Science-Backed Tools

Fasting Calculator

Four calculators in one. Plan your fasting window, calculate your calorie needs, track your metabolic stage, or plan a 5:2 or ADF fasting week.

Fasting Window Calculator

Enter your fast start time and choose a protocol. We'll calculate your eating window, show a 24-hour timeline, and tell you when each fasting stage begins.

Fast starts
—
today
Eating window opens
—
tomorrow
Eating window closes
—
tomorrow
Your 24-hour day
12am3am6am9am 12pm3pm6pm9pm12am
Fasting
Eating window
When key fasting stages begin
Note: Stage timings are approximate and vary by individual, meal composition, and activity level. These are commonly cited ranges based on metabolic research.

Calorie & TDEE Calculator

Uses the Mifflin-St Jeor equation — the most validated formula for estimating resting metabolic rate — combined with your activity level to calculate your total daily energy expenditure and goal-based calorie targets.

Please enter an age between 15 and 90.
Please enter a weight between 30 and 300 kg.
Please enter a height between 100 and 250 cm.
BMR
—
kcal/day at rest
TDEE
—
kcal/day total burn
Target calories
—
kcal/day
Your daily target breakdown
Recommended macronutrient ranges
How fasting helps
Mifflin-St Jeor equation: Male: (10 × kg) + (6.25 × cm) − (5 × age) + 5. Female: (10 × kg) + (6.25 × cm) − (5 × age) − 161. This formula has the strongest validation in peer-reviewed literature for non-athletic adults. Individual results vary — treat this as a starting estimate and adjust based on real-world results over 2–4 weeks.

Fasting Stage Tracker

Enter how many hours you have been fasting to see which metabolic stage you are currently in, what is happening in your body, and what comes next.

Please enter a value between 0 and 168 hours.
All fasting stages
Important: Stage timings are population averages based on metabolic research. Your personal response depends on insulin sensitivity, last meal composition, activity level, and individual metabolism. These ranges are directional, not diagnostic.

5:2 & ADF Planner

Calorie-restriction fasting methods require specific targets on fast days. Enter your TDEE (use the Calorie & TDEE calculator above) and choose your method to get exact daily targets and a weekly meal plan framework.

Please enter a TDEE between 1000 and 5000 kcal.
Normal day calories
—
kcal
Fast day calories
—
kcal
Weekly deficit
—
kcal/week
Your weekly schedule
Fast day meal guide
5:2: Fast days should be non-consecutive. ADF: On modified ADF (recommended for beginners), fast days allow 25% of TDEE rather than complete fasting. Both methods have peer-reviewed evidence for weight loss comparable to continuous calorie restriction, with potential additional benefits for metabolic health markers.
Before you take these numbers too literally

These numbers are a starting point, not a prescription. The calculator uses validated formulas, but it doesn't know your medical history, your medications, or what your body actually does with food. Talk to your doctor before making changes to your eating routine — and be especially careful with any form of fasting if you:

  • are pregnant, breastfeeding, or trying to conceive;
  • are under 18 or over 80;
  • have a BMI in the “underweight” category;
  • have a history of, or are at risk for, disordered eating;
  • are dealing with chronic stress or adrenal issues;
  • have nutritional deficiencies;
  • have an existing medical condition like diabetes, insulin resistance, or heart disease; or
  • take medications that need to be taken with food, or that affect blood sugar or blood pressure.