Intermittent Fasting Calculator
Tell us when you want your first meal. We’ll show you exactly when to stop eating and when your fast ends, for whichever schedule you pick.
Intermittent fasting means restricting food to a set window each day rather than changing what you eat. The most common pattern, 16:8, is 16 hours without food and an 8-hour eating window. The fasting itself is not magic. In trials where calories are matched, fasting and normal eating produce similar fat loss. It works when the shorter window leads you to eat less overall.
Eating window opens
12:00
Stop eating at
20:00
Fast ends
12:00
next day
Before you start: intermittent fasting is not suitable during pregnancy or breastfeeding, under 18, or with a history of disordered eating. If you take medication with food, particularly insulin or a sulfonylurea, speak to your prescriber first, because fasting can cause blood glucose to fall too low.
How the schedules compare
| Schedule | Fast | Eat | Best for |
|---|---|---|---|
| 12:12 | 12h | 12h | A first step. Barely different from normal eating. |
| 14:10 | 14h | 10h | Noticeable but easy to live with |
| 16:8 | 16h | 8h | The default. Best balance of effect and sustainability. |
| 18:6 | 18h | 6h | Once 16:8 feels routine |
| 20:4 | 20h | 4h | Hard to hit protein and micronutrient needs |
| OMAD | 23h | 1h | Rarely necessary. The easiest schedule to under-eat on. |
Longer fasts are not proportionally better. Past roughly 16 hours the main thing that changes is how difficult it becomes to eat enough protein.
What happens during a fast
A rough timeline of what your body is doing as the hours pass. Treat the boundaries as gradual rather than sharp, since they depend on your last meal, your activity and how adapted you are.
| Hours in | What is happening |
|---|---|
| 0 to 4 | Digesting and absorbing your last meal. Blood glucose and insulin are elevated, then settle. |
| 4 to 12 | Insulin falls. Your body runs mostly on stored liver glycogen. Hunger often peaks around your usual mealtime and then passes. |
| 12 to 16 | Liver glycogen runs low and fat becomes a larger share of fuel. This is where most 16:8 fasts end. |
| 16 to 24 | Ketone production rises. Some people report clearer focus here, others just feel hungry. |
| 24+ | Autophagy markers increase, though nearly all of the evidence for meaningful benefit comes from animal studies rather than humans. |
Autophagy gets quoted a great deal online, usually with a precise hour attached. In humans the timing has not been established, and the studies people cite were done in mice. It is a reason to be curious, not a reason to extend your fast.
What a 16:8 day looks like
An example built around a noon to 8pm window, hitting roughly 2,000 calories and 150 g of protein. Two solid meals plus a snack is usually easier to manage than three small ones.
Break the fast
Eggs with spinach and whole grain toast, or Greek yogurt with berries and nuts. Protein and fiber first keeps the afternoon steady.
Snack
Cottage cheese and fruit, or a protein shake if you train in the afternoon. This is the meal people skip, and skipping it is why dinner gets out of hand.
Dinner
Salmon or chicken with rice and a large pile of vegetables. Make it the biggest meal, since a full stomach makes the overnight fast much easier.
Window closes
Water, herbal tea or black coffee from here. Most of the fast happens while you are asleep, which is the entire trick.
Starting without hating it
Most people who quit in week one jumped straight to 16:8 from three meals and snacks. Moving the window gradually works far better.
Week one: 12:12. Stop eating after dinner and delay breakfast slightly. For a lot of people this is close to what they already do.
Week two: 14:10. Push breakfast back an hour or two. Black coffee or tea in the morning helps, and so does having something to do.
Week three: 16:8. By now the morning hunger has usually faded, because ghrelin adjusts to your new pattern within a week or two.
Then stop. There is no prize for going further. If 16:8 is comfortable and your intake is where you want it, you are done.
Mistakes that undo the whole thing
Treating the window as a free pass
Eight hours is plenty of time to eat well past maintenance. If the scale is not moving, the window is not the problem. Check your intake against the calorie calculator.
Letting protein slide
Fewer meals means fewer chances to eat protein, and it is the first thing to fall. Work out your target with the protein calculator and split it deliberately.
Forgetting that food is hydration
Roughly a fifth of your fluid normally comes from meals, so a long fasting window quietly reduces intake. Headaches during a fast are often thirst or low sodium rather than hunger.
Going too long, too soon
Jumping to 20:4 in week one produces a miserable two weeks and an abandoned plan. The schedule you keep for six months beats the strict one you quit in ten days.
Eating very late
A window that drifts to 4pm until midnight technically counts, but late heavy meals tend to disrupt sleep, and poor sleep makes the next day’s hunger considerably worse.
What the research actually supports
For fat loss, fasting performs about as well as ordinary calorie restriction and no better, once calories are matched. Its advantage is practical rather than metabolic: fewer meals to plan, fewer decisions, less snacking.
For blood sugar, results are more encouraging. Time-restricted eating improves insulin sensitivity in several trials, with earlier windows tending to do better than later ones. If this is your reason for trying it, lean toward eating earlier in the day.
For longevity, the honest answer is that nobody knows. The animal data is genuinely interesting. The human data is short-term and measures markers rather than lifespan. Anyone telling you fasting adds years is describing mice.
For muscle, a 2016 trial of trained men on 16:8 found they kept their muscle and strength over eight weeks while losing fat. Protein intake was held high throughout, which is the part that tends to get left out when the study gets quoted.