Intermittent Fasting Calculator
Find your eating window and fasting schedule for any IF protocol.
Intermittent Fasting Calculator
Set your eating window start time and IF protocol. Get your eating window, fasting window, and suggested break-fast time.
Black coffee, plain tea, and water do not break a fast. Electrolytes without calories are generally fine. Avoid intense training in the first week of a new protocol. The research on IF shows benefits mainly from calorie reduction — not the fasting itself — so hit your nutrition targets within your eating window.
About this calculator
Intermittent fasting (IF) is a time-restricted eating pattern, not a diet — it specifies when to eat rather than what to eat. The 16:8 protocol (16 hours fasted, 8-hour eating window) is the most widely practised. Proposed mechanisms include improved insulin sensitivity, elevated autophagy during the fasted state, reduced overall calorie intake through shortened eating windows, and circadian rhythm alignment when fasting periods overlap with nighttime sleep. A 2022 meta-analysis in the New England Journal of Medicine found that time-restricted eating produced similar weight loss to calorie restriction alone; the primary advantage for many people is simplicity — fewer decisions about meals rather than a metabolic magic.
How to interpret your results
Your schedule shows your eating window open and close times for the selected protocol. The most important variable is where your eating window falls relative to your sleep and wake times. Eating within 1–2 hours of waking and finishing 2–3 hours before bed aligns eating with peak circadian metabolic activity — this appears to provide additional metabolic benefits beyond the window itself. Starting with a 14:10 protocol (10-hour eating window) for 2 weeks before progressing to 16:8 reduces initial hunger and adherence challenges. If training falls outside your eating window, a small protein-containing snack (20–30 g protein, ~150 kcal) around training is unlikely to meaningfully disrupt the physiological benefits of IF.
The science and formula
Time-restricted eating (TRE) was investigated systematically by Satchidananda Panda and colleagues at the Salk Institute, who found in animal models that confining all calorie intake to an 8–12 hour window improved metabolic health markers independent of calorie intake. Proposed human mechanisms include: improved insulin sensitivity through extended fasting periods; elevated autophagy (cellular self-cleaning) during fasting — shown to increase after 14–16 hours without food; circadian rhythm alignment — human metabolism is more efficient during daytime hours; and spontaneous calorie reduction from fewer eating opportunities. The CALERIE study and multiple RCTs confirm that IF produces equivalent fat loss to continuous restriction when total calories are matched.
Important limitations
IF is not appropriate for: pregnant or breastfeeding women, people with a history of eating disorders, type 1 diabetics or insulin-dependent type 2 diabetics, those with hypoglycaemia, and underweight individuals. Early research suggesting dramatic metabolic improvements beyond simple calorie restriction has not been consistently replicated in human RCTs with careful calorie matching. Adherence is the primary determinant of success — IF that disrupts social eating or causes binge behaviour in the eating window will not produce the intended results. Some athletes find large eating windows (14:10 or less restriction) better for training performance and recovery.
Frequently Asked Questions
Does intermittent fasting work for fat loss?
IF produces fat loss primarily through reducing total calorie intake — a shorter eating window typically leads to fewer calories consumed. Meta-analyses show comparable fat loss between IF and continuous calorie restriction when calories are matched. IF may be more sustainable for people who find meal skipping easier than ongoing calorie counting.
Will I lose muscle on intermittent fasting?
Muscle loss risk on IF is low if protein intake is adequate (1.6–2.2 g/kg/day) and resistance training is maintained. Studies show no greater muscle loss with IF versus continuous calorie restriction at equal protein intakes. Consuming most of your protein in your eating window is sufficient.
Can I drink coffee or tea during the fasting window?
Black coffee and plain tea (no milk, cream, or sugar) are generally considered acceptable during fasting periods. They contain negligible calories and do not significantly raise insulin. However, they do break a strict "clean fast." For autophagy benefits, plain water is the only confirmed safe option.
What is the best IF protocol for beginners?
Start with 14:10 (14 hours fasted, 10-hour eating window) — typically skipping eating before 9am and after 7pm. After 2 weeks, progress to 16:8 if desired. The step-down approach reduces the hunger and energy dip side effects of more aggressive protocols. Most people find skipping breakfast easier than skipping dinner due to lower morning hunger hormones.
Should I train in a fasted state?
Fasted training (e.g., morning workouts before breaking the fast) is fine for aerobic and moderate-intensity work. For high-intensity training or heavy strength sessions, performance may be better with pre-workout nutrition. Research is mixed — most studies find no performance difference for moderate-intensity exercise, but some show impaired power output for maximal efforts in a truly fasted state.
Does IF affect hormones?
Extended fasting (16+ hours) can temporarily reduce testosterone and thyroid hormones in some individuals, particularly in combination with very low calorie intake. Aggressive IF protocols (20:4 or OMAD) combined with large calorie deficits can suppress luteinising hormone and reduce sex hormone production. A moderate 16:8 protocol at maintenance calories or a mild deficit typically does not produce clinically significant hormonal disruption in healthy adults.
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