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Intermittent Fasting Calculator

Enter your last meal time to see where you are in your fast: your current metabolic stage, time remaining, and when each stage of fasting begins.

If your last meal was yesterday, enter the time - we will calculate automatically.

16h fasting8h eating

The 16:8 protocol: a 16-hour fast and an 8-hour eating window across one day

4 min read·Holistic Health

What is intermittent fasting?

Intermittent fasting is an eating pattern that alternates set periods of fasting with a defined eating window. Unlike most diets, it does not tell you what to eat. It tells you when. The most common arrangement is 16:8, a 16-hour fast paired with an 8-hour eating window, though longer fasts are also practised.

The interest in the approach comes from what happens during the fasting hours. When food and the insulin response that follows it are absent for long enough, the body draws down its glycogen stores and shifts toward burning stored fat for fuel. As the fast extends, insulin settles to a low baseline, growth hormone rises, and cellular maintenance processes that are suppressed in the fed state become more active.

It is worth being clear about scope. The evidence for intermittent fasting is genuine but still developing, individual responses vary widely, and this tool is an informational tracker, not medical advice. It is not appropriate for everyone, and anyone with a health condition, a history of disordered eating, or who is pregnant or on medication should speak with a clinician before fasting.

How the calculation works

You enter the time of your last meal and choose a protocol: 16:8, 18:6, 20:4, a 24-hour fast, or a custom number of hours. If the entered time is later than the current time, the tracker assumes the meal was the day before and calculates accordingly.

From that point it measures elapsed hours against your chosen target, shows how much of the fast remains and when it completes, and maps your position onto a sequence of metabolic stages: Fed State (0 to 4 hours), Early Fasting (4 to 8), Fat Burning (8 to 12), Metabolic Shift (12 to 18), Autophagy Begins (18 to 24), and Deep Autophagy (24 hours and beyond). The stage you are currently in is highlighted, with the rest shown as a timeline.

Worked example: Your last meal was at 8:00pm and you choose the 16:8 protocol. The tracker counts 16 hours forward, completing the fast at 12:00pm the next day. If the current time is 9:00am, that is 13 hours elapsed, placing you in the Metabolic Shift stage (12 to 18 hours), with 3 hours remaining until your eating window opens.

Frequently asked questions

What can I consume during the fasting window?

The aim during the fast is to avoid anything that triggers a meaningful insulin or digestive response. Water, plain black coffee, and unsweetened tea are generally considered acceptable and can make the fast easier to sustain. Plain sparkling water is fine. What breaks a fast in practice is calories and the insulin response: anything with sugar, milk or cream, sweeteners that provoke an insulin response, or bone broth and similar caloric drinks moves you back toward the fed state. If your primary goal is autophagy rather than weight management, a stricter water-only approach is often preferred.

Is intermittent fasting safe for everyone?

No. Intermittent fasting is not appropriate for people with a history of eating disorders, those who are pregnant or breastfeeding, people with type 1 diabetes or who take insulin, underweight individuals, children and adolescents, and anyone whose medical condition or medication requires regular food intake. Even for healthy adults, very long fasts carry more risk and are best approached with knowledge and, ideally, medical guidance. If any of these apply to you, treat this tool as educational only and consult a healthcare provider before changing how you eat.

Which IF protocol should I start with?

For most people new to fasting, 16:8 is the sensible entry point. It is the gentlest of the common protocols and usually requires only skipping breakfast and eating between roughly noon and 8pm, with much of the fasting window covered by sleep. Once 16:8 feels comfortable and sustainable over several weeks, some people extend to 18:6 or 20:4 for a longer fasting window. Longer or more aggressive protocols are not better by default; the protocol you can hold consistently without strain is the one that produces results.

Fasting works best alongside the rest of your routine. The TDEE Calculator shows the daily energy target that should still be met inside your eating window. The Water Intake Calculator covers the hydration that matters even more during the fasting hours. And the BMR Calculator gives the baseline metabolic rate that anchors any change to how and when you eat.

Explore Related Tools

The 16:8 Protocol - Why It Became the Default

The 16:8 protocol achieved widespread adoption largely because it maps naturally onto existing behavior for many people: if your last meal is at 8pm and you skip breakfast, eating lunch at noon gives you a 16-hour fast with no calendar disruption. For most people, the 8 sleeping hours cover more than half the fasting window.

The metabolic benefits - improved insulin sensitivity, increased fat oxidation, growth hormone elevation - begin meaningfully after the 12-hour mark, making 16:8 the minimum protocol for those seeking these effects. Shorter eating windows (18:6, 20:4) amplify the benefits and extend the autophagy window.