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Fasting Calculator — Free Intermittent Fasting Timer & Tracker | Calcgator

Intermittent Fasting
Calculator & Timer

Choose your fasting protocol, set your start time and get a live countdown with fasting phases, eating window and health benefits. The simplest fasting calculator you'll ever use.

✓ 6 fasting protocols ✓ Live countdown timer ✓ Fasting phase tracker 📱 Works on mobile ⚡ No signup
⏱️
Fasting Calculator
Pick your protocol, set your time — start fasting
🍽️ Choose Your Fasting Protocol
🕐 16 : 8 16h fast · 8h eat
🔥 18 : 6 18h fast · 6h eat
⚡ 20 : 4 20h fast · 4h eat
🏆 OMAD 23h fast · 1h eat
🌱 14 : 10 14h fast · 10h eat
✏️ Custom Set your own
Fasting Hours
hours
Eating Hours
hours
🕐 When Did You Start Fasting?
▾
00:00:00
elapsed
Fed State
🚫 Fasting Window
8:00 PM – 12:00 PM
16 hours
🍽️ Eating Window
12:00 PM – 8:00 PM
8 hours
Fed0–4h
Early Fast4–12h
Fat Burning12–16h
Ketosis16–24h
Autophagy24h+
0h
Elapsed
16h
Remaining
0%
Complete
💡
Fed State (0–4 hours): Your body is digesting and absorbing nutrients from your last meal. Blood sugar and insulin levels are elevated. This is not yet fasting.
🔬 Fasting Phases & Benefits
🍽️ Fed State 0–4 hours Digesting food, insulin elevated
⏳ Early Fast 4–12 hours Insulin drops, glycogen depleting
🔥 Fat Burning 12–16 hours Glycogen depleted, body burns fat
⚡ Ketosis 16–24 hours Ketone production, mental clarity
🧬 Autophagy 24+ hours Cellular repair and recycling
🕐 Fast History
No fasts recorded yet — start your first fast above.
💡 Fasting Tips
💧
Stay hydrated
Drink water, black coffee or plain tea. Dehydration is the #1 reason people feel bad while fasting.
🧂
Electrolytes matter
Add a pinch of salt to water if fasting over 16 hours. Low sodium causes headaches and fatigue.
😴
Sleep counts as fasting
Most of your fasting window happens while you sleep. Start your fast after dinner for the easiest experience.
🚫
What breaks a fast
Anything with calories — milk, juice, sugar, gum with sugar. Black coffee and plain tea are fine.
📊 Protocol Reference
14:10Beginner
16:8Most popular
18:6Intermediate
20:4Advanced
OMAD1 meal/day
Fat burn starts~12 hours
Ketosis starts~16 hours
Times are approximate and vary by individual metabolism, activity level and diet.
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The science behind intermittent fasting

🔥
After 12 hours

Fat Burning Begins

After roughly 12 hours of fasting, your liver glycogen stores are depleted. Your body switches its primary fuel source from glucose to stored body fat. This metabolic switch is the core mechanism that makes intermittent fasting effective for weight management.

🧬
After 24 hours

Autophagy & Cell Repair

Extended fasting triggers autophagy — your body's cellular recycling system. Damaged proteins and organelles are broken down and reused. Research links autophagy to reduced inflammation, improved immune function, and potential longevity benefits.

⚡
3.6–14% increase

Metabolism Boost, Not Slowdown

Contrary to popular belief, short-term fasting increases metabolic rate by 3.6–14% due to elevated norepinephrine. Your body does not enter "starvation mode" during a 16–24 hour fast. Metabolism only slows with prolonged caloric restriction over weeks.

Complete Fasting Phase Timeline

0–4h
🍽️ Fed State
Body is digesting and absorbing nutrients. Insulin and blood sugar levels are elevated. No fasting benefits yet.
Digesting
4–12h
⏳ Early Fasting
Insulin levels begin to drop. Body starts using liver glycogen for energy. Blood sugar stabilises. Growth hormone starts rising.
Glycogen use
12–16h
🔥 Fat Burning Zone
Liver glycogen is depleted. Body switches to burning stored fat for fuel. This is the metabolic switch — the key benefit of intermittent fasting.
Fat oxidation
16–24h
⚡ Ketosis
Liver produces ketone bodies from fat. Brain uses ketones for fuel — many people report increased mental clarity and focus during this phase.
Ketone production
24h+
🧬 Autophagy
Deep cellular cleanup begins. Damaged cells and proteins are recycled. Linked to reduced inflammation, immune support and longevity. Only for experienced fasters.
Cell repair
FAQ

Got questions?
We've got answers.

Everything you need to know about intermittent fasting, protocols, safety and what to expect — answered below.

Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. The most popular method is 16:8 — fast for 16 hours and eat within an 8-hour window. It does not specify what to eat, only when to eat. Use the fasting calculator above to plan your schedule.
The 16:8 method is the most beginner-friendly. Fast for 16 hours (including sleep) and eat within an 8-hour window — for example, eating from 12 PM to 8 PM. Most people adapt within 3–5 days. If 16 hours feels too long, start with 14:10 and work your way up.
Water, black coffee (no sugar, no milk), plain green tea, and herbal tea are all fine during a fast. Anything with calories — juice, milk, soda, sweetened drinks — will break your fast. Sparkling water is also perfectly fine.
Your body typically starts burning stored fat after 12–14 hours of fasting, once glycogen stores in the liver are depleted. This is why most IF protocols recommend at least 16 hours of fasting — to ensure you spend meaningful time in the fat-burning zone. The fasting timer above tracks this phase for you.
For most healthy adults, intermittent fasting is safe and well-researched. However, it is not recommended for pregnant or breastfeeding women, children, people with eating disorders, or those with certain medical conditions like diabetes. Always consult your doctor before starting any fasting protocol.
No. Short-term fasting (16–24 hours) does not slow metabolism. Studies show it can actually increase metabolic rate by 3.6–14% due to increased norepinephrine. Prolonged caloric restriction (weeks of very low calories) is what slows metabolism — not intermittent fasting.