Intermittent Fasting Calculator

Know exactly when to break your fast.

Pick your fasting protocol and when you stopped eating. We'll do the math.

i Not medical advice. Breaking Fast is an informational tool built by an enthusiast, not a healthcare professional. Please consult your doctor before starting any fasting practice, especially if you have medical conditions, are pregnant, or take medication.
Step 1

How long is your fast?

16h
Step 2

When did you stop eating?

:
Scroll each column to adjust
Your result

You can break your fast:

0%
00:00:00
Remaining
Fast started
Duration 16 hours
Break your fast on

Your history

Streak: 0 days
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What's happening inside

Your body moves through stages as you fast.

A general timeline based on published research. Individual responses vary.

4h
Digestion ends
Food has left the stomach. Insulin levels generally start to drop.
Now
8h
Glycogen burning
The body typically shifts toward stored glucose for energy.
Now
12h
Metabolic switch
Glycogen runs low for many adults. Fat oxidation begins to accelerate.
Now
16h
Autophagy begins
Research suggests cellular cleanup processes start to ramp up here.
Now
18h
Ketosis
The liver may begin producing ketones. Some people report mental clarity.
Now
24h
Deep autophagy
Studies suggest cellular renewal advances. Growth hormone tends to rise.
Now

Stay hydrated.

Tap a glass each time you drink water. Eight glasses is a common reference, but listen to your body.

0 / 8 glasses

Common ways to break a fast.

General practices many people find useful. Your nutritionist or doctor can tailor recommendations to you.

  1. i.Many people start with water — sometimes with a pinch of sea salt after longer fasts — to gently rehydrate.
  2. ii.Lean protein (eggs, fish, chicken) is a common first food.
  3. iii.Healthy fats such as avocado, olive oil, or nuts are often added.
  4. iv.Refined sugars and ultra-processed carbs may cause discomfort after long fasts.
  5. v.Eating slowly and stopping before feeling completely full can help avoid bloating.
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Questions

Frequently asked questions.

Common questions about intermittent fasting. None of this is medical advice — when in doubt, consult a professional.

A note before you read on. The articles below summarize publicly available research and common practices. They are not personalized medical advice. If you are pregnant, breastfeeding, under 18, have diabetes, eating disorders, or take medications, please talk with a healthcare professional before fasting.
Learn

The science behind intermittent fasting.

Short reads summarizing published research. For practical guidance specific to your situation, consult a professional.

Fundamentals · 4 min read

What intermittent fasting actually is.

Intermittent fasting describes an eating pattern rather than a diet — it focuses on when you eat instead of what. Most protocols alternate between an eating window (typically 4 to 10 hours) and a fasting window (14 to 20 hours). During the fasting window, the body generally works through stored glucose and gradually transitions toward fat as a fuel source.

Published research has explored connections to insulin sensitivity, inflammation markers, weight management, and autophagy — a cellular cleanup process where the body breaks down and recycles damaged cells. Work by Yoshinori Ohsumi (Nobel Prize, 2016) and follow-up studies suggest autophagy tends to increase meaningfully after 16 hours of fasting in many adults, though individual responses vary.

The 16:8 protocol is popular largely because it is sustainable: for many people it amounts to skipping breakfast, eating lunch around midday, and finishing dinner by early evening. Adaptation usually takes one to two weeks, but this varies.

Methodology · 5 min read

Choosing a protocol.

A common starting point is 12:12 — twelve hours of eating, twelve of fasting. In practice this often means simply not snacking after dinner. Many people then extend gradually, adding an hour or two per week until they find a rhythm that fits their life.

14:10 works well for people who feel weak skipping breakfast altogether. 16:8 is the most widely studied protocol and is often considered approachable. Both fit easily around social meals.

More aggressive approaches like 18:6, 20:4, and OMAD (One Meal A Day) push further into autophagy and ketosis but require more careful nutrient planning. OMAD in particular concentrates all daily nutrition into a single meal, which can risk deficiencies in protein, fiber, and micronutrients. Extended fasts beyond 24 hours are typically discussed with a healthcare professional first.

Paying attention to how your body responds matters more than the specific number of hours. Persistent dizziness, irritability, or sleep disruption are signs worth taking seriously — consider shortening your window or consulting a professional.

Common pitfalls · 3 min read

Patterns that limit results.

Overeating in the eating window. Fasting alone doesn't change total calorie balance. If the same calories are consumed in a compressed window, weight loss tends to stall. Metabolic effects may still apply, but body composition changes won't.

Caloric drinks during the fast. Cream in coffee, juices, and even some "zero-calorie" drinks with sweeteners can affect insulin response and the metabolic state of the fast. Most practitioners stick to water, black coffee, and plain tea.

Electrolyte depletion. Longer fasts can reduce sodium, potassium, and magnesium. A pinch of sea salt in water during fasts longer than 18 hours is a common practice that may help with headaches and fatigue.

Starting too aggressively. Jumping from frequent meals straight to OMAD often backfires. Most people adapt better by adding an hour per week until they reach their target.

Underestimating sleep. Poor sleep raises cortisol and ghrelin, which tends to make fasting feel harder. Many find that protecting seven to nine hours of sleep makes longer protocols more sustainable.

Tools and resources

What others recommend.

Products and books frequently mentioned by the fasting community. We may earn a small commission on purchases — it helps keep this tool free.

Disclosure: Some links below are affiliate links. We only include items we'd recommend regardless.
Book

The Complete Guide to Fasting

Jason Fung's accessible deep-dive into the science and practice of fasting. The most-cited starting point in the community.

View on Amazon →
Supplement

LMNT Electrolytes

Zero-sugar electrolyte drink mix many fasters use to prevent headaches and fatigue on longer fasts.

View on Amazon →
Gear

Smart Body Composition Scale

Withings or Renpho — both well-reviewed for tracking weight and body fat over time, syncing with health apps.

View on Amazon →
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