About Intermittent Fasting Calculator

This intermittent fasting calculator helps you do two things at once:

• Track your fast with a simple countdown timer (client-side, in your browser).

• Understand what’s happening at different fasting durations through an educational timeline, fasting phases, and practical guidance.

It is designed to work for both beginners (who want a clear “what do I do next?” experience) and experienced fasters (who want a fast timeline, a clean goal slider, and useful safety context).

This tool is educational and not medical advice.

What this intermittent fasting calculator shows

1) Fasting duration (hours)

You can set a fasting goal using common protocols (for example 16:8, 18:6, OMAD) or choose a custom duration. The calculator supports short fasts and multi-day fasts, and visually maps hours across a timeline.

2) A fasting timeline with phases

The timeline is color-coded to represent different fasting phases. These labels are educational, not guarantees—your exact timeline depends on diet, glycogen, training status, sleep, stress, and individual response.

3) A “where you are now” marker (during a running fast)

When a fast is running, the calculator shows:

• A thin marker that represents current progress (elapsed time).

• A thick marker that represents your goal end point (target duration).

• A dimmed zone showing what’s beyond your goal, so it’s obvious what is “outside the plan.”

4) A ketosis likelihood indicator (educational)

The calculator includes a ketosis meter intended as a rough educational signal, not a substitute for blood ketone testing. Ketone levels can vary widely between people.

5) Optional weight loss estimates

If you choose to use the “Weight Loss Estimates” section, you can enter basic body data and activity level. The calculator then produces a conservative estimate of potential fat loss based on an estimated energy deficit.

This is optional and intentionally cautious because real-world fat loss is often lower than simple math suggests.

6) Optional safety checks

Safety checks allow you to flag higher-risk situations (for example under 18, pregnancy, breastfeeding, diabetes medications). If any apply, prolonged fasting may be unsafe.

How to use the fasting timer

Step 1: Pick a fasting protocol (or go custom)

Use the “Common Protocols” pills to quickly select a typical intermittent fasting pattern. The default is set to 18 hours, which is a common starting point for people who want a meaningful fast without pushing into more extreme territory.

If you drag the timeline handle, the calculator switches to Custom automatically.

Step 2: Set your start time (or leave it as “Now”)

• If you are about to begin fasting, leave the start time as Now.

• If your fast began earlier, you can set the time to match when you actually started.

Once the fast is running, the start time display changes to:

• “Fast Started On …” so you can see the logged time clearly.

Step 3: Click “Start Fasting”

The interface switches into a running state:

• The circle is replaced by a countdown timer to avoid layout shift.

• The timeline shows your current progress plus your goal.

Step 4: Adjust your goal while fasting (if needed)

If your plan changes, you can simply drag the thick goal marker during the fast. This updates:

• The countdown

• The % complete

• The “goal” readout

• The dimmed “outside goal” portion of the timeline

Step 5: Stop fasting

Click Stop Fasting to end the running timer.

Understanding fasting phases

This intermittent fasting calculator breaks fasting into phases so the experience is easier to interpret. Phases are educational “buckets,” not strict biological switches.

Typical phases include:

• Fed / Absorptive (0–4 hours)
  Your body is mostly processing recent food. Insulin is generally higher than later phases.

• Post-Absorptive (4–12 hours)
  Your body transitions toward stored fuel. Liver glycogen plays a bigger role in maintaining blood glucose.

• Early Fasting (12–18 hours)
  Glycogen tends to decline further. Fat oxidation generally increases for many people.

• Ketosis Rising (18–24 hours)
  Ketones may rise more noticeably, especially if your diet is lower carb or you are active.

• Deepening Ketosis (24–48 hours)
  Reliance on stored fuel often increases. This is also where people frequently notice hydration and electrolyte issues.

• Prolonged / Extended (48–72+ hours and beyond)
  Potential benefits and risks both increase. Refeeding strategy and medical context matter more.

The calculator intentionally uses conservative wording in the longer-range phases because safety becomes more important the longer a fast goes.

Electrolytes, hydration, training, and refeeding

This tool includes practical reminders because most fasting issues are not “willpower” problems—they are often hydration, sleep, electrolyte, or pacing problems.

General guidance many fasters find useful:

• Hydration: Drink water consistently rather than in big bursts.

• Electrolytes: For longer fasts, electrolytes are commonly discussed (sodium, potassium, magnesium). Needs vary and certain conditions/medications can make electrolyte strategies unsafe. If you have kidney disease, blood pressure issues, or take medications, consult a clinician first.

• Training: Consider reducing intensity during longer fasts. Light movement is often better tolerated than maximal strength work.

• Breaking a fast: Many people do better breaking a longer fast with a smaller, gentle meal instead of an oversized “refeed.”

How weight loss estimates are calculated

The “Weight Loss Estimates” section is optional and conservative. It aims to avoid over-promising.

In general, the calculator:

• Estimates daily energy burn (TDEE) using a lean-mass method when body fat % is provided, and adjusts by activity.

• Estimates a calorie deficit during the fasting period.

• Applies a conservative “real-world” reduction factor because energy burn often drops and behavior changes during fasting.

• Converts energy deficit to fat loss using a standard approximation (7700 kcal per kg of fat).

Important notes:

• Scale weight changes during fasting are heavily affected by water, glycogen, sodium, and GI content, especially over 24–72 hours.

• The estimate is most meaningful as a ballpark educational number, not a prediction.

Safety notes and who should not fast

This intermittent fasting calculator includes safety flags because fasting is not appropriate for everyone.

Prolonged fasting may be unsafe if you are:

• Under 18

• Pregnant

• Breastfeeding

• Using glucose-lowering medications (risk of hypoglycemia)

• Managing kidney disease

• Have a history of eating disorders

• Underweight (low BMI) or have conditions that can worsen during fasting (for example gout/high uric acid)

If any of these apply, follow clinician guidance first.

What this tool is best for

• Picking an intermittent fasting protocol and tracking it consistently

• Learning the “big picture” of fasting phases without overwhelm

• Adjusting your goal mid-fast without restarting

• Getting optional, conservative estimates (if you choose to enter body data)

• Staying aware of safety considerations