Intermittent Fasting Window Formula
Intermittent fasting is a meal-timing strategy, not a fat-burning hack. Stockman et al. 2018 meta-analysis (n=17 trials) found IF produces fat loss equivalent to standard caloric restriction — neither faster nor more durable. What matters: positioning the eating window so your training session falls inside it. Eating 1-2h before training preserves performance; training fasted depletes glycogen and costs strength outputs ~5-15%.
Formula
Copy the exact expression or work through it step by step below.
eating_window_start = training_time − pre_training_meal_buffer
pre_training_meal_buffer ≈ 1-2 hours (lifters), 0.5-1h (endurance)
eating_window_length = 8 hours (16:8 protocol)
protein_dose_per_meal = total_protein_target / meals_in_window
meals_in_window: 2-3 (8h window) or 3-4 (10h window) Variables
training_time
Scheduled training time
Wall-clock time of your main session. Anchor the eating window around this — performance falls off if you train past hour 6+ of a fast.
pre_training_meal_buffer
Hours between meal and training
Time to digest. Lifters: 1-2h (heavier meals). Endurance: 30-90 min (lighter, carb-skewed).
eating_window_length
Hours per day to eat
8h is the canonical 16:8. Variations: 10:14, 6:18 (more aggressive), 4:20 (warrior diet — research-light, hard to hit protein targets).
protein_dose_per_meal
Protein per meal
Protein synthesis maxes out around 30-40g per dose for muscle protein synthesis (Schoenfeld 2018). A 16:8 window with 180g protein target = 60g/meal × 3 meals. Distribution matters.
Step By Step
- 1
Decide your training time. This anchors everything.
Evening lifter, training at 18:00.
- 2
Set pre-training meal 1-2h before training.
Meal 1 at 16:00 (training at 18:00 → 2h buffer).
- 3
Set post-training meal 1-2h after training (or as soon as practical).
Meal 2 at 19:30. Post-training, larger protein dose.
- 4
Anchor eating window: start = first meal, end = first meal + 8h (or your chosen window).
16:00 to 22:00 + a third meal. Window = 6h or extend to 10h (15:00-22:00).
- 5
Hit protein dosing. Total target ÷ meals.
180g target ÷ 3 meals = 60g per meal. 60g = ~225g chicken breast or 6 large eggs.
Worked Example
85 kg intermediate lifter, 16:8 IF protocol, trains 18:00 on weekdays, 180g protein/day target
Training time
18:00
Window length
8 hours (16:8)
Pre-training buffer
2 hours
Protein target
180g/day
Eating window: 16:00 - 24:00 Meal 1: 16:00 (pre-training, 60g protein, carb-paired) Meal 2: 19:30 (post-training, 70g protein) Meal 3: 22:30 (50g protein, lighter) Protein total: 60 + 70 + 50 = 180g ✓
Eating window 16:00-24:00 places the main meal 2h before training. Performance preserved vs full-day eating. The 16h fast covers sleep + morning — feasible if morning coffee (no calories) is acceptable. Compared to a 12:8 (12pm-8pm) window: this delays first meal until 16:00, which works for evening trainers but fails for morning trainers.
Common Variations
Try These Tools
Run the numbers next
Macro Calculator
Convert calorie targets into protein, carbs, and fat grams for your goal.
Protein Intake Calculator
Get daily protein targets based on training level and goal.
TDEE Calculator
Estimate your daily energy expenditure with Mifflin-St Jeor + activity factors.
Sources & References
- Stockman, Thomas, Burke & Apovian (2018). Intermittent fasting: is the wait worth the weight? — Current Obesity Reports — meta-analysis of intermittent fasting trials
- Tinsley & La Bounty (2015). Effects of intermittent fasting on body composition and clinical health markers in humans. — Nutrition Reviews — IF body composition review
- Schoenfeld & Aragon (2018). How much protein can the body use in a single meal for muscle-building? — Journal of the International Society of Sports Nutrition — per-meal protein dosing
- Harris et al. (2018). Intermittent fasting interventions for treatment of overweight and obesity in adults. — JBI Database of Systematic Reviews — clinical fasting outcomes