Intermittent fasting calculator

How your intermittent fasting window is calculated

This is clock arithmetic, not a formula with body composition inputs. Every intermittent fasting protocol is written as two numbers that add up to 24: hours fasting, then hours eating. The calculator takes the eating hours for the protocol you pick, adds them to the time of your first meal, and that sum is when your eating window closes. Everything else in the 24 hours is the fast.

For a 16:8 protocol with a first meal at 12:00, the eating window runs from 12:00 to 20:00, eight hours later. The fast is the remaining sixteen hours, from 20:00 to 12:00 the next day. If your first meal is late enough that the eating window would run past midnight, the calculator wraps it into the next day rather than showing an impossible time, so a first meal at 20:00 on the same 16:8 protocol gives an eating window of 20:00 to 04:00 and a fast from 04:00 to 20:00.

The bar underneath the result shows the same two numbers as a proportion of the full 24 hours, so you can see at a glance how much of your day is eating window and how much is fast, whatever time you start.

The intermittent fasting protocols explained

All of the protocols in the calculator are daily patterns: the same eating window at roughly the same time every day. They differ only in how long that window is.

14:10. A 10 hour eating window and a 14 hour fast. The gentlest daily protocol, close to a normal overnight gap stretched a little longer. A reasonable place to start if you have never restricted eating times before.

16:8. An 8 hour eating window and a 16 hour fast. The protocol most people mean when they say intermittent fasting, and the eating window length used in both trials described below.

18:6. A 6 hour eating window and an 18 hour fast. A meaningfully harder step down from 16:8, usually meaning two meals rather than three.

20:4. A 4 hour eating window and a 20 hour fast, sometimes called the Warrior Diet. Most days become one large meal and a small second one.

OMAD, 23:1. One meal a day, eaten inside a single hour. The most restrictive protocol here, and the hardest to hit protein and micronutrient targets on without planning the meal carefully.

None of these is inherently better for weight loss than another. A shorter eating window mainly makes it easier to eat less without counting, because there is less time in the day to eat in. It does not add anything on top of that, which is the point covered in the next section.

Fasting and eating windows at a glance

The table shows what each protocol looks like with a first meal at 12:00, the calculator default. Enter your own first meal time above to shift every column.

Why intermittent fasting can help you lose weight

The honest version: intermittent fasting works mainly by making a calorie deficit easier to hit for some people, not through anything special about the timing itself. Compress eating into a shorter window and there is simply less time to snack, graze or add extra meals, so many people end up eating less without deliberately counting anything. Total calorie intake, not the clock, is still what decides whether you lose weight. Work out your own numbers with the calorie deficit calculator or TDEE calculator.

Two randomised trials tested eating windows directly, and both point the same way. The TREAT trial (Lowe et al, 2020) put 116 adults with overweight or obesity on either a 16:8 eating window (eating freely from 12:00 to 20:00) or three structured meals a day, with no calorie targets given to either group. After twelve weeks, the time restricted group lost 0.94 kg and the structured meal group lost 0.68 kg, a difference of 0.26 kg that was not statistically significant, and the authors found no difference in estimated energy intake between the groups. In other words, an eating window on its own, without eating less, did little.

A larger trial (Liu et al, 2022) tested time restricted eating against an equal calorie target rather than against no target at all. Over 12 months, 139 adults with obesity were assigned to an 8 hour eating window plus calorie restriction, or the same calorie restriction with no eating window. Both groups followed the same calorie targets. The time restricted group lost 8.0 kg and the calorie restriction only group lost 6.3 kg; the reported net difference of 1.8 kg was not statistically significant, and body fat results were consistent with that. When the calorie target is matched, adding an eating window on top did not produce significantly more weight or fat loss.

Put together, the two trials point the same way from different angles: an eating window without eating less did not beat normal meal timing, and an eating window added to the same calorie target did not significantly beat that target alone. If counting calories directly works better for you than restricting the time you eat in, these trials give no reason to force a fasting window on top.

What breaks a fast

There are two different questions hiding under this one: what breaks the fast you are trying to keep for a specific protocol, and what breaks the metabolic state fasting is trying to reach. They do not have the same answer.

For the protocol. Anything with meaningful calories, food or drink, starts your eating window for that protocol. Black coffee, plain tea and water have negligible calories and do not count against your eating window for practical purposes. Milk, sugar, cream or a splash of juice in your coffee do add calories, however small, and strictly speaking start the window.

