Choose your fasting protocol, set your last meal time, and see exactly when your eating window opens and what fasting zone you are in.
Intermittent fasting (IF) is an eating pattern that cycles between periods of eating and fasting. It does not prescribe specific foods but rather when you eat. The most common methods are 16:8 (fast for 16 hours, eat within an 8-hour window), 18:6 (fast 18, eat 6), 20:4 (fast 20, eat 4), and OMAD (one meal a day, approximately 23:1). During the fasting window, only water, plain tea, and black coffee are typically consumed.
Select your fasting protocol (16:8, 18:6, 20:4, or custom) and enter the time you started your fast. The timer shows your current fasting duration, time remaining, progress percentage, and a visual indicator of which metabolic phase you are in. It also tracks your fasting streak across multiple days. Read our full Intermittent Fasting Guide for protocols, benefits, and tips.
0-4 hours: Digestion and absorption of your last meal. Blood sugar and insulin rise. 4-8 hours: Blood sugar normalizes, insulin drops. Your body begins shifting from glucose to stored glycogen. 8-12 hours: Glycogen stores begin depleting. Fat burning increases. 12-16 hours: Significant fat oxidation. Growth hormone secretion increases. 16-24 hours: Autophagy (cellular cleanup) ramps up. Ketone production increases. These are approximate timelines and vary by individual.
Intermittent fasting (IF) encompasses several protocols. The 16:8 method (16 hours fasting, 8 hours eating) is the most popular and typically involves skipping breakfast, eating from noon to 8 PM. The 5:2 method involves eating normally 5 days per week and restricting to 500-600 calories on 2 non-consecutive days. The Eat-Stop-Eat method involves one or two 24-hour fasts per week. Extended fasting (36-72 hours) is practiced less commonly and carries greater risks, including electrolyte imbalances.
Research on intermittent fasting has shown potential benefits including improved insulin sensitivity, reduced inflammation markers, and modest weight loss. A 2019 review in the New England Journal of Medicine found evidence supporting IF for metabolic health, though the authors noted that long-term data is limited. Weight loss from IF appears to result primarily from overall calorie reduction rather than any metabolic advantage unique to fasting itself. People with diabetes, eating disorders, pregnancy, or who are underweight should consult a healthcare provider before attempting any fasting protocol.
In the first 4-8 hours after eating, your body digests and absorbs nutrients. Between 8-12 hours, glycogen stores begin to deplete and the body shifts toward fat oxidation. After 12-16 hours, fat burning increases and some studies suggest cellular autophagy (cleanup of damaged cell components) begins. After 24 hours, glycogen is largely depleted and the body relies primarily on fat for fuel. The Hydration Calculator is important during fasting because water, black coffee, and plain tea are generally considered acceptable during fasting windows, and adequate hydration is essential.
The most popular intermittent fasting protocols are 16:8 (16 hours fasting, 8-hour eating window), 18:6, 20:4, and 24-hour fasts. This timer tracks your current fast from the moment you start, showing elapsed time and how far you are through your target window.
Most of the research-backed benefits of fasting (improved insulin sensitivity, autophagy, reduced inflammation) require fasts of at least 14-16 hours. Shorter fasts may still help with calorie reduction but are less likely to trigger the metabolic changes that make IF distinct from simple calorie restriction. For the full evidence breakdown, see our intermittent fasting guide.