Intermittent Fasting Calculator
Find your ideal fasting schedule with a 4-question quiz. Live countdown timer, 6 proven protocols, and a personalized daily eating window.
Quick quiz
Suggested starting point
12:12 — Gentle circadian reset. 12:12 is a solid, sustainable starting point.
Answer all 4 questions above for a personalized recommendation.
Choose your schedule
Daily schedule
Live countdown
What happens in your body?
Hour-by-hour biological changes during your 12-hour fast. Greyed-out stages are beyond your current protocol.
Your body digests the last meal and absorbs nutrients. Blood glucose and insulin are elevated.
Insulin drops, glycogen stores begin depleting. Your body starts transitioning from glucose to fat as fuel.
Glycogen is largely depleted. The liver produces ketones and fat oxidation increases significantly.
HGH surges, supporting muscle preservation and cellular repair. Real metabolic benefits begin here.
Cells activate autophagy — breaking down and recycling damaged proteins and organelles. A key longevity mechanism.
Deep ketosis and peak fat burning. Many people report heightened mental clarity during this phase.
Autophagy peaks. Significant metabolic reset, reduced inflammation, and immune regeneration are underway.
Tips for 12:12
The three most important practical guidelines for your chosen protocol.
Finish dinner by 8 PM and eat breakfast at 8 AM — you may already be doing this without realising it.
This is a circadian-aligned fast. Avoid late-night snacking, which disrupts your body clock even within the 12-hour window.
Once comfortable after 2–3 weeks, extend to 14:10 by simply delaying breakfast by 2 hours.
How to use
- 1
Answer 4 quick questions about your goal, lifestyle, experience, and exercise timing.
- 2
Get your personalized protocol recommendation — or choose any of the 6 fasting schedules.
- 3
Set your wake-up time to build your daily eating window schedule and start the live countdown timer.
Intermittent Fasting Calculator — Personalized Fasting Schedule
Free intermittent fasting calculator. Get a personalized protocol (12:12, 14:10, 16:8, 18:6, 20:4, OMAD) with live countdown timer and daily eating window. No signup, works in your browser.
Skycally's Intermittent Fasting Calculator turns a short 4-question quiz into a personalized fasting schedule you can start today. Tell it your goal, lifestyle, experience level, and when you exercise, and it recommends one of six proven protocols — from a gentle 12:12 circadian reset to advanced OMAD (23:1) — with a clear rationale for why that window fits you.
Once you pick a protocol and set your wake-up time, the calculator builds your daily eating and fasting windows and starts a live countdown timer. The timer updates every second, shows a progress ring for the current phase, and can optionally beep when you transition between eating and fasting. Every calculation runs 100% in your browser — no account, no data sent to a server, and your preferences persist locally between visits.
Intermittent fasting is not one-size-fits-all. Beginners typically do best with 14:10 or 16:8, while advanced fasters may prefer 18:6 or 20:4. Morning exercisers usually need a shorter fasting window to fuel training; those chasing longevity and metabolic benefits often lean longer. The calculator weighs all these factors so you skip the guesswork and start with a schedule that fits your real life.
The hour-by-hour body timeline shows exactly what is happening inside your body at each stage of the fast — from the initial glycogen depletion at hour 8, to the onset of autophagy at hour 14, to the peak cellular repair that occurs between hours 18 and 24. Most fasting tools tell you when to eat; this one tells you why each additional hour matters. Combine your fasting schedule with precise nutrition targets using our Calorie Calculator, and align your sleep window with your fast using the Sleep Calculator to maximise the hours you fast while asleep.
Frequently Asked Questions
Which intermittent fasting schedule is best for beginners?
Most beginners do well with 14:10 or 16:8 — long enough to see metabolic benefits, short enough to fit real life. Start with 14:10 for the first 1–2 weeks, then extend to 16:8 if it feels sustainable.
Can I exercise while fasting?
Yes. Low-to-moderate intensity training (walking, easy cycling, mobility) works well fasted. For heavy strength or high-intensity sessions, most people perform better training near the start or middle of their eating window.
Does the timer keep running when the page is closed?
The countdown re-computes from your wake-up time and protocol every time you open the page, so you always see accurate remaining time — no background process needed.
Is intermittent fasting safe for everyone?
Intermittent fasting is generally safe for healthy adults, but it is not recommended during pregnancy, breastfeeding, for people with a history of eating disorders, or those on medications that require food. Consult your doctor before starting.
What is autophagy and when does it start?
Autophagy is the body's cellular self-cleaning process — it breaks down and recycles damaged cell components. Research suggests autophagy begins around 14–16 hours of fasting in most people, with peak activity between 18 and 24 hours. This is one reason protocols like 18:6, 20:4, and OMAD have gained popularity among longevity researchers.
Can I drink coffee or tea during the fasting window?
Black coffee, plain tea, and water do not break a fast because they contain negligible calories and do not trigger an insulin response. Adding milk, sugar, cream, or sweeteners — even artificial ones — can disrupt the fasted state. Stick to plain versions during your fasting hours.
What is the best intermittent fasting schedule for fat loss?
16:8 is the most evidence-backed protocol for fat loss in most people. It creates a meaningful caloric restriction window while being sustainable long-term. Those with more experience often find 18:6 accelerates results. Breaking your fast with protein and fat rather than carbohydrates significantly improves fat oxidation.
How long does it take to see results from intermittent fasting?
Most people notice improved energy and reduced hunger within the first 1–2 weeks as the body adapts. Measurable changes in body composition typically appear within 4–8 weeks of consistent practice. Metabolic benefits such as improved insulin sensitivity develop over months. Consistency matters more than the specific protocol chosen.
Combine your fasting schedule with precise nutrition using our Calorie Calculator — know exactly how much to eat during your window.
Optimise your fasting hours by aligning them with your sleep schedule using the Sleep Calculator, and track your hydration during the fast with our Water Intake Calculator.
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