Intermittent Fasting Calculator
Get a personalised intermittent fasting schedule based on your lifestyle, goal, and experience level. Covers all popular protocols: 16:8, 18:6, 20:4, OMAD, and 5:2.
⏱️ IF Protocols Compared
16h fast · 8h eating — Most popular — skip breakfast, eat noon–8pm
18h fast · 6h eating — Narrower window — e.g., 1pm–7pm
20h fast · 4h eating — Warrior diet — one large meal + small meal
23h fast · 1h eating — One meal a day — maximum autophagy
2 days/wk · 5 normal days — Eat normally 5 days, 500–600 kcal on 2 days
All protocols produce similar weight loss results when total calorie intake is equal. The best protocol is the one you can sustain consistently. Start with 16:8 and adjust based on how you feel.
✅ What's Allowed During a Fast
- ✓Water (still or sparkling)
- ✓Black coffee (no milk/sugar)
- ✓Plain green or herbal tea
- ✓Apple cider vinegar (diluted)
- ✓Salt/electrolytes if needed
❌ Breaks Your Fast
- ✕Any food (even small snacks)
- ✕Milk or cream in coffee
- ✕Sugar or sweeteners
- ✕Juice or smoothies
- ✕Protein shakes / BCAAs
The Science Behind Intermittent Fasting
Intermittent fasting works through several biological mechanisms. During fasting, insulin levels drop significantly, which shifts the body from glucose-burning to fat-burning mode. After 12–16 hours without food, the body increasingly relies on stored fat for energy. Extended fasts (18+ hours) activate autophagy — a cellular "cleanup" process where the body breaks down and recycles damaged proteins and organelles.
Research published in the New England Journal of Medicine (2019) found that IF produces benefits beyond simple calorie reduction, including improved insulin sensitivity, reduced inflammation, enhanced brain function, and cellular repair processes that may slow ageing. These benefits appear to be triggered by the fasting period itself, not just the calorie deficit.
Intermittent Fasting for Weight Loss
IF is effective for weight loss primarily because it naturally reduces calorie intake — most people eat 20–30% fewer calories when restricted to a shorter eating window. A 2020 meta-analysis in the Annual Review of Nutrition found that IF produces 3–8% body weight loss over 3–24 weeks, comparable to traditional calorie restriction. The key advantage: many people find it easier to stick to a time-restricted eating pattern than to count calories indefinitely.
For best results, combine IF with a structured nutrition plan. Use our Calorie Calculator to find your daily target, our Macro Calculator to balance your macronutrients, and our Protein Calculator to ensure you're eating enough protein to preserve muscle during your eating window.
Common Mistakes When Starting IF
Overeating during the eating window: IF is not a licence to binge. You still need to eat at or below your calorie target for weight loss. Not eating enough protein: Aim for 1.6–2.2g/kg/day, concentrated in your eating window. Starting too aggressively: Jump from no fasting to 20:4 and you'll likely quit within a week. Start with 14:10 or 16:8 and gradually extend. Ignoring hydration: Use our Water Intake Calculator to stay hydrated — thirst is often mistaken for hunger during fasts.