Formula, in Fat Loss

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%.

4 variables. Published May 14, 2026. By AI Fit Hub.

Education, not medical advice. The result comes only from the published formula and your inputs. How we check our formulas.

Formula

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

Morning trainer (06:00): eating window 06:00-14:00. Pre-training meal: small (15-25g protein, 30g carbs) 30 min before. Larger meal post-training. Protein distribution: 20% / 50% / 30%.

Eat-Stop-Eat (Pilon 2009): 24h fasts 1-2x per week, normal eating otherwise. Same caloric outcome as daily 16:8 per Tinsley 2017 review; preference-driven.

Lean Gains (Berkhan): 16:8 with longer fasts on rest days, shorter on training days (8 hours becomes 10 hours on training days). Marginal evidence advantage; mostly adherence trick.

5:2 (Mosley): 2 days of ~500 kcal, 5 days of maintenance. Weekly caloric deficit same as daily mild deficit per Harris 2018; adherence varies by personality.

Run the numbers next

Nutrition Macro Calculator
Convert calorie targets into protein, carbs, and fat grams for your goal.
Nutrition Protein Intake Calculator
Get daily protein targets based on training level and goal.
Nutrition TDEE Calculator
Estimate your daily energy expenditure with Mifflin-St Jeor + activity factors.

Sources

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