Intermittent Fasting Calculator: Eating Window, Calories & Protein
Get your personalised IF schedule — eating window times, daily calorie target, protein goal, and per-meal breakdowns — for every major fasting protocol.
Intermittent Fasting Protocol Comparison
| Protocol | Fasting Window | Difficulty | Best For |
|---|---|---|---|
| 16:8 | 16 hours | Easy | Beginners, daily practice |
| 18:6 | 18 hours | Moderate | Accelerated fat loss |
| 20:4 (Warrior) | 20 hours | Hard | Experienced fasters |
| OMAD | 23 hours | Very Hard | Short-term, disciplined |
| 5:2 | 2 days/week | Moderate | Those who prefer weekly flexibility |
How to Hit Your Protein Goals While Fasting
The biggest nutritional challenge with intermittent fasting is not calories — it is protein. Compressing all your protein into a shortened eating window requires deliberate planning. Research shows that protein absorption is efficient even in larger bolus doses, so eating 50–60g of protein per meal is not wasted.
Prioritise protein at every meal
Build every meal around a protein source first. Eggs, chicken, fish, Greek yoghurt, and cottage cheese are dense, fast-to-prepare options.
Use protein shakes strategically
A 30–40g protein shake at the start or end of your eating window is the easiest way to close a protein gap without significant caloric load.
Don't skip the last meal
A protein-rich final meal before the fast window begins slows amino acid delivery through the night, supporting overnight muscle protein synthesis.
Track for the first 2 weeks
Most people dramatically underestimate how much protein they eat without tracking. Use any food logging app for the first 2 weeks to calibrate your intake.
Frequently Asked Questions
Which intermittent fasting protocol is best for weight loss?
16:8 is the most researched and sustainable protocol for most people — 16 hours of fasting with an 8-hour eating window. It produces a natural calorie restriction in most people without requiring strict calorie counting, and is easy to maintain long-term. 18:6 and 20:4 accelerate results by shortening the eating window further, but compliance decreases with more aggressive restriction. The 5:2 protocol (500 kcal on 2 days) is effective for those who prefer normal eating most of the week.
How much protein do I need while intermittent fasting?
Protein requirements do not change with intermittent fasting — the same daily targets apply (1.6–2.2g/kg for muscle retention). The challenge is consuming sufficient protein in a compressed eating window. For 16:8, spreading protein across 3 meals is straightforward. For OMAD (one meal a day), hitting 150–180g of protein in a single meal is difficult and may require liquid protein sources. Protein absorption is not limited to a single bolus — your body continues processing amino acids between meals.
Will intermittent fasting cause muscle loss?
Short-term fasting (under 24 hours) does not significantly increase muscle protein breakdown in most people. The primary risk factor for muscle loss during IF is insufficient protein during the eating window and excessive caloric deficit, not the fasting itself. Maintain resistance training and hit your daily protein targets to preserve muscle effectively.
Can I drink coffee or water during the fasting window?
Black coffee, plain tea, and water do not break a fast in terms of metabolic effects — they contain negligible calories and do not trigger an insulin response. However, anything containing calories (milk, sugar, cream, bulletproof coffee with fats) technically breaks a fast by triggering metabolic processes. For strict autophagy protocols, even black coffee may be avoided.
When should I work out during intermittent fasting?
Training in a fasted state is fine for most people and may enhance fat oxidation during the session. However, performance (particularly strength and power) can be slightly reduced when fasted. If performance is your priority, schedule training near the start of your eating window, or consume a small protein-only snack before training. Post-workout is generally the best time for your highest-protein meal.
What breaks an intermittent fast?
Strictly speaking, any caloric intake breaks a fast. In practical terms, foods and drinks that significantly raise insulin and blood sugar end the fasted state. This includes all meals, sugary drinks, juice, and milk. Non-caloric or near-zero-calorie options like black coffee, plain tea, water, sparkling water, and electrolyte supplements (without carbs) are widely considered acceptable during fasting windows.