INTERMITTENT FASTING
CALCULATOR
Pick a fasting protocol and eating-window start time to get your personalized fasting and eating schedule, including your last-meal and break-fast times and a daily calorie target.
What Is Intermittent Fasting?
Intermittent fasting (IF) is an eating pattern that cycles between defined periods of fasting and eating. Unlike diets that restrict specific foods or macronutrients, IF focuses on when you eat rather than what you eat. The most common protocols are 16:8 (16 hours fasting, 8-hour eating window), 18:6, 20:4 (sometimes called the Warrior Diet), 5:2 (normal eating on 5 days, roughly 500–600 calories on the other 2 days), and OMAD (one meal a day). This calculator does not diagnose or treat any condition — it simply converts your chosen protocol and start time into clock times you can follow, plus a rough calorie target based on the daily calorie needs (TDEE) you enter.
How This Calculator Works
Enter your eating-window start time and your estimated total daily energy expenditure (TDEE) — the number of calories your body burns in a day at your current activity level. If you don't already know your TDEE, our TDEE Calculator can estimate it from your age, sex, weight, height, and activity level. The tool then adds the eating-window length for your chosen protocol to your start time to find when your eating window ends, and subtracts the fasting-window length to find when your next fast breaks. For the 5:2 protocol, the calorie figure shown for "eating days" reflects a simple average intended to keep weekly intake in the same range as eating at your TDEE every day; on the two lower-calorie days most people following 5:2 aim for roughly 500–600 calories rather than a full fast.
Choosing a Protocol
There is no single "best" fasting schedule — the right protocol depends on your daily routine, hunger tolerance, and goals. 16:8 is the most widely used starting point because it fits around a typical sleep schedule and simply means skipping breakfast or dinner. 18:6 and 20:4 compress the eating window further and are generally attempted by people who already have experience with shorter fasts. 5:2 keeps five days of normal eating and restricts calories (not a full fast) on two non-consecutive days, which some people find easier to sustain socially. OMAD is the most restrictive pattern on this list and is generally not recommended for beginners.
What the Research Says
Randomized controlled trials and meta-analyses generally find that intermittent fasting produces weight loss that is comparable to continuous calorie restriction when total calorie intake is matched, rather than showing a unique metabolic advantage from meal timing alone. Some studies report modest additional improvements in markers like insulin sensitivity, but the evidence base is still developing and results vary between individuals. IF is one tool among several for managing calorie intake — it is not guaranteed to produce a specific amount of weight loss, and it does not replace medical treatment for any condition.
Who Should Be Cautious With Fasting
Intermittent fasting is not appropriate for everyone. People who are pregnant or breastfeeding, have a current or past eating disorder, have type 1 diabetes or take glucose-lowering medication, are underweight, or are under 18 should not start an IF protocol without first talking to a qualified healthcare provider. If you feel dizzy, unusually fatigued, or unwell while fasting, stop and eat, and speak with a doctor before continuing.