For the metabolic switch. A 2019 review in the New England Journal of Medicine (de Cabo and Mattson, 2019) describes a metabolic switch, where the body moves from using mostly glucose towards using fatty acids and ketones once liver glycogen runs low. The review links that to energy restriction of 10 to 14 hours or more, notes that blood ketones in humans start to rise within 8 to 12 hours of fasting, and gives an 18 hour fast with a 6 hour eating window as an example regimen a doctor might prescribe. How far into the switch you get on a given day still depends on what and how much you last ate and how active you are, so the clock alone does not tell you.

Either way, a fast breaking a few minutes early because of a splash of milk is not something to worry about. The protocol is a practical eating pattern, not a medical procedure with a hard cutoff.

Training while fasting

Training in a fasted state is a personal preference more than a performance requirement. Some people train well first thing before their eating window opens; others feel flat and perform worse without food beforehand. Neither is wrong, and it is worth testing on yourself rather than assuming either way.

A few practical points apply regardless of preference. Water, and electrolytes on a longer or hotter session, still matter during a fast; fasting affects food, not fluids. If you train fasted, schedule your first meal of the eating window to land soon after the session so protein and carbohydrate are not delayed for hours. For strength training specifically, several hours without protein beforehand is unlikely to matter for a single session, but stringing together many low protein days on a short eating window can make it harder to hit a daily protein target that supports recovery.

If performance drops noticeably on fasted training and does not improve after a couple of weeks of adjusting, that is a signal to shift the eating window earlier or move training inside it, not a reason to push through indefinitely.

Who should not try intermittent fasting

Intermittent fasting is not appropriate for everyone, and a few groups should avoid it or only try it with medical guidance.

Pregnant or breastfeeding. Nutrient and energy needs are higher and more time sensitive than usual; this is not the time to restrict an eating window.

A history of disordered eating. Structured restriction, even framed around timing rather than calories, can reinforce harmful patterns for someone recovering from or prone to an eating disorder.

On medication that can cause low blood sugar, including insulin or sulfonylureas for diabetes. Skipping meals on these medications can cause dangerous hypoglycaemia; any change to eating pattern needs to go through the prescriber first.

Outside of these groups, most healthy adults can try intermittent fasting safely, but speak to a doctor first if you have an existing medical condition or take regular medication.

Common mistakes

Eating whatever fits in the window. A shorter eating window does not cancel out a calorie surplus. Two large, calorie dense meals can outstrip what three smaller meals would have added up to.

Skipping protein planning on a short window. OMAD and 20:4 leave one or two meals to carry an entire day of protein. Plan the meal around a protein source first, not last.

Choosing the hardest protocol first. Jumping straight to OMAD or 20:4 without ever trying 16:8 makes it more likely you quit in the first week. Start with the gentlest protocol that still feels like a change.

Assuming the fast itself burns fat faster. In the matched calorie trial above, adding a fasting window to the same calorie target did not produce significantly more weight loss.

Ignoring hydration during the fast. Coffee and tea count towards fluid intake, but plain water still matters, especially on longer protocols or on training days.

What about the 5:2 diet?

The 5:2 diet is a weekly pattern, not a daily clock pattern, so it does not fit the calculator above and is worth covering separately. On five days of the week you eat normally, with no restriction on timing. On two non-consecutive days, you eat a sharply reduced amount, typically around 500 to 700 kcal, spread across one or two small meals.

The underlying mechanism is the same as any other intermittent fasting protocol: it creates a weekly calorie deficit by concentrating the restriction into two days instead of spreading a smaller cut across all seven. Some people find two clearly defined restricted days easier to plan around than a daily reduction; others find two low calorie days harder to get through than a modest cut every day. The de Cabo and Mattson review describes two trials in overweight women where 5:2 and a 25 percent daily calorie cut produced the same weight loss over six months.

How coaches use an intermittent fasting calculator

For a coach, intermittent fasting is a scheduling tool, not a separate nutrition strategy from the calorie target underneath it. The useful question with a client asking about it is whether a shorter eating window will help them stick to the deficit they already need, not whether the fast itself will do extra work.

A simple approach: agree the calorie and protein target first, then let the client choose whichever eating window makes that target easiest to hit consistently, adjusting the protocol rather than the target if adherence slips.

Scraler keeps that in one place. Set a calorie and protein target per client with the meal planner, let them log meals against it, and check adherence in check-ins instead of guessing whether a fasting window is actually helping.