel.
- 4–12 hEarly fasting
Insulin falls. Your liver releases stored sugar (glycogen) to keep blood sugar steady.
- 12–24 hMetabolic switch
As liver glycogen runs low, you burn more fat and start making ketones. In studies this happens somewhere between 12 and 36 hours.
- 24 h+Longer fast
Fat burning and ketones keep rising. Fasting past 24 hours is not right for everyone: talk to a doctor first.
The toolkit
The calculators most fasting sites have, plus tools for the questions they skip: what breaks a fast for your goal, shift work, your cycle, symptoms, and breaking a long fast safely.
Chooser
Which schedule suits me?
Six questions, one schedule and eating window to try.
Planner
5:2 and alternate-day planner
Pick your fast days and see the next four weeks.
Database
Does it break my fast?
60 foods, drinks and medicines, answered against your actual goal.
Planner
Shift-work planner
A window that survives night shifts and rotations, not a fixed 6pm rule.
Planner
Women's cycle planner
Phase-adjusted windows for your cycle, perimenopause and beyond.
Diagnostic
Fasting troubleshooter
Headache, stall, insomnia? Ranked causes, fixes and safety triage.
Guide
Break-fast & refeed
What to eat first, sized to your fast — with refeeding safety.
Timer
Offline fasting timer
Works with no signal or account; keeps your history on your device.
Calculators
Body stats
TDEE, BMR, protein, body fat and BMI — with the maths shown.
Scheduler
Ramadan schedule
Suhoor and iftar times for 42 cities or your location, four calculation methods.
Calculator
Water fast calculator
Energy used over a 24 to 72 hour fast, with safety checks first.
Printable
Printable schedule
One or four weeks, filled in or blank, A4 or Letter.
Every tool is free and works in your browser. The guides explain the questions behind them.
Figures checked at the source
Every figure is checked against its official source before it goes on the site, and the check is dated.
Honest on the unknowns
Where the evidence is thin, the page says so, and no page claims fasting treats or cures anything.
Safety first
Tools refuse unsafe targets, and every health page says who should not fast and when to see a doctor.
Questions about the calculator
How does the intermittent fasting calculator work?
Enter the time you finished your last meal and pick a schedule. The calculator adds your fasting hours to that time to show when you can eat again, and the rest of the 24 hours is your eating window. It works across midnight and counts down live.
Which schedules can I calculate?
Daily schedules from 12:12 to one meal a day (OMAD): 12:12, 14:10, 16:8, 18:6, 20:4 and OMAD. For 5:2, 4:3 and alternate-day fasting, use the 5:2 and alternate-day planner, which marks your fast days on a calendar.
Can I add my eating window to my calendar?
Yes. Press "Add eating window to calendar" and the calculator downloads a calendar file that opens in Google Calendar, Apple Calendar or Outlook.
Is it free, and do I need an account?
Every tool on the site is free, with no sign-up and no advertising. The numbers you enter stay in your browser.
Which fasting schedule should I start with?
If you are new to fasting, a 12-hour overnight fast is a gentle start. The schedule chooser asks six questions about your day and suggests a schedule and eating window, or tells you to talk to a doctor first if fasting may not be safe for you.