Intermittent fasting calculator

What intermittent fasting actually does

Intermittent fasting (IF) is a protocol that cycles between eating windows and fasting windows. It is not a diet in the macronutrient sense — it doesn't tell you what to eat, only when. Most of its fat-loss benefit comes from a trivial mechanism: compressing the eating window removes a few hundred late-night calories for many people. The mTOR-signaling, autophagy, and insulin-sensitivity effects are real but smaller than most headlines claim.

The protocols, ranked by difficulty

• 12:12 (gentle): finish dinner by 8pm, don't eat again until 8am. Most adults do this without labeling it fasting.
• 14:10: a slight step up. Easy to sustain without meaningful hunger.
• 16:8 (Leangains): the most popular version. Skip breakfast or early dinner. Most research uses this window.
• 18:6: harder. Typically used by people already adapted to 16:8.
• 20:4 (Warrior): small snacks allowed within the window, main meal near the end.
• OMAD (One Meal A Day, 23:1): single meal. Works for some, wrecks adherence and social eating for most.

What's in this calculator

Pick a protocol, set your last-meal time, and see when your fast ends and your eating window closes. Most people plan backward from their preferred end-of-fast meal — if you want to break the fast at noon on a 16:8, last meal the night before needs to finish by 8pm.

What breaks a fast (and what doesn't)

• Breaks a fast: any caloric food or drink. Milk in coffee, creamer, protein shakes, gums with sugar, anything over ~50 kcal.
• Does NOT break a fast: water, plain black coffee, plain tea, herbal tea, electrolyte salts without sugar.
• Gray zone: artificial sweeteners, bone broth, vinegar. Depend on which effect of fasting you care about. For weight loss, not meaningful. For strict autophagy, break it.

What IF is good at

• Calorie control without tracking: for people who don't want to count macros, compressing the window often drops 200–500 kcal/day automatically.
• Adherence simplicity: one rule (when to eat) beats many rules (what/how much).
• Insulin sensitivity: modest improvements, more pronounced in overweight or prediabetic populations.
• Autophagy: cellular cleanup process that appears to ramp up past ~16 hours of fasting. Hard to measure directly in humans.

What IF is not good at

• Muscle building: compressing protein into 8 hours can hurt muscle protein synthesis distribution. See the protein intake calculator.
• Endurance performance: fasted training past 60 min often compromises high-intensity efforts. Check the running pace calculator.
• Relationships with food: people with a history of disordered eating should proceed cautiously or avoid IF entirely.
• Women's hormonal health: some women experience menstrual irregularities with longer fasts. Use shorter windows (14:10, 12:12) if this applies.

How to start IF without hating it

• Build up: 12:12 → 14:10 → 16:8 over 2–3 weeks. Sudden 16:8 is harder than it needs to be.
• Keep your fluid intake high — see the water intake calculator. Most "hunger" in the first 14 hours is mild dehydration.
• Don't binge when the window opens. Eat your planned calories (see the TDEE calculator) normally.
• Plan protein: 30–40 g per meal inside the window, on each of 2–3 meals. Use the macro calculator for splits.
• Caffeine helps in the first two weeks; don't let it push bedtime — see the sleep calculator.

IF and training

Fasted training is fine for easy cardio and moderate weight sessions. Hard intervals, max-effort lifts (1RM), and endurance sessions over 90 minutes often perform better fed. A practical compromise: break the fast with 20–30 g protein and a small carb source ~60 min before the hardest session of the day.

Is IF "metabolically superior"?

Head-to-head trials comparing IF to calorie-matched continuous eating generally show similar fat loss and similar metabolic improvements. The advantage of IF is behavioral, not metabolic. If compressing the window helps you eat fewer calories without tracking, that's real. If it leads to binges or social isolation, it's counterproductive.

Who should not do IF

• Anyone with a history of eating disorders.
• Pregnant or breastfeeding people (see the due date calculator).
• Type 1 diabetics without medical supervision.
• Underweight individuals.
• Children and adolescents.

FAQ

Can I drink coffee during my fast?

Plain black coffee, yes. Adding anything (cream, milk, sweetener, butter) breaks a strict fast. Coffee with a splash of half-and-half (< 10 kcal) is in the gray zone and won't disrupt weight loss but may blunt other fasting effects.

Does IF preserve muscle better than regular dieting?

No — not when total protein is lower than recommended because of the compressed window. Hit protein targets from the protein calculator and muscle retention should match continuous-eating approaches.

Is 16:8 magic?

No. It's a popular number because it fits lifestyles well (skip breakfast, eat lunch and dinner). The research doesn't show a sharp threshold at 16 hours specifically.

Will IF wreck my metabolism?

Not at the intakes and timeframes typically used. Multi-day fasts can temporarily lower basal rate; daily 16:8 does not.