Intermittent Fasting Calculator UK
Set your intermittent fasting schedule, find your eating and fasting windows, and calculate personalised calorie targets based on your body and goals.
Step 1: Choose Your Fasting Protocol
Leangains
16 hrs fasting, 8 hrs eating. Best for beginners.
18:6 Method
18 hrs fasting, 6 hrs eating. More advanced.
Warrior Diet
20 hrs fasting, 4 hrs eating. Advanced users.
One Meal a Day
23 hrs fasting, 1 meal. Expert level.
Fast Diet
5 normal days, 2 x 500 kcal days.
Gentle Start
14 hrs fasting, 10 hrs eating. Easiest entry.
Step 2: Set Your Eating Window
Step 3: Calculate Your Calorie Target (Optional)
Intermittent Fasting Methods Compared
| Protocol | Fast Duration | Eating Window | Difficulty | Best For |
|---|---|---|---|---|
| 14:10 | 14 hours | 10 hours | Easy | Complete beginners |
| 16:8 (Leangains) | 16 hours | 8 hours | Moderate | Most people |
| 18:6 | 18 hours | 6 hours | Moderate-Hard | Experienced IF users |
| 20:4 (Warrior) | 20 hours | 4 hours | Hard | Advanced practitioners |
| OMAD | 23 hours | 1 hour | Very Hard | Expert level only |
| 5:2 (Fast Diet) | 2 days/week (500 kcal) | 5 normal days | Moderate | Those preferring weekly structure |
| Alternate Day Fasting | Every other day (500 kcal) | Every other day | Hard | Significant weight loss |
The Science Behind Intermittent Fasting
Intermittent fasting works through several distinct physiological mechanisms. During the fasting state, insulin levels drop significantly within 2–4 hours after the last meal, which facilitates the mobilisation of stored fat (lipolysis). After 12–16 hours of fasting, glycogen stores become substantially depleted, and the body increasingly relies on fat oxidation for energy — a process called metabolic switching.
From approximately 16–18 hours of fasting, autophagy — the cellular cleaning process — is significantly upregulated. Autophagy involves cells breaking down and recycling damaged proteins, organelles, and cellular debris. Research by Yoshinori Ohsumi (2016 Nobel Prize in Physiology or Medicine) demonstrated the fundamental importance of autophagy for cellular health. Extended fasting (24–72 hours) is associated with even more pronounced autophagy activation, though daily IF protocols like 16:8 do contribute meaningfully to this process.
Key hormonal changes during IF include: reduced IGF-1 (insulin-like growth factor 1), increased norepinephrine (which raises metabolic rate by 3.6–14%), and increased human growth hormone (HGH), which helps preserve lean muscle mass during weight loss.
16:8 Intermittent Fasting: A UK Practical Guide
The 16:8 method is by far the most popular and researched IF protocol in the UK. Here is a practical guide to implementing it:
Common UK 16:8 schedules:
- Eating 10:00–18:00 (early eaters — finish before evening)
- Eating 12:00–20:00 (most popular — skip breakfast)
- Eating 14:00–22:00 (evening eaters — skip breakfast and lunch)
Research suggests that earlier eating windows (e.g., 8:00–16:00) produce slightly better metabolic outcomes than late eating windows, as they align better with the body's circadian rhythm. Insulin sensitivity is highest in the morning and decreases through the day, meaning the same meal eaten at 8am produces less insulin spike than the same meal at 8pm. However, the most important factor is adherence — the schedule you can maintain consistently will produce better results than the theoretically optimal one you abandon after 2 weeks.
What to Eat During the Eating Window
Intermittent fasting does not prescribe specific foods, but for best results during your eating window prioritise: lean proteins (chicken breast, turkey, fish, eggs, Greek yoghurt, legumes) at 25–35% of calories; complex carbohydrates (oats, sweet potato, brown rice, wholegrain bread) at 40–50% of calories; healthy fats (avocado, olive oil, nuts) at 25–35% of calories; and vegetables and fruit for fibre, vitamins, and minerals. UK-specific tips: take advantage of NHS Change4Life food swaps; porridge with berries is an excellent first meal for breaking a fast; avoid the temptation to overconsume during the eating window, which is the most common reason IF fails.
The 5:2 Diet and Dr Michael Mosley's Legacy
The 5:2 diet became a cultural phenomenon in the UK following Dr Michael Mosley's 2012 BBC Horizon documentary Eat, Fast and Live Longer and the subsequent bestselling book The Fast Diet. Dr Mosley, who sadly died in 2024, was instrumental in making intermittent fasting accessible to UK audiences. The 5:2 protocol involves eating normally for 5 days (within maintenance calories) and restricting to 500 calories (women) or 600 calories (men) on 2 non-consecutive fast days. Studies from the Genesis Breast Cancer Prevention Centre in Manchester showed 5:2 fasting led to similar weight loss as continuous calorie restriction, but with greater improvements in insulin sensitivity in some participants. The diet remains popular in the UK and is recognised by the NHS as a legitimate weight management approach.
Breaking a Fast: What to Eat First
After an extended fast, start with foods that are gentle on the digestive system and do not cause a sharp insulin spike. Good options include: a small portion of protein (2 eggs, some Greek yoghurt), a handful of nuts, a small piece of fruit, or a bowl of broth. Avoid very large, high-sugar meals immediately after breaking a fast, as the insulin response may be more pronounced after extended fasting. Gradually transition back to normal-sized meals over 20–30 minutes.