How intermittent fasting windows work
Intermittent fasting isn’t a diet so much as a schedule — you eat within a set window each day and fast the rest. The numbers name the split: 16:8 means 16 hours fasting and an 8-hour eating window. This calculator turns whichever protocol you pick, plus the time you break your fast, into your exact eating and fasting hours.
The most common protocols, from gentlest to most aggressive:
| Protocol | Eating window | Who it suits |
|---|---|---|
| 14:10 | 10 hours | Beginners; easiest to sustain |
| 16:8 | 8 hours | The popular default — usually skips breakfast |
| 18:6 | 6 hours | Once 16:8 feels easy |
| 20:4 | 4 hours | Experienced fasters (“warrior”) |
| 23:1 (OMAD) | ~1 hour | One meal a day; hard to hit nutrition targets |
Start gentle. Most people do best beginning at 14:10 or 16:8 and only tightening the window if it stays comfortable. A window you can hold every day beats an aggressive one you abandon in a week.
Why fasting can help with weight
The main reason intermittent fasting works for weight loss is unglamorous: a shorter eating window usually means fewer total calories, because there’s simply less time to eat. Some people also find a defined window easier to stick to than counting calories all day. There’s ongoing research into metabolic effects beyond calorie reduction — insulin sensitivity, cellular repair — but the calorie effect is the reliable one.
What breaks a fast is anything with meaningful calories. Water, black coffee, and plain tea are fine and won’t interrupt the fasting window; anything with sugar, cream, or sweetener — or anything you chew — ends it.
Intermittent fasting on a GLP-1
Here’s where you need to be thoughtful. GLP-1 medications already do much of what fasting does — they blunt appetite and cut how much you eat. Stacking a tight fasting window on top can backfire in a specific way: you compress an already-small appetite into a few hours and end up unable to get enough protein in, which is exactly the nutrient you most need to protect muscle during rapid weight loss.
If you’re on a GLP-1 and want to fast, the sensible version is a gentle window — 14:10 or 16:8 — used mainly to structure your day, with protein as the priority at every meal. Aggressive protocols like 20:4 or OMAD make the protein problem worse and are rarely worth it here. And because both a GLP-1 and fasting can lower blood sugar, anyone taking one for type 2 diabetes (or on insulin or a sulfonylurea) should talk to their prescriber before combining them — the risk of going too low is real.
Making it sustainable
Ease in, hydrate through the fast, and judge a protocol by whether you can hold it on a normal week, not a perfect one. If a window leaves you lightheaded, ravenous, or unable to hit your protein, it’s too aggressive — widen it. This tool is a scheduling aid and general education, not medical advice; if you have a history of disordered eating, are pregnant or breastfeeding, or manage a condition with medication, check with a clinician before starting.