Intermittent Fasting Window Calculator

Intermittent Fasting Guide

What should I know about Intermittent Fasting Protocols?

16:8: most researched and popular protocol. Fast 16 hours (including sleep), eat within an 8-hour window. A typical schedule: last meal 8pm, first meal 12pm. 18:6: more restrictive eating window. Often used when 16:8 results plateau. 20:4: significantly restricts eating opportunities — requires meal planning. OMAD: one meal per day. Very restrictive, difficult to meet nutritional needs. 5:2: eat normally 5 days per week. On 2 non-consecutive days, eat only 500-600 kcal. Each protocol can achieve similar results for weight management if total weekly calorie intake ends up similar — the main differences are in how sustainable and socially convenient each pattern is for a given lifestyle.

What Breaks a Fast?

Breaks a fast: any caloric food or drink. Milk or cream in coffee. Fruit juice. Alcohol. Energy drinks with calories. Does NOT break a fast: water. Plain black coffee or tea (no milk, no sugar). Sparkling water. Electrolyte drinks with zero calories. Plain herbal tea. Black coffee in particular does not break a fast and may support fasting by reducing hunger. Chewing gum: technically zero calories but insulin response may be triggered in some people. Medications: most medications are fine to take during a fast, but some specifically require food to avoid stomach upset or to aid absorption — check with a pharmacist if unsure.

What do I need to know about Evidence for Intermittent Fasting?

Weight loss: IF produces similar weight loss to continuous calorie restriction when total calories are matched — the advantage is that some people find it easier to restrict calories within a compressed window. Metabolic benefits: some evidence for improved insulin sensitivity, reduced fasting insulin, and autophagy (cellular repair) during extended fasting. Conflicting evidence: some studies show muscle loss risk with extended fasting without adequate protein. Recent research suggests that the timing of eating may matter less than the total calorie deficit achieved, and that intermittent fasting works primarily by making that deficit easier to sustain for some people, rather than through any unique metabolic effect.

What should I know about Practical Fasting Tips?

Starting out: begin with a 12-hour fast (e.g. 8pm to 8am) — this is the baseline most people already achieve naturally. Gradually extend to 14 hours, then 16. Breaking the fast: break with a protein-rich meal to maximise satiety through the eating window. Coffee or tea (black) in the morning significantly reduces hunger and supports extending the fast comfortably. Electrolytes: extended fasts can deplete sodium and potassium. A pinch of salt in water or a zero-calorie electrolyte supplement supports appetite control without breaking the fast in the way food does.

What should I know about Who Should Avoid Intermittent Fasting?

Intermittent fasting isn't appropriate for everyone, and certain groups should avoid it or only attempt it under medical supervision. People with a current or past history of eating disorders should generally avoid structured fasting protocols, since restrictive eating windows can reinforce disordered eating patterns. Pregnant or breastfeeding women have increased and relatively constant nutritional needs that extended fasting windows can conflict with, and most guidance advises against fasting during these periods. People with diabetes, particularly those on insulin or sulfonylurea medications, face a real risk of hypoglycemia (dangerously low blood sugar) if fasting windows aren't carefully coordinated with medication timing, and should only fast under a doctor's guidance with medication adjustments as needed. Children and adolescents, who need consistent nutrition and calories for growth, are generally advised against adult-style fasting protocols. People with a history of gallstones can sometimes find fasting triggers symptoms, since reduced, irregular eating affects bile flow. Anyone on regular medication that needs to be taken with food, or managing a chronic condition where stable blood sugar or nutrient timing matters, should discuss fasting with their GP or a registered dietitian before starting, rather than assuming a popular protocol is automatically safe for their situation.

What do I need to know about Electrolytes and Hydration While Fasting?

Fasting windows, especially longer ones (16+ hours or multi-day fasts), increase the risk of mild dehydration and electrolyte imbalance, partly because a meaningful amount of daily fluid and mineral intake normally comes from food itself, not just drinks. Water, black coffee, and plain tea are generally considered acceptable during a fasting window without breaking the fast's metabolic intent, and staying well hydrated is important since early fasting-related headaches and fatigue are often actually dehydration rather than true hunger. Sodium, potassium, and magnesium levels can drop during longer fasts, particularly as the body excretes more sodium once glycogen stores (which hold water alongside stored carbohydrate) are depleted — this is a common cause of the headaches, light-headedness, and muscle cramps some people experience in their first week or two of intermittent fasting. A pinch of salt in water, a small amount of a sugar-free electrolyte supplement, or a cup of broth (for longer fasts, check whether this fits the specific protocol being followed) can help offset this without adding meaningful calories. These symptoms typically ease within the first couple of weeks as the body adapts, but persistent or severe symptoms are a sign to break the fast and reassess rather than push through, particularly for anyone new to fasting.

Worked Example: Calculating a 16:8 Eating Window

The 16:8 protocol means 16 hours fasting and an 8-hour eating window each day. Worked example: someone who wakes at 7am and wants their last meal finished by 8pm needs their eating window to start at noon (8pm minus 8 hours = 12pm), making the fast run from 8pm the previous evening to noon the next day, a 16-hour gap. For the more moderate 14:10 protocol with the same 8pm last-meal target, the eating window would instead open at 10am (8pm minus 10 hours), giving a 14-hour fast. Working backward from whichever meal time is easiest to keep consistent, usually the evening meal, is generally simpler than trying to force a fixed start time that doesn't fit someone's actual daily schedule.

Fasting and Medication: Why Timing Needs Medical Input

Several common medications are meant to be taken with food, and skipping meals for extended stretches can change both how a drug is absorbed and how a person's body responds, something that is particularly important for anyone managing diabetes, since blood sugar medications and insulin are typically timed around meals specifically to avoid hypoglycaemia (dangerously low blood sugar), which can become more likely if a dose is taken without the meal it's designed to accompany. People on blood pressure medication, those who are pregnant or breastfeeding, anyone with a history of disordered eating, and anyone taking medication where the prescribing information specifies taking it with food should talk to a doctor or pharmacist before starting any intermittent fasting protocol, rather than adjusting fasting and medication timing on their own.

What Counts as Breaking a Fast, in Practical Terms

Most intermittent fasting approaches treat water, plain black coffee and plain tea as acceptable during the fasting window, since they contain negligible or no calories and don't meaningfully trigger an insulin response, while anything containing calories, including cream, sugar, or sweeteners added to coffee, and anything containing protein or fat, technically breaks the fast in the strict sense, even in small amounts. Some people take a more relaxed practical approach and allow a small amount of milk or a sugar-free sweetener without feeling it undermines their goals, which is a personal choice rather than a universally agreed rule, since the various intermittent fasting protocols themselves differ on how strictly "zero calories" needs to be interpreted.

Why Consistency Matters More Than the Exact Protocol Chosen

Research and clinical guidance on intermittent fasting generally suggests that the specific protocol chosen, 14:10, 16:8 or another variant, matters less for most people's results than simply being able to follow a chosen pattern consistently over weeks and months, since an aggressive schedule abandoned after a few days typically achieves less than a more moderate one sustained long-term. Choosing a window that realistically fits existing sleep, work and social patterns, rather than the most restrictive option available, is accordingly a more evidence-aligned starting point for most people than picking the shortest possible eating window purely because it sounds more effective.