Plan your eating and fasting windows with projected calorie reduction
This calculator provides general guidance only. Intermittent fasting is not suitable for everyone, including pregnant or breastfeeding women, children, and people with eating disorders or diabetes. Consult your GP before starting any fasting regimen.
Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. Rather than specifying which foods to eat, it focuses on when you eat. The approach has grown significantly in popularity over the past decade, supported by a growing body of research suggesting benefits for weight management, metabolic health, and even cognitive function. The NHS recognises intermittent fasting as one approach to weight management, though it emphasises that overall calorie intake and food quality remain the most important factors. The most common form of intermittent fasting is time-restricted eating, where you confine all meals to a specific window each day. The 16:8 protocol -- fasting for 16 hours and eating within an 8-hour window -- is the most popular starting point because it essentially involves skipping breakfast and stopping eating after dinner. More aggressive protocols like 18:6 and 20:4 shorten the eating window further. Weekly protocols like 5:2 (eating normally for five days and restricting to 500-600 calories on two non-consecutive days) take a different approach, creating a weekly calorie deficit rather than a daily one. This calculator helps you plan your fasting schedule by showing your exact eating and fasting windows, estimating how many meals fit within your window, calculating calorie distribution per meal, and projecting potential weight loss based on the calorie reduction associated with your chosen protocol. It is important to understand that weight loss from intermittent fasting comes from the calorie deficit it creates, not from fasting itself -- if you overeat during your eating window, you will not lose weight regardless of how long you fast.
To plan your intermittent fasting schedule: 1. Select your fasting protocol. If you are new to intermittent fasting, start with 16:8. This is the most sustainable protocol and typically involves eating between noon and 8pm. More experienced fasters may prefer 18:6 or 20:4. The 5:2 and Eat-Stop-Eat protocols suit those who prefer normal eating most days with occasional full restriction. 2. Choose your first meal time. This determines when your eating window opens. For 16:8 with a first meal at 12:00, your last meal should be finished by 20:00. Adjust this based on your work schedule and social commitments -- consistency matters more than the exact time. 3. Enter your daily calorie target. This should be your target intake during normal eating days. If you do not know your daily calorie needs, use our TDEE calculator first. 4. Enter your current weight. This is used to project potential monthly weight loss based on the calorie deficit created by your fasting protocol. 5. Review your results. The calculator shows your eating and fasting windows, suggested number of meals, calories per meal, weekly calorie reduction, and projected monthly weight loss. Use the chart to visualise your daily or weekly calorie distribution pattern.
The calculations differ based on the protocol type: **Time-restricted protocols (16:8, 18:6, 20:4):** The eating window is calculated by adding the eating hours to your first meal time. For example, 16:8 starting at 12:00 gives an eating window of 12:00 to 20:00. Meals per day are estimated at roughly one meal every 3 hours within the window (8 hours / 3 = 2 meals for 16:8). The weekly calorie reduction assumes a conservative 10% reduction in overall intake, which research suggests is typical for time-restricted eating due to the shortened eating opportunity. **5:2 protocol:** Weekly calories = (5 x normal daily calories) + (2 x 500 calories). For someone eating 2,000 calories normally: weekly total = (5 x 2,000) + (2 x 500) = 11,000. Normal weekly total would be 7 x 2,000 = 14,000. Weekly deficit = 14,000 - 11,000 = 3,000 calories. **Weight loss projection:** Monthly weight loss is estimated using the principle that approximately 7,700 calories equals 1 kg of body weight. A weekly deficit of 3,000 calories translates to roughly 3,000 x (30/7) / 7,700 = 1.7 kg per month. These are theoretical projections -- actual results depend on adherence, food choices, activity level, and metabolic adaptation.
Intermittent fasting is not appropriate for everyone. The NHS advises against fasting for pregnant or breastfeeding women, children and teenagers, people with type 1 diabetes (type 2 diabetics should consult their GP first), those with a history of eating disorders, and people who are underweight. If you take medications that must be taken with food at specific times, you may need to adjust your protocol. Always consult your GP before starting a fasting regimen, especially if you have any existing health conditions.