Intermittent Fasting Calculator
Most 16:8 schedules assume you wake at 6 and sleep at 10. Pick a protocol, enter your real wake and bedtime, and get an eating window that fits your actual day — with a full fasting timeline and an honest read on what the research does and doesn't support.
Your day
This tool schedules the clock, not your health. Time-restricted eating is not appropriate for everyone — see the FAQ below, and check with a clinician if you take glucose-lowering medication, are pregnant or breastfeeding, or have any history of disordered eating.
How the Schedule Is Built
The calculator anchors your window to three fixed points. First, your wake time: the window never opens sooner than one hour after waking, because rolling straight from bed into breakfast shrinks the fasted portion of the morning — the part most people find easiest. Second, your bedtime: the window closes at least three hours before lights out, which is the standard sleep-hygiene recommendation — late meals are linked to worse sleep quality and reflux, and poor sleep is one of the strongest appetite dysregulators there is. Third, your protocol: the fasting hours (14, 16, 18, 20, or 23) determine how many hours of eating remain to place between those two buffers.
When both placements are possible, the calculator shows the one you picked; if your available span between the buffers is shorter than the eating window, it tells you directly and suggests a gentler protocol. A 16:8 window needs 8 available hours between "1 h after waking" and "3 h before bed" — meaning roughly a 12-hour span from waking to bedtime. Shift workers: the math is identical, just rotate the clock — the fast runs through your sleep whichever way it faces.
Which protocol to start with
| Protocol | Fasting / eating | Best for | Difficulty |
|---|---|---|---|
| 14:10 | 14 h / 10 h | First-timers, big breakfast people, anyone coming off grazing all day | Easy |
| 16:8 | 16 h / 8 h | The default. Fits most work schedules; skip breakfast, eat lunch through dinner | Moderate |
| 18:6 | 18 h / 6 h | People already comfortable on 16:8 who want tighter structure | Hard |
| 20:4 | 20 h / 4 h | Advanced; usually one large meal plus a small second. Hard to hit protein targets | Hard |
| OMAD (23:1) | 23 h / 1 h | Rarely necessary. Single meal makes protein, fiber, and social life all harder | Hardest |
Longer fasts are not better fasts. Head-to-head trials of different fasting windows show similar results when calories match — adherence decides the outcome, so pick the gentlest schedule you can hold for months.
What breaks a fast
The practical rule: under ~5 calories and no meaningful insulin response keeps the fast intact. That covers water (plain, sparkling, infused with nothing caloric), black coffee, and unsweetened tea. It does not cover milk or cream in coffee, juice, sugar-free gum by the handful, BCAAs, or the "just a bite while cooking" that quietly restarts digestion. The honest corollary: if a splash of milk in your coffee is the difference between holding the schedule for months and quitting in three weeks, the splash wins — a near-fast that lasts beats a perfect fast you abandon.
One genuinely underrated item is fluid volume. Roughly 20-30% of daily water normally arrives in food; fast through breakfast and that contribution disappears for the morning. Most "day one fasting headaches" are dehydration, not caffeine withdrawal. Run your numbers through the water intake calculator and front-load a large glass at wake-up — before the coffee, not with it.
Where fasting meets meal prep
Compressed eating windows raise the stakes on food quality: when you have 8 hours instead of 14 to eat, every meal has to pull its weight. That is exactly what meal prep is for. Sunday's containers put a high-protein lunch and dinner inside the window with zero weekday decisions, which removes the two most common failure points — the vending-machine lunch when the window is open but the fridge is empty, and the 9 p.m. raid when it isn't. Check your protein target with the protein calculator first: inside a compressed window, protein needs stay the same (about 1.6 g per kg of body weight) but the per-meal amounts get bigger, and under-shooting protein while fasting is the one combination that risks muscle loss.
The honest part: what fasting does and doesn't do
A landmark 2019 review in the New England Journal of Medicine concluded that time-restricted eating produces weight loss primarily because the eating window makes people eat less — not because of fasting-specific fat-burning, "autophagy timing," or metabolic tricks. Controlled trials comparing 16:8 against ordinary calorie restriction find roughly equal results when calories match. Fasting is a scheduling tool that makes a calorie deficit easier to hold for some people — no magic, but also no harm for most. If the schedule makes you obsessive, exhausted, or prone to overeating when the window opens, that is a real signal to eat on a normal schedule instead. Electrolytes are the one supplement with a legitimate fasting case: on longer fasts or during the first adaptation week, a sugar-free electrolyte powder in water covers sodium, potassium, and magnesium without breaking the fast.
Frequently asked questions
What can I have during the fasting window?
Water, black coffee, and unsweetened tea — anything under about 5 calories. Milk in coffee, juice, and BCAAs all end the fast in practical terms. The most overlooked item is water itself: food normally supplies a fifth to a third of your daily fluid, so fasting mornings need deliberate hydration. Day-one fasting headaches are usually dehydration, not hunger.
Will fasting make me lose muscle?
Not by itself — studies show muscle is preserved when protein stays high within the eating window and resistance training continues. The real risk is the reverse: compressed windows make under-eating protein easier. Get your daily gram target, split it across the meals your window allows, and keep lifting.
Does my eating window have to be identical every day?
No. Consistency builds the habit, but a one-to-two-hour shift for a weekend dinner is fine — flexible schedules show similar results to rigid ones. The number worth protecting is roughly 14-16 fasting hours most days, not the exact clock times.
Who shouldn't do intermittent fasting?
Anyone on glucose-lowering medication, people who are pregnant or breastfeeding, anyone with a history of disordered eating, and growing children or teens — all need medical supervision or a different approach entirely. And fasting is not superior to ordinary calorie restriction: if the schedule breeds binge-restrict cycles, eating normally is the better tool.
Where these numbers come from
- de Cabo & Mattson, NEJM 2019 — the benchmark review of intermittent fasting: effects, mechanisms, and the honest state of the evidence.
- Harvard Health Publishing — clinical perspective on time-restricted eating, benefits, and who should be cautious.
- NIH / NIDDK — weight-management fundamentals: the calorie balance that any eating schedule, fasting included, has to work through